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  THE
  PREPARATION & MOUNTING
  OF
  MICROSCOPIC OBJECTS.


  BY
  THOMAS DAVIES.


  LONDON:
  ROBERT HARDWICKE, 192, PICCADILLY.
  AND ALL BOOKSELLERS.




_A New Edition, Revised and Enlarged._

_Price 2s. 6d. plain; 4s. coloured._

HALF-HOURS WITH THE MICROSCOPE. A Popular Guide to the Use of the
Microscope as a Means of Amusement and Instruction. With Drawings of
250 objects from Nature, by TUFFEN WEST. By E. LANKESTER, M.D., F.R.S.


CONTENTS:

      Half an hour on the Structure.
      Half an hour in the Garden.
      Half an hour in the Country.
      Half an hour at the Pondside.
      Half an hour at the Seaside.
      Half an hour Indoors.
      Appendix: the Preparation and Mounting of Objects.


_Fcp. 8vo., 6s. beautifully illustrated by hand-painting._

RUST, SMUT, MILDEW, AND MOULD under the MICROSCOPE: a Plain and Easy
Guide to the Study of Microscopic Fungi. By M. C. COOKE. Coloured
Plates of over 300 Figures.


London: ROBERT HARDWICKE, 192, Piccadilly; and all Booksellers.




PREFACE.


In bringing this Handbook before the public, the Author believes that
he is supplying a want which has been long felt. Much information
concerning the “Preparation and Mounting of Microscopic Objects” has
been already published; but mostly as supplementary chapters only,
in books written professedly upon the Microscope. From this it is
evident that it was necessary to consult a number of works in order
to obtain anything like a complete knowledge of the subject. These
pages, however, will be found to comprise all the most approved methods
of mounting, together with the results of the Author’s experience,
and that of many of his friends, in every department of microscopic
manipulation; and as it is intended to assist the beginner as well
as the advanced student, the very rudiments of the art have not been
omitted.

As there is a diversity of opinion as to the best mode of proceeding in
certain cases, numerous quotations have been made. Wherever this has
been done, the Author believes that he has acknowledged the source from
which he has taken the information; and he here tenders his sincere
thanks to those friends who so freely allowed him to make use of their
works. Should, however, anyone find his own process in these pages
_unacknowledged_, the author can only plead oversight, and his regret
that such should have been the case.




CONTENTS

                                                                    PAGE
  PREFACE                                                            iii


  CHAPTER I.

  APPARATUS.                                                           1


  CHAPTER II.

  TO PREPARE AND MOUNT OBJECTS “DRY.”                                 22


  CHAPTER III.

  MOUNTING IN CANADA BALSAM.                                          56


  CHAPTER IV.

  PRESERVATIVE LIQUIDS, ETC., PARTICULARLY WHERE CELLS ARE USED.      83


  CHAPTER V.

  SECTIONS AND HOW TO CUT THEM, WITH SOME REMARKS ON DISSECTION.      96


  CHAPTER VI.

  INJECTION.                                                         122


  CHAPTER VII.

  MISCELLANEOUS.                                                     140


  INDEX.                                                             153




THE

PREPARATION AND MOUNTING

OF

MICROSCOPIC OBJECTS.




CHAPTER I.

APPARATUS.


Before entering into the subject of the setting of Objects for
the Microscope, the student must be convinced of the necessity of
cleanliness in everything relating to the use of that instrument. In
no branch is this more apparent than in the _preparation_ of objects;
because a slide which would be considered perfectly clean when viewed
in the ordinary way is seen to be far otherwise when magnified some
hundreds of diameters; and those constant enemies, the floating
particles of dust, are everywhere present, and it is only by unpleasant
experience that we fully learn what _cleanliness is_.

Any object which is to be viewed under the microscope must, of course,
be supported in some way--this is now usually done by placing it upon a
glass slide, which on account of the transparency has a great advantage
over other substances. These “slides” are almost always made of one
size, viz., three inches long by one broad, generally having the edges
ground so as to remove all danger of scratching or cutting any object
with which they may come in contact. The glass must be very good,
else the surface will always present the appearance of uncleanliness
and dust. This dusty look is very common amongst the cheaper kinds of
slides, because they are usually “sheet” glass; but is seldom found
in those of the quality known amongst dealers by the name of “patent
plate.” This latter is more expensive at first, but in the end there
is little difference in the cost, as so many of the cheaper slides
cannot be used for delicate work if the mounted object is to be seen in
perfection. These slides vary considerably in thickness; care should,
therefore, be taken to sort them, so that the more delicate objects
with which the higher powers are to be used maybe mounted upon the
_thinnest_, as the light employed in the illumination is then less
interfered with. To aid the microscopist in this work, a metal circle
may be procured, having a number of different sized openings on the
outer edge, by which the glass slides can be measured. These openings
are numbered, and the slides may be separated according to these
numbers; so that when mounting any object there will be no need of a
long search for that glass which is best suited to it.

When fresh from the dealer’s hands, these slides are generally
covered with dust, &c., which may be removed by well washing in clean
rain-water; but if the impurity is obstinate, a little washing soda
may be added, care being taken, however, that every trace of this is
removed by subsequent waters, otherwise the crystals will afterwards
form upon the surface. A clean linen cloth should be used to dry the
slides, after which they may be laid by for use. Immediately, however,
before being used for the reception of objects by any of the following
processes, all dust must be removed by rubbing the surface with clean
wash-leather or a piece of cambric, and, _if needful_, breathing upon
it, and then using the leather or cambric until perfectly dry. Any
small particles left upon the surface may generally be removed by
blowing gently upon it, taking care to allow no damp to remain.

We have before said, that any object to be viewed in the microscope
must have its support; but if this object is to be preserved, care
must be taken that it is defended from the dust and other impurities.
For this purpose it is necessary to use some transparent cover, the
most usual at one time being a plate of mica, on account of its
thinness; this substance is now, however, never used, thin glass being
substituted, which answers admirably. Sometimes it is required to
“_take up_” as little space as possible, owing to the shortness of
focus of the object-glasses. It can be procured of any thickness, from
one-fiftieth to one-two-hundred-and-fiftieth of an inch. On account
of its want of strength it is difficult to cut, as it is very liable
to “_fly_” from the point of the diamond. To overcome this tendency
as much as possible, it must be laid upon a thicker piece, previously
made wet with water, which causes the thin glass to adhere more firmly,
and consequently to bear the pressure required in cutting the covers.
The process of cutting being so difficult, especially with the thinner
kinds, little or nothing is gained by cutting those which can be got
from the dealers, as the loss and breakage is necessarily greater in
the hands of an amateur. It is convenient, however, to have on hand a
few larger pieces, from which unusual sizes may be cut when required.

If the pieces required are _rectangular_, no other apparatus will be
required save a diamond and a flat rule; but if _circles_ are wanted,
a machine for that purpose should be used (of which no description
is necessary here). There are, however, other contrivances which
answer tolerably well. One method is, to cut out from a thick piece of
cardboard a circle rather larger than the size wanted. Dr. Carpenter
recommends metal rings with a piece of wire soldered on either side;
and this, perhaps, is the best, as cardboard is apt to become rough at
the edge when much used. A friend of mine uses thin brass plates with
circles of various sizes “turned” through them, and a small raised
handle placed at one end. The diamond must be passed round the inner
edge, and so managed as to meet again in the same line, in order that
the circle may be true, after which they may be readily disengaged. The
sizes usually kept in stock by the dealers are one-half, five-eighths,
and three-quarters inch diameter; but other sizes may be had to order.

For the information of the beginner it may be mentioned here that the
price of the circles is a little more than of the squares; but this
is modified in some degree by the circle being rather lighter. If
appearance, however, is cared for at all, the circles look much neater
upon the slides when not covered with the ornamental papers; but if
these last are used (as will shortly be described) the squares are
equally serviceable.

As before mentioned, the thin glass is made of various thicknesses,
and the beginner will wish to know which to use. For objects requiring
no higher power than the one-inch object-glass, the thicker kinds
serve well enough; for the half-inch the medium thickness will be
required; while, for higher powers, the thinnest covers must be used.
The “test-objects” for the highest powers require to be brought so
near to the object-glass that they admit of the very thinnest covering
only, and are usually mounted betwixt glasses which a beginner would
not be able to use without frequent breakage; but if these objects were
mounted with the common covers, they would be really worthless with the
powers which they require to show them satisfactorily.

It may be desirable to know how such small differences as those betwixt
the various thin glass covers can be measured. For this purpose there
are two or three sorts of apparatus, all, however, depending upon the
same principle. The description of one, therefore, will be sufficient.
Upon a small stand is a short metal _lever_ (as it may be termed) which
returns by a spring to one certain position, where it is in contact
with a fixed piece of metal. At the other end this lever is connected
with a “finger,” which moves round a dial like that of a watch,
whereupon are figures at fixed distances. When the lever is separated
from the metal which is stationary, the other end being connected
with the “finger” of the dial, that “finger” is moved in proportion
to the distance of the separation. The thin glass is, therefore,
thrust betwixt the end of the lever and fixed metal, and each piece is
measured by the figures on the dial in stated and accurate degrees.
This kind of apparatus, however, is expensive, and when not at our
command, the thin glass may be placed edgewise in the stage forceps,
and measured very accurately with the micrometer.

Cleanliness with the thin glass is, perhaps, more necessary than with
the slides, especially when covering objects which are to be used with
a high power; but it is far more difficult to attain, on account of
the liability to breakage. The usual method of cleaning these covers
is as follows:--Two discs of wood, about two inches in diameter, are
procured, one side of each being perfectly flat and covered with clean
wash-leather. To the other side of these a small knob is firmly affixed
as a handle, or where practicable the whole may be made out of a solid
piece. In cleaning thin glass, it should be placed betwixt the covered
sides of the discs, and may then be safely rubbed with a sufficient
pressure, and so cleaned on both sides by the leather. If, however,
the glass be greasy, as is sometimes the case, it must be first washed
with a strong solution of potash, infusion of nut-galls, or any of
the commonly used grease-removing liquids; and with _some_ impurities
water, with the addition of a few drops of strong acid, will be found
very useful, but this last is not often required.

The above method of cleaning thin glass should always be used by
beginners; but after some experience the hand becomes so sensitive
that the above apparatus is often dispensed with, and the glasses,
however thin, may be safely cleaned betwixt the fingers and thumb with
a cambric handkerchief, having first slightly damped the ends of the
fingers employed to obtain firm hold. When the dirt is very obstinate,
breathing upon the glass greatly facilitates its removal, and the sense
of touch becomes so delicate that the breakage is inconsiderable; but
this method cannot be recommended to novices, as nothing but time spent
in delicate manipulation can give the sensitiveness required.

It was before mentioned that the ordinary glass slides were sometimes
worthless, _especially for fine objects_, from having a rough surface,
which presented a dusty appearance under the microscope. This
imperfection exists in some _thin glass_ also, and it is irremediable;
so that it is useless to attempt to cleanse it; nevertheless, care
should be taken not to mistake dirty glass for this roughness, lest
good glass be laid aside for a fault which does not really belong to it.

When any object which it is desired to mount is of considerable
thickness, or will not bear pressure, it is evident that a wall must
be raised around it to support the thin glass--this is usually termed
a “cell.” There are various descriptions of these, according to the
class of objects they are required to protect; and here may be given a
description of those which are most generally used in mounting “dry”
objects, leaving those required for the preservation of liquids until
we come to the consideration of that mode of mounting. Many have made
use of the following slides. Two pieces of hard wood of the usual size
(3 in. by 1 in.), not exceeding one-sixteenth of an inch in thickness,
are taken, and a hole is then drilled in the middle of one of these
of the size required. The two pieces are then united by glue or other
cement, and left under pressure until thoroughly dry, when the cell is
fit for use. Others substitute cardboard for the lower piece of wood,
which is less tedious, and is strong enough for every purpose. This
class of “cell” is, of course, fitted for opaque objects only where
no light is required from below; and as almost all such are better
seen when on a dark background, it is usual to fix a small piece of
black paper at the bottom of the cell upon which to place them. For
very small objects the “grain” which all such paper has when magnified
detracts a little from the merit of this background; and lately I
have used a small piece of thin glass covered on the back with black
varnish, and placed the object upon the smooth untouched side.

Another method of making these cells is as follows:--Two “punches,”
similar to those used for cutting “gun-wads,” are procured, of such
sizes that with the smaller may be cut out the centre of the larger,
leaving a ring whose side is not less than one-eighth of an inch wide.
These rings may be readily made, the only difficulty being to keep the
sides parallel; but a little care will make this easy enough. For this
purpose close-grained cardboard may be conveniently used. It must have
a well-glazed surface, else the varnish or cement used in affixing the
thin glass cover sinks into the substance, and the adherence is very
imperfect. When this takes place it is easily remedied by brushing over
the surface of the cardboard a strong solution of gum or isinglass; and
this application, perhaps, closes also the pores of the card, and so
serves a double purpose. But, of course, the gum must be _perfectly_
dried before the ring is used.

For cardboard, gutta-percha has been substituted, but cannot be
recommended, as it always becomes brittle after a certain time, never
adheres to the glass with the required firmness, and its shape is
altered when worked with even a little heat. Leather is often used, and
is very convenient; it should be chosen, however, of a close texture,
and free from oil, grease, and all those substances which are laid upon
it by the “dressers.”

Rings of cardboard, &c., have been rejected by persons of great
experience, because they are of such a nature that dampness can
penetrate them. This fault can be almost, if not totally, removed by
immersing them in some strong varnish, such as the asphalt varnish
hereinafter mentioned; but they must be left long enough when affixed
to the glass slide to become _perfectly dry_, and this will require a
much longer time than at first would be supposed.

There has, however, been lately brought out what is termed the _ivory
cell_. This is a ring of ivory-like substance, which may be easily and
firmly fixed to the glass slide by any of the commonly-used cements,
and so forms a beautiful cell for any dry objects. They are made of
different sizes, and are not expensive.

Sometimes slides are used which are made by taking a thin slip of wood
of the usual size (3 in. by 1 in.), in the centre of which is cut a
circular hole large enough to receive the object. A piece of thin
glass is fixed underneath the slide, forming a cell for the object,
which may then be covered and finished like an ordinary slide. This
has the advantage of serving for transparent objects for which the
before-mentioned wooden slides are unsuitable. A slight modification
of this plan is often used where the thickness of the objects is
inconsiderable, especially with some of the Diatomaceæ, often termed
“test-objects.” The wooden slide is cut with the central opening as
above, and two pieces of thin glass are laid upon it, betwixt which the
diatoms or other objects are placed, and kept in their proper position
by a paper cover. This arrangement is a good one, insomuch as the very
small portion of glass through which the light passes on its way to the
microscope from the reflector causes the refraction or interference to
be reduced to the lowest point.

A novice would naturally think the appearance of some of the slides
above mentioned very slovenly and unfinished; but they are often
covered with ornamental papers, which may be procured at almost every
optician’s, at a cost little more than nominal, and of innumerable
patterns and colours. How to use these will be described in another
place.

It is very probable that a beginner would ask his friend what kind of
slides he would advise him to use. Almost all those made of wood are
liable to warp more or less, even when the two pieces are separate or
of different kinds; those of cardboard and wood are generally free
from this fault, yet the slides, being opaque, prevent the employment
of the Lieberkuhn. To some extent glass slides, when covered with
ornamental papers, are liable to the same objection, as the light is
partly hindered. And sometimes the dampness from the paste, or other
substance used to affix the papers, penetrates to the object, and so
spoils it, though this may be rendered less frequent by first attaching
the _thin_ glass to the slide by some harder cement. Much time,
however, is taken up by the labour of covering the slides, which is a
matter of consideration with some. Certainly, the cost of the glass
slides was formerly great; but now they are reasonable enough in this
respect, so that this objection is removed. It is, therefore, well to
use glass slides, except where the thin glasses are employed for tests,
&c., as above. When the thin glass circles are placed upon the slides,
and the edge is varnished with black or coloured rings, the appearance
of finish is perfect. The trouble is much less than with most of
the other methods, and the illumination of the object very slightly
interfered with.

To varnish the edges of these covers, make circles of any liquid upon
the glass slide, and perform any other “circular” work mentioned
hereafter, the little instrument known as “Shadbolt’s turntable” is
almost indispensable. It is made as follows:--At one end of a small
piece of hard wood is fixed an iron pivot about one-eighth inch thick,
projecting half an inch from the wood, which serves as a centre upon
which a round brass table three inches in diameter revolves. On the
surface of this are two springs, about one and a half inches apart,
under which the slide is forced and so kept in position, whilst the
central part is left open to be worked upon. The centre is marked, and
two circles half an inch and one inch in diameter are usually deeply
engraved upon the table to serve as guides in placing the slide, that
the ring may be drawn in the right position. When the slide is placed
upon the “table” underneath the springs, a camel-hair pencil is filled
with the varnish, or other medium used, and applied to the surface
of the glass; the table is then made to revolve, and a circle is
consequently produced, the diameter of which it is easy to regulate.

Many objects for the microscope may be seriously injured by allowing
the fingers to touch them--many more are so minute that they cannot
be removed in this way at all, and often it is necessary to take
from a mass of small grains, as in sand, some particular particle.
To accomplish this, there are two or three contrivances recommended:
one by means of split bristles, many of which will readily be found
in any shaving brush when it has been well used. The bristles when
pressed upon any hard surface open, and when the pressure is removed
close again with a spring; but the use of these is limited. Camel-hair
pencils are of great service for this and many other purposes to the
microscopist. In _very_ fine work they are sometimes required so small
that all the hairs with the exception of one or two finer pointed ones
are removed. A few of various sizes should always be kept on hand.

Equally necessary are fine pointed needles. They are very readily put
up for use by thrusting the “eye” end into a common penholder, so as to
be firm. The points may be readily renewed, when injured, on a common
whetstone; but when out of use they may be protected by being thrust
into a piece of cork.

Knives of various kinds are required in some branches of microscopic
work, but these will be described where “dissection,” &c., is treated
at some length, as also various forms of scissors. In the most simple
objects, however, scissors of the usual kind are necessary. Two or
three sizes should always be kept at hand, sharp and in good order.

A set of glass tubes, kept in a case of some sort to prevent breakage,
should form part of our “fittings” and be always cleaned immediately
after use. These are generally from six to ten inches long and from
one-eighth to a quarter of an inch in diameter. One of these should
be straight and equal in width at both ends; one should be drawn out
gradually to a fine point; another should be pointed as the last, but
be slightly curved at the compressed end, in order to reach points
otherwise unattainable. It is well to have these tubes of various
widths at the points, as in some waters the finer would be inevitably
stopped. For other purposes the fine ones are very useful, especially
in the transfer of “preservative liquids” which will come under notice
in another chapter.

Forceps are required in almost all microscopic manipulations, and
consequently are scarcely ever omitted from the microscopic box,
even the most meagrely furnished; but of these there are various
modifications, which for certain purposes are more convenient than the
usual form. The ordinary metal ones are employed for taking up small
objects, thin glass, &c.; but when slides are to be held over a lamp,
or in any position where the fingers cannot conveniently be used, a
different instrument must be found. Of these there are many kinds; but
Mr. Page’s wooden forceps serve the purpose very well. Two pieces of
elastic wood are strongly bound together at one end, so that they may
be easily opened at the other, closing again by their own elasticity.
Through the first of these pieces is loosely passed a brass stud,
resembling a small screw, and fastened in the second, and through the
second a similar stud is taken and fixed in the first--so that on
pressure of the studs the two strips of wood are opened to admit a
slide or other object required to be held in position. The wood strips
are generally used three or four inches long, one inch wide, and about
one-eighth inch thick.

Again, some objects when placed upon the glass slide are of such an
elastic nature that no cement will secure the thin glass covering until
it becomes hard. This difficulty may be overcome by various methods.
The following are as good and simple as any. Take two pieces of wood
about two inches long, three-quarters wide, and one-quarter thick;
and a small rounded piece one inch long and one-quarter in diameter;
place this latter betwixt the two larger pieces. Over one end of the
two combined pass an india-rubber band. This will give a continual
pressure, and may be opened by bringing the two pieces together at
the other end; the pressure may be readily made uniform by paring the
points at the inner sides, and may be regulated by the strength of the
india-rubber band. These bands may be made cheaply, and of any power,
by procuring a piece of india-rubber tubing of the width required, and
cutting off certain breadths. Another very simple method of getting
this pressure is mentioned in the “Micrographic Dictionary.” Two pieces
of whalebone of the length required are tied together firmly at each
end. It is evident that any object placed betwixt them will be subject
to continual pressure. The power of this may be regulated by the
thickness and length of the whalebone. This simple contrivance is very
useful.

Common watch-glasses should always be kept at hand. They are certainly
the cheapest, and their transparency makes them very convenient
reservoirs in which objects may be steeped in any liquid; as it
saves much trouble to examine cursorily under the microscope, when
the air-bubbles are expelled from insects, &c., &c. They are readily
cleaned, and serve very well as covers, when turned upside down,
to protect any objects from the dust. For this latter purpose Dr.
Carpenter recommends the use of a number of bell-glasses, especially
when one object must be left for a time (which often happens) in order
that another may be proceeded with. Wine glasses, when the “legs” are
broken, may thus be rendered very useful.

As heat is necessary in mounting many objects, a lamp will be required.
Where gas is used, the small lamp known as “Bunsen’s” is the most
convenient and inexpensive. It gives great heat, is free from smoke,
and is readily affixed to the common gas-burner by a few feet of
india-rubber tubing. The _light_ from these lamps is small, but this is
little or no drawback to their use. Where gas is not available, the
common spirit-lamps may be used, which are very clean and answer every
purpose.

In applying the required heat to the slides, covers, &c., it is
necessary in many cases to ensure uniformity, otherwise there is danger
of the glass being broken. For this purpose a brass plate at least
three inches wide, somewhat longer, and one-eighth of an inch thick,
must be procured. It should then be affixed to a stand, so that it may
be readily moved higher or lower, in order that the distance from the
lamp may be changed at will, and thus the degree of heat more easily
regulated. This has also the advantage of enabling the operator to
allow his slides, &c., to cool more gradually, which, in some cases, is
absolutely necessary,--as in fusing some of the salts, &c.

In order to get rid of air-bubbles, which are frequently disagreeable
enemies to the mounter of objects, an air-pump is often very useful.
This is made by covering a circular plate of metal with a bell-glass,
both of which are ground so finely at the edges that greasing the place
of contact renders it air-tight. The pump is then joined to the metal
plate underneath, and worked with a small handle like a common syringe.
By turning a small milled head the air may be allowed to re-enter when
it is required to remove the bell-glass and examine or perform any
operation upon the object. The mode of using this instrument will be
described hereafter, but it may be here stated that substitutes have
been devised for this useful apparatus; but as it is now to be obtained
at a low cost, it is hardly worth while to consider them. Much time is,
in many instances, certainly saved by its use, as a very long immersion
in the liquids would be required to expel the bubbles, where the
air-pump would remove them in an hour.

The next thing to be considered is what may be termed CEMENTS, some
of which are necessary in every method of mounting objects for the
microscope. Of these will be given the composition where it is
probable the young student can make use of it; but many of them are
so universally kept as to be obtainable almost anywhere; and when
small quantities only are required, economy suffers more from home
manufacture than from paying the maker’s profit.

Amongst these, CANADA BALSAM may, perhaps, be termed the most
necessary, as it is generally used for the preservation of many
transparent objects. It is a thick liquid resin of a light amber
colour, which on exposure to the atmosphere becomes dry and hard
even to brittleness. For this reason it is seldom used as a _cement_
alone where the surface of contact is small, as it would be apt to be
displaced by any sudden shock, especially when old. In the ordinary
method of using, however, it serves the double purpose of preserving
the object and fixing the thin glass cover; whilst the comparatively
large space upon which it lies lessens the risk of displacement. By
keeping, this substance becomes thicker; but a very little warmth will
render it liquid enough to use even when to some extent this change has
taken place. When heated, however, for some time and allowed to cool,
it becomes hardened to any degree, which may be readily regulated by
the length of time it has been exposed, and the amount of heat to which
it has been subjected. On account of this property it is often used
with chloroform: the balsam is exposed to heat until, on cooling, it
assumes a glassy appearance; it is then dissolved in pure chloroform
until it becomes of the consistence of thick varnish. This liquid is
very convenient in some cases; as air-bubbles are much more easily
got rid of than when undiluted Canada balsam is used. It also dries
readily, as the chloroform evaporates very quickly, for which reason it
must be preserved in a closely-stoppered bottle. It has been said that
this mixture becomes _cloudy_ with long keeping, but I have not found
it so in any cases where I have used it. Should it, however, become so,
a little heat will readily dispel the opacity. The ordinary balsam,
if exposed much to the air whilst being used, becomes thicker, as has
been already stated. It may be reduced to the required consistency
with common turpentine, but I have often found this in some degree
injurious to the transparency of the balsam, and the amalgamation of
the two is by no means perfect. (See also Chapter III.) The cheapness
of the article renders it no extravagance to use it always undiluted;
and when preserved in a bottle with a hollow cover fitting tightly
around the neck, both surfaces being finely ground, it remains fit for
use much longer than in the ordinary jar.

ASPHALTUM.--This substance is dissolved in linseed oil, turpentine, or
naphtha, and is often termed “Brunswick black.” It is easily worked,
but is not generally deemed a _trustworthy_ cement, as after a time it
is readily loosened from its ground. It is, however, very useful for
some purposes (such as “finishing” the slides), as it dries quickly. I
shall, however, mention a modification of this cement a little further
on.

MARINE GLUE.--No cement is more useful or trustworthy for certain
purposes than this. It is made in various proportions; but one really
good mixture is--equal parts of india-rubber and gum shellac; these are
dissolved in mineral naphtha with heat. It is, however, much better to
get it from the opticians or others who keep it. It requires heat in
the application, as will be explained in Chapter IV.; but it is soluble
in few, if any, liquids used by the microscopist, and for that reason
is serviceable in the manufacture of cells, &c. Where two pieces of
glass are to be firmly cemented together, it is almost always employed;
and in all glass troughs, plates with ledges, &c., the beginner may
find examples of its use.

GOLD SIZE.--This substance may always be procured at any colourman’s
shop. The process of its preparation is long and tedious. It is,
therefore, not necessary to describe it here. Dr. Carpenter says that
it is very durable, and may be used with almost any preservative
liquids, as it is acted upon by very few of them, turpentine being its
only true solvent. If too thin, it may be exposed for awhile to the
open air, which by evaporation gradually thickens it. Care must be
taken, however, not to render it too thick, as it will then be useless.
A small quantity should be kept on hand, as it is much more adhesive
when _old_.

LIQUID GLUE is another of these cements, which is made by dissolving
gum shellac in naphtha in such quantity that it may be of the required
consistency. This cement appears to me almost worthless in ordinary
work, as its adherence can never be relied upon; but it is so often
used and recommended that an enumeration of cements might be deemed
incomplete without it. Even when employed simply for varnishing the
outside of the glass covers, for appearance’ sake alone, it invariably
chips. Where, however, oil is used as a preservative liquid, it serves
very well to attach the thin glass; but when this is accomplished,
another varnish less liable to “chip” must always be laid upon it. (See
Chapter IV.)

BLACK JAPAN.--This is prepared from oil of turpentine, linseed oil,
amber, gum anime, and asphalt. It is troublesome to make, and therefore
it is much better to procure it at the shops. It is a really good
cement, and serves very well to make shallow cells for liquids, as will
be described in Chapter IV. The finished cell should be exposed for a
short time to the heat of what is usually termed a “cool oven.” This
renders it very durable, and many very careful manipulators make use of
it for their preparations.

ELECTRICAL CEMENT.--This will be found very good for some purposes
hereinafter described. To make it, melt together--

  5 parts of resin.
  1     ”    beeswax.
  1     ”    red ochre.[A]

    [A] Dr. Griffiths says that the addition of 2 parts of Canada
        balsam renders this cement much more adhesive to the glass.

It must be used whilst hot, and as long as it retains even slight
warmth can be readily moulded into any form. It is often employed in
making shallow cells for liquids, as before mentioned.

GUM-WATER is an article which nobody should ever be without; but
labels, or indeed any substance, affixed to glass with common gum, are
so liable to leave it spontaneously, especially when kept very dry,
that I have lately added five or six drops of glycerine to an ounce of
the gum solution. This addition has rendered it very trustworthy even
on glass, and now I never use it without. This solution cannot be kept
long without undergoing fermentation, to prevent which the addition
of a small quantity of any essential oil (as oil of cloves, &c.),
or one-fourth of its volume of alcohol, may be made, which will not
interfere in any way with its use.

There is what is sometimes termed an _extra adhesive_ gum-water, which
is made with the addition of isinglass, thus:--Dissolve two drachms of
isinglass in four ounces of distilled vinegar; add as much gum arabic
as will give it the required consistency. This will keep very well, but
is apt to become thinner, when a little more gum may be added.

All these, except one or two, are liquid, and must be kept in
_stoppered_ bottles, or, at least, as free from the action of the air
as possible.

When any two substances are to be united firmly, I have termed the
medium employed “a cement;” but often the appearance of the slides is
thought to be improved by drawing a coloured ring upon them, extending
partly on the cover and partly on the slide, hiding the junction of the
two. The medium used in these cases I term A VARNISH, and hereinafter
mention one or two. Of course, the tenacity is not required to be so
perfect as in the _cements_.

SEALING-WAX VARNISH is prepared by coarsely powdering sealing wax,
and adding spirits of wine; it is then digested at a gentle heat to
the required thickness. This is very frequently used to “finish” the
slides, as before mentioned, and can easily be made of any colour by
employing different kinds of sealing-wax; but is very liable to “chip”
and leave the glass.

BLACK VARNISH is readily prepared by adding a small quantity of
lamp-black to gold-size and mixing intimately. Dr. Carpenter recommends
this as a good finishing varnish, drying quickly and being free from
that brittleness which renders some of the others almost worthless; but
it should not be used in the first process when mounting objects in
fluid.

Amongst these different cements and varnishes I worked a long time
without coming to any decision as to their comparative qualities,
though making innumerable experiments. The harder kinds were
continually cracking, and the softer possessed but little adhesive
power. To find hardness and adhesiveness united was my object, and the
following possesses these qualities in a great degree:--

  India-rubber            ½ drachm.
  Asphaltum               4 oz.
  Mineral naphtha        10  ”

Dissolve the india-rubber in the naphtha, then add the asphaltum--if
necessary heat must be employed.

This is often used by photographers as a black varnish for glass, and
never cracks, whilst it is very adhesive. Dr. Carpenter, however,
states that his experience has not been favourable to it; but I have
used it in great quantities and have never found it to leave the glass
in a single instance when used in the above proportions. The objections
to it are, however, I think easily explained, when it is known that
there are many kinds of pitch, &c., from coal, sold by the name of
asphaltum, some of which are worthless in making a microscopic cement.
When used for this purpose, the asphaltum must be genuine and of the
best quality that can be bought. The above mixture serves a double
purpose--to unite the cell to the slide, and also as a “finishing”
varnish. But it is perhaps more convenient to have two bottles of this
cement, one of which is thicker than common varnish, to use for uniting
the cell, &c.; the other liquid enough to flow readily, which may be
employed as a surface varnish in finishing the slides.

The brushes or camel-hair pencils should always be cleaned after use;
but with the asphalt varnish above mentioned it is sufficient to wipe
off as carefully as possible the superfluous quantity which adheres to
the pencil, as, when again made use of, the varnish will readily soften
it; but, of course, it will be necessary to keep separate brushes for
certain purposes.

Here it may be observed that every object should be labelled with name
and any other descriptive item as soon as mounted. There are many
little differences in the methods of doing this. Some write with a
diamond upon the slide itself; but this has the disadvantage of being
not so easily seen. For this reason a small piece of paper is usually
affixed to one end of the slide, on which is written what is required.
These labels may be bought of different colours and designs; but the
most simple are quite as good, and very readily procured. Take a sheet
of thin writing paper and brush over one side a strong solution of gum,
with the addition of a few drops of glycerine as above recommended;
allow this to dry, and then with a common gun-punch stamp out the
circles, which may be affixed to the slides by simply damping the
gummed surface, taking care to write the required name, &c., upon it
before damping it, or else allowing it to become perfectly dry first.

There is one difficulty which a beginner often experiences in
sorting and mounting certain specimens under the microscope, viz.,
the _inversion_ of the objects; and it is often stated to be almost
impossible to work without an erector. But this difficulty soon
vanishes, the young student becoming used to working what at first
seems in contradiction to his sight.

Let it be understood, that in giving the description of those articles
which are usually esteemed _necessary_ in the various parts of
microscopic manipulation, I do not mean to say that without many of
these no work of any value can be done. There are, as all will allow,
certain forms of apparatus which aid the operator considerably; but
the cost may be too great for him. A little thought, however, will
frequently overcome this difficulty, by enabling him to make, or
get made, for himself, at a comparatively light expense, something
which will accomplish all he desires. As an example of this, a friend
of mine made what he terms his “universal stand,” to carry various
condensers, &c., &c., in the following way:--Take a steel or brass
wire, three-sixteenths or one-quarter inch thick and six or eight
inches long; “tap” into a _solid_, or make rough and fasten with melted
lead into a _hollow_, ball. (The foot of a cabinet or work-box answers
the purpose very well.) In the centre of a round piece of tough board,
three inches in diameter, make a hemispherical cavity to fit half of
the ball, and bore a hole through from the middle of this cavity, to
allow the wire to pass. Take another piece of board, about four inches
in diameter, either round or square, and one-and-a-half or two inches
thick, make a similar cavity in its centre to receive the other half
of the ball, but only so deep as to allow the ball to fit tightly when
the two pieces of board are screwed together, which last operation
must be done with three or four screws. Let the hole for the wire in
the upper part be made conical (base upwards), and so large as only to
prevent the ball from escaping from its socket, in order that the shaft
may move about as freely as possible. Turn a cavity, or make holes,
in the bottom of the under piece, and fill with lead to give weight
and steadiness. This, painted green bronze and varnished, looks neat;
and by having pieces of gutta-percha tubing to fit the shaft, a great
variety of apparatus may be attached to it.

Again, a “condenser” is often required for the illumination of opaque
objects. My ingenious friend uses an “engraver’s bottle” (price 6d.),
fills it with water, and suspends it betwixt the light and the object.
Where the light is very yellow, he tints the water with indigo, and so
removes the objectionable colour.

I merely mention these as examples of what may be done by a little
thoughtful contrivance, and to remove the idea that nothing is of
much value save that which is the work of professional workmen, and
consequently expensive.




CHAPTER II.

TO PREPARE AND MOUNT OBJECTS “DRY.”


The term “dry” is used when the object to be mounted is not immersed
in any liquid or medium, but preserved in its natural state, unless it
requires cleaning and drying.

I have before stated that thorough cleanliness is necessary in the
mounting of all microscopic objects. I may here add that almost every
kind of substance used by the microscopist suffers from careless
handling. Many leaves with fine hairs are robbed of half their beauty,
or the hairs, perhaps, forced into totally different shapes and groups;
many insects lose their scales, which constitute their chief value to
the microscopist; even the glass itself distinctly shows the marks of
the fingers if left uncleaned. Every object must also be _thoroughly
dry_, otherwise dampness will arise and become condensed in small
drops upon the inner surface of the thin glass cover. This defect is
frequently met with in slides which have been mounted quickly; the
objects not being thoroughly dry when enclosed in the cell. Many of the
cheap slides are thus rendered worthless. Even with every care it is
not possible to get rid of this annoyance occasionally.

For the purpose of mounting opaque objects “dry,” _discs_ were at one
time very commonly made use of. These are circular pieces of cork,
leather, or other soft substance, from one-quarter to half inch in
diameter, blackened with varnish or covered with black paper, on which
the object is fixed by gum or some other adhesive substance. They are
usually pierced longitudinally by a strong pin, which serves for the
forceps to lay hold of when being placed under the microscope for
examination. Sometimes objects are affixed to both sides of the disc,
which is readily turned when under the object-glass. The advantage of
this method of mounting is the ease with which the disc may be moved,
and so present every part of the object to the eye save that by which
it is fastened to the disc. On this account it is often made use of
when some particular subject is undergoing investigation, as a number
of specimens may be placed upon the discs with very little labour,
displaying all the parts. But where exposure to the atmosphere or small
particles of dust will injure an object, no advantage which the discs
may possess should be considered, and an ordinary covered cell should
be employed. Small pill-boxes have been used, to the bottom of which a
piece of cork has been glued to afford a ground for the pin or other
mode of attachment; but this is liable to _some_ of the same faults as
the disc, and it would be unwise to use these for permanent objects.

Messrs. Smith and Beck have lately invented, and are now making, a
beautiful small apparatus, by means of which the disc supporting the
object can be worked with little or no trouble into any position that
may prove most convenient, whilst a perforated cylinder serves for the
reception of the discs when out of use, and fits into a case to protect
them from dust. A pair of forceps is made for the express purpose of
removing them from the case and placing them in the holder.

All dry objects, however, which are to be preserved should be mounted
on glass slides in one of the cells (described in Chapter I.) best
suited to them. Where the object is to be free from pressure, care
must be taken that the cell is deep enough to ensure this. When the
depth required is but small, it is often sufficient to omit the card,
leather, or other circles, and with the “turn-table” before described,
by means of a thick varnish and camel-hair pencil, to form a ring
of the desired depth; but should the varnish not be of sufficient
substance to give such “walls” at once, the first application may be
allowed to dry, and a second made upon it. A number of these may be
prepared at the same time, and laid by for use. When liquids are used
(see Chapter IV.), Dr. Carpenter recommends gold-size as a good varnish
for the purpose, and this may be used in “dry” mountings also. I have
used the asphaltum and india-rubber (mentioned in Chapter I.), and
found it to be everything I could wish. The cells, however, must be
_thoroughly dry_, and when they will bear the heat they should be baked
for an hour at least in a tolerably cool oven, by which treatment the
latter becomes a first-rate medium. All dry objects which will not bear
pressure must be firmly fastened to the slide, otherwise the necessary
movements very often injure them, by destroying the fine hairs, &c.
For this purpose thin varnishes are often used, and will serve well
enough for large objects, but many smaller ones are lost by adopting
this plan, as for a time, which may be deemed long enough to harden
the varnish, they exhibit no defect, but in a while a “wall” of the
plastic gum gathers around them, which refracts the light, and thus
leads the student to false conclusions. In all _finer_ work, where it
is necessary to use any method of fixing them to the slide, a solution
of common gum, with the addition of a few drops of glycerine (Chapter
I.), will be found to serve the purpose perfectly. It must, however,
be carefully filtered through blotting paper, otherwise the minute
particles in the solution interfere with the object, giving the slide a
dusty appearance when under the microscope.

When mounting an object in any of these cells, the glass must be
thoroughly cleaned, which may be done with a cambric handkerchief,
after the washing mentioned in Chapter I. _If the object be large_,
the point of a fine camel-hair pencil should be dipped into the gum
solution, and a minute quantity of the liquid deposited in the cell
where the object is to be placed, but not to cover a greater surface
than the object will totally hide from sight. This drop of gum must be
allowed to dry, which will take a few minutes. Breathe then upon it two
or three times, holding the slide not far from the mouth, which will
render the surface adhesive. Then draw a camel-hair pencil through the
lips, so as to moisten it slightly (when anything small will adhere to
it quite firmly enough), touch the object and place it upon the gum in
the desired position. This must be done immediately to ensure perfect
stability, otherwise the gum will become at least partially dry and
only retain the object imperfectly.

When, however, the objects are so minute that it would be impossible to
deposit atoms of gum small enough for each one to cover, a different
method of proceeding must be adopted. In this case a small portion of
the same gum solution should be placed upon the slide, and by means
of any small instrument--a long needle will serve the purpose very
well--spread over the surface which will be required. The quantity thus
extended will be very small, but by breathing upon it may be prevented
drying whilst being dispersed. This, like the forementioned, should be
then allowed to dry; and whilst the objects are being placed on the
prepared surface, breathing upon it as before will restore the power of
adherence.

When gum or other liquid cement has been used to fix the objects to
the glass, the thin covers must not be applied until the slide has
been _thoroughly dried_, and all fear of dampness arising from the use
of the solution done away with. Warmth may be safely applied for the
purpose, as objects fastened by this method are seldom, if ever, found
to be loosened by it. As objects are met with of every thickness, the
cells will be required of different depths. There is no difficulty in
accommodating ourselves in this--the deeper cells may be readily cut
out of thick leather, card, or other substance preferred (as mentioned
in Chapter I.). Cardboard is easily procured of almost any thickness;
but sometimes it is convenient to find a thinner substance even than
this. When thin glass is laid upon a drop of any liquid upon a slide,
every one must have observed how readily the liquid spreads betwixt
the two: just so when any thin varnish is used to surround an object
of little substance, excessive care is needed lest the varnish should
extend betwixt the cover and slide, and so render it worthless. The
slightest wall, however, prevents this from taking place, so that a
ring of common paper may be used, and serve a double purpose where the
objects require no deeper cell than this forms.

Many objects, however, are of such tenuity--as the leaves of many
mosses, some of the Diatomaceæ, scales of insects, &c.--that no cell is
requisite excepting that which is necessarily formed by the medium used
to attach the thin glass cover to the slide; and where the slide is
covered by the ornamental papers mentioned in Chapter I., and pressure
does not injure the object, even this is omitted, the thin glass being
kept in position by the cover; but slides mounted in this manner are
frequently injured by dampness, which soon condenses upon the inner
surfaces and interferes both with the object and the clearness of its
appearance.

The thin glass, then, is to be united to the slide, so as to form a
perfect protection from dust, dampness, or other injurious matter,
and yet allow a thoroughly distinct view of the object. This is to be
done by applying to the glass slide round the object some adhesive
substance, and with the forceps placing the thin glass cover (quite dry
and clean) upon it. A gentle pressure round the edge will then ensure
a perfect adhesion, and with ordinary care there will be little or no
danger of breakage. For this purpose gold-size is frequently used. The
asphalt and india-rubber varnish also will be found both durable and
serviceable. Whatever cement may be used, it is well to allow it to
become in some measure “fixed” and dried; but where no cell or “wall”
is upon the slide, this is _quite necessary_, otherwise the varnish
will be almost certain to extend, as before mentioned, and ruin the
object. It may be stated here that gold-size differs greatly in its
drying powers, according to its age, mode of preparation, &c. (Chapter
IV.)

Should any object be enclosed which requires to be kept flat during
the drying of the cement, it will be necessary to use some of the
contrivances mentioned in Chapter I.

When the slide is thus far advanced, there remains the “finishing”
only. Should the student, however, have no time to complete his work
at once, he may safely leave it at this stage until he has a number
of slides which he may finish at the same time. There are different
methods of doing this, some of which may be here described.

If ornamental papers are preferred, a small circle must be cut out
from the centre a little less than the thin glass which covers the
object. Another piece of coloured paper is made of the same size, and
a similar circle taken from its centre also, or both may be cut at the
same time. The slide is then covered round the edges with paper of any
plain colour, so that it may extend about one-eighth of an inch over
the glass on every side. The ornamental paper is then pasted on the
“object” surface of the glass, so that the circle shows the object as
nearly in the centre as possible, and covers the edges of the thin
glass. The other coloured paper is then affixed underneath with the
circle coinciding with that above. And here I may observe, that when
this method is used there is no necessity for the edges of the slide to
be “ground,” as all danger of scratching, &c., is done away with by the
paper cover.

Many now use paper covers, about one and a half inches long, on the
upper side of the slide only, with the centre cut out as before, with
no other purpose than that of hiding the edge of the thin glass where
it is united to the slide.

The method of “finishing,” however, which is mostly used at the present
time, is to lay a coating of varnish upon the edge of the thin glass,
and extend it some little way on the slide. When a black circle is
required, nothing serves the purpose better than the gold-size and
lamp-black, or the asphalt and india-rubber varnish, neither of which
is liable to chip; but when used for this, the latter should be rather
thinner, as before advised. Some of these varnishes are preferred of
different colours, which may be made by using the different kinds of
sealing-wax, as described in Chapter I.; but they are always liable
to the defects there mentioned. This circle cannot be made in any
other way but by one of those contrivances which have now centred in
Shadbolt’s turntable. A very little practice will enable the young
student to place his slide so that the circle may be uniform with the
edge of the thin glass.

The slide is now complete, except the addition of the name and any
other particulars which may be desirable. For this purpose one of the
methods described in Chapter I. must be employed.

Amongst the various classes of microscopic objects now receiving
general attention, the Diatomaceæ may be placed in a prominent
position. They afford endless opportunities of research, and some
very elaborate works have already been issued concerning them.
Professor Smith’s may be mentioned as one containing, perhaps, the
best illustrations. The young student may wish to know what a diatom
is. The “Micrographic Dictionary” gives the following definition:--“A
family of confervoid Algæ, of very peculiar character, consisting of
microscopic brittle organisms.” They are now looked upon by almost all
of our scientific men as belonging to the _vegetable_ kingdom, though
some few still assign them to the animal. They are almost invariably
exceedingly small, so that the unaided eye can perceive nothing on
a prepared slide of these organisms but minute dust. Each separate
portion, which is usually seen when mounted, is termed a “frustule,”
or “testule:” this consists of two similar parts, composed of silica,
between and sometimes around which is a mass of viscid matter called
the “endochrome.” They are found in almost every description of water,
according to the variety: some prefer sea-water, others fresh, and
many are seen nowhere but in that which is a mixture of both, as the
mouths of rivers, &c. Ditches, ponds, cisterns, and indeed almost every
_reservoir_, yield abundance of these forms. They are not, however,
confined to “present” life; but, owing to the almost indestructible
nature of their siliceous covering, they are found in a fossil state
in certain earths in great abundance, and are often termed “fossil
Infusoria.” Upon these frustules are generally to be seen lines,
or “markings,” of different degrees of minuteness, the delicacy of
which often serves the purpose of testing the defining power of the
object-glasses. Some of the frustules are triangular, others circular,
and, indeed, of almost every conceivable shape, many of them presenting
us with exquisitely beautiful designs.

The markings, however, are seldom seen well, if at all, until the
frustules are properly prepared, the different methods of accomplishing
which will be given a little further on.

The _collection_ of the fresh diatoms is so closely connected with
their _preservation_, that a few notes may be given upon it before we
pass on. For this purpose a number of small bottles must be provided,
which may be placed in a tin box, with a separate apartment for each,
so that all chance of breakage may be done away with. The diatoms
are generally of a light brown colour; and where they are observed
in the water, the bottle may be so placed, with the mouth closed by
the finger, that when the finger is withdrawn the water will rush in,
carrying the diatoms also. If they are seen upon plants, stones, or
any other substance, they may generally be detached and placed in the
bottle. When there is a green covering upon the surface of the water, a
great quantity of diatoms is usually found amongst it; as also upon the
surface of the mud in those ponds where they abound. In these cases, a
broad flat spoon will be found very useful, and one is now made with a
covering upon the broader portion of it to protect the enclosed matter
from being so readily carried off whilst bringing it to the surface
again. Where there is any depth of water, and the spoon will not reach
the surface of the mud, the bottle must be united to a long rod, and
being then carried through the upper portion with the mouth downwards,
no water will be received into it; but on reaching the spot required,
the bottle-mouth may be turned up, and thus become filled with what is
nearest.

From the stomachs of common fish--as the cod, sole, haddock, &c.--many
specimens of Diatomaceæ may be obtained, but especially from the crab,
oyster, mussel, and other shell-fish. Professor Smith states that from
these curious receptacles he has taken some with which he has not
elsewhere met. To remove them from any of the small shell-fish, it is
necessary to take the fish or stomach from the shell, and immerse it
in strong hot acid (nitric is the best) until the animal matter is
dissolved, when the residue must be washed and treated as the ordinary
Diatomaceæ hereinafter described.

Many diatoms are seen best when mounted in a dry state, the minute
markings becoming much more indistinct if immersed in liquid or balsam;
and for this reason those which are used as test objects are usually
mounted _dry_. Many kinds also are now prepared in this way, as opaque
objects to be examined with the lieberkuhn, and are exquisitely
beautiful. Others, however, are almost invariably mounted in balsam;
but as these will be again referred to in Chapter III., and require
the same treatment to fit them for the slide, it will not be out of
place to describe the cleaning and preparation of them here. As before
stated, there is much matter surrounding them which must be got rid of
before the “_siliceous_ covering” can be shown perfectly. As, however,
we may first wish to become acquainted in some degree with what we have
to do, it is well to take a small piece of _talc_, and place a few of
the diatoms upon it. This may be held over the flame of the spirit-lamp
until all the surrounding matter is burnt away, and a tolerable idea
may be thus obtained as to the quality of our treasure.

In some cases it is well to use this burning operation alone in
_mounting_ specimens of diatoms, when they may be placed in their
natural state upon the thin glass, burnt for awhile upon the platinum
plate, hereafter described, and mounted dry or in balsam.

In the preparation and cleaning of Diatomaceæ, there is little
satisfaction unless these operations have been successfully performed,
as a very small portion of foreign matter seriously interferes with
the object. The mode of preparing them varies even amongst the most
experienced. It will be found, therefore, most satisfactory to examine
the principal of these separately, although it may be at the risk of
some little repetition.

The method which is the most frequently made use of is the
following:--Place the “gathering” containing the Diatomaceæ in a small
glass or porcelain vessel, add strong nitric acid, and, by the aid
of Bunsen’s burner or spirit-lamp, boil for some minutes. From time
to time a drop of the mixture may be put upon a slide, and examined
under the microscope to see if all foreign matter be got rid of. When
the valves are clean, the vessel containing them must be filled with
water, and the whole left for an hour or two, so that all the diatoms
may settle perfectly. The liquid must then be poured off carefully,
or drained away by the aid of a syphon, so that none of the diatoms
are removed with it. Indeed, it is well to examine the liquid drained
off each time with the microscope, as the finer forms are frequently
lost in the washings. The vessel must then be refilled with pure
water, allowed to settle, and drained as before. This washing must be
repeated until a drop being placed upon a slide and evaporated leaves
no crystals. When it is desirable to preserve the diatoms in this state
before mounting (which process will be described in another place),
they may be placed in a small phial with a little distilled water.

There are many cases in which the above method will not effect a
_perfect_ cleansing, as certain substances with which the diatoms are
frequently mixed are not soluble in _nitric acid_. For this reason
the following method is resorted to:--Take a quantity of the matter
containing the Diatomaceæ and wash first with pure water, to get rid of
all the impurities possible. Allow this to settle perfectly and decant
the water. Add hydrochloric acid gradually, and when all effervescence
has subsided, boil for some minutes by aid of the lamp. When cool and
the particles have subsided, decant the hydrochloric and add nitric
acid. The boiling must then be repeated until a drop of the liquid when
placed under the microscope shows the valves or “frustules” clean.
After allowing the diatoms to settle, the acid must be decanted, and
pure water substituted. The washing must be repeated as in the former
process until all the remains of crystals or acid are removed, when the
specimens may be preserved in small phials as stated above.

Such are the usual modes of treating the Diatomaceæ, but there are
certain cases in which particular methods are required to give anything
like perfect results. Persons of great experience combine a variety of
treatments, and thus obtain better and more uniform specimens. Perhaps
it will be advantageous to give the young student the process adopted
by one of the most successful preparers of these objects; but I will
first state the different methods of mounting the cleaned diatoms dry:
how to employ Canada balsam and fluid in their preservation will be
elsewhere described.

It was before stated that the diatoms when cleansed might be preserved
in small phials of distilled water. When required for mounting, shake
the phial, and with a thin glass tube or rod take up a drop of the
fluid and spread it upon the surface of the slide in the desired
position. This must then be allowed to dry gradually, or by the aid
of the lamp if necessary, without being shaken or interfered with,
otherwise uniformity of dispersion will be prevented. When _thoroughly_
dry, a thin ring of one of the adhesive varnishes--gold-size will be
found as good as any--may be drawn round the diatoms, and allowed to
dry in a slight degree. The slide and thin glass cover should then be
warmed and the latter gently pressed upon the ring of varnish until the
adhesion all round is complete.

As some of the diatoms require object-glasses of extremely high power,
and, consequently, short focus, to show them, they must be as close to
the outer surface of the cover as possible. For this reason they are
sometimes placed upon the under side of the thin glass, as follows.
Clean the surfaces of the slide and cover, and with the rod or pipe
place the liquid containing the diatoms upon the thin glass, and dry as
before. Trace the ring to receive the cover upon the slide, and when
almost dry, warm both and proceed as above. Whichever of these methods
is employed, the outer ring of coloured varnish may be applied as
elsewhere described and the slide finished.

The diatoms are also sometimes mounted betwixt two thin glasses, as
described in Chapter I., so that the light by which they are examined
may receive as little interference as possible, and that an achromatic
condenser may be brought into focus under the slide.

Of the various modes of cleaning and mounting the Diatomaceæ, I believe
that the following may be safely recommended as affording results of
the very best quality. My friend, Mr. T. G. Rylands, gave it to me as
that which he prefers, and I can safely say that his numerous slides
are at least equal to any I have ever seen. I will give it just as I
received it from him, though there may be some little repetition of
what has been said elsewhere, as he does not appropriate any part of it
as his own. He says:--In this branch of mounting, general rules alone
can be laid down, because the gatherings may contain iron, lime, fine
silt, or vegetable matter under conditions for special treatment, and
consequently the first step should be to experiment on various kinds.

In gathering diatoms much labour is saved by judgment and care;
hence it is desirable to get acquainted with them in their growing
condition, so that when recognised upon the sands or other spots they
may be carefully removed by the aid of the spoon or small tin scoop
before described. When growing upon algæ or other plants, the plants
and diatoms together may be carried home, in which case they must be
simply drained and not washed or pressed, in order that the diatoms
be not lost. As it is always desirable to examine the gathering on
the ground, a “Gairdner’s hand microscope” with powers from 80 to 200
diameters will be found very useful. The best gatherings are those
which represent one species abundantly. Those which are mixed may
be rejected, unless they are seen to contain something valuable or
important, as the object should be not so much to supply microscopical
curiosities as to collect material which is available for the study of
nature.

The gathering when carried home should always be carefully examined
before anything is done with it; not only on account of the additional
information thus acquired, but also because it often happens that a
specimen should be mounted in fluid (see Chapter IV.) in the condition
in which it is gathered, as well as cleaned and mounted in balsam
(Chapter III.) and dry.

Where the gathering is taken from sand, the whole may be shaken up
in water as a preliminary operation, when much of the sand will
be separated by its own weight. The lime test, however, should be
applied, viz.--a small portion of hydrochloric acid, and if there be
effervescence it must be dissolved out by this means. From Algæ and
other weeds diatoms may be detached by agitating the whole together in
a weak solution of nitric acid--about one of pure acid to twenty or
thirty of water, as it must be sufficiently weak to free the diatoms
without destroying the matter to which they adhere. The diatoms
may then be separated by sifting through coarse muslin, which will
retain the Algæ, &c. The process of cleaning will vary according to
circumstances. Some gatherings require to be boiled only a few minutes
in nitric acid; but the more general plan where they are mixed with
organic or other foreign matter, is to boil them in pure sulphuric
acid until they cease to grow darker in colour (usually from a half
to one minute), and then to add, drop by drop to avoid explosions,
a cold saturated solution of chlorate of potash until the colour is
discharged, or, in case the colour does not disappear, the quantity
of the solution used is at least equal to that of the acid. This
operation is best performed in a wide-mouthed ordinary beaker glass,[B]
a test-tube being too narrow. The mixture whilst boiling should be
poured into thirty times its bulk of cold water, and the whole allowed
to subside. The fluid must then be carefully decanted and the vessel
re-supplied once or twice with pure water, so as to get rid of all the
acid. The gathering may then be transferred to a small boiling glass
or test-tube, and--the water being carefully decanted--boiled in the
smallest available quantity of nitric acid, and washed as before. This
last process has been found necessary from the frequent appearance of
minute crystals, which cannot otherwise be readily disposed of without
the loss of a considerable proportion of diatoms.

    [B] These glasses are round, about six inches high, and usually
        contain about eight ounces. They are rather wider at the
        bottom, tapering gradually to the top, and may be generally
        procured at the chemists, &c.

I may here mention that the washing glasses used by Mr. Rylands are
stoppered conical bottles varying in capacity from two ounces to one
quart; the conical form being employed to prevent the adherence of
anything to the side; they are “stoppered” to render them available in
the shaking process about to be described.

The gathering, freed from acid, is now put into two inches depth of
water, shaken vigorously for a minute or two, and allowed to subside
for half an hour, after which the turbid fluid must be carefully
decanted. This operation must be repeated until all the matter is
removed which will not settle in half an hour. The fluid removed should
be examined by a drop being put upon a slide, as in some cases very
light diatoms have been found to come off almost pure in one or more of
these earlier washings. The quantity of water and time of subsidence
given may be taken generally, but may require to be modified according
to circumstances and the judgment of the operator. By the repetition
and variation of this process--the _shaking_ being the most important
part--the gathering, if a _pure_ one, will be sufficiently clean. If,
however, it contains a variety of species and forms, it may require to
be divided into _different densities_.

In some cases, however, it is best to divide the gathering as a
_preliminary_ operation, which may be done by agitating it in a
quantity of water and decanting what does not readily subside. The
heavier and the lighter portions are then to be treated as two
separate boilings. But when the cleansing has been carried to the
above stage and this division is required, the plan must be somewhat
as follows:--The gathering must be shaken in a test-tube with six
inches of water, and then allowed to subside until one inch at the top
remains pure. About three inches are then to be carefully withdrawn by
a pipette, when the tube may be filled up and the operation repeated.
The three lower inches also may then be decanted and examined. The
gathering is thus divided into three portions, viz.--that which was
withdrawn by the pipette, that which remained floating in the lower
three inches of water in the tube, and that which had settled at the
bottom. An examination of these will inform the operator how to obtain
that particular density of gathering which he desires, and how far it
is worth while to refine this process of elutriation; for in cases of
necessity any one, or all three, of these densities may be operated
upon in the same way to separate a particular diatom.

As occasional aids, it may be remarked, that in some cases _liquor
ammoniæ_ may be used in place of water, as it often separates fine
dirt, which is not otherwise easily got rid of. Some fossil deposits
require to be treated with a boiling solution of carbonate of soda to
disintegrate them; but this operation requires great care, lest the
alkali should destroy the diatoms. Vegetable silicates also sometimes
require to be removed by a solution of carbonate of soda; but as the
frustules of the diatoms themselves are but _vegetable silica_, even
more care is required in this case. It may be well to mention, that
some diatoms are so imperfectly siliceous that they will not bear
_boiling_ in acid at all. Some of these may be allowed to stand in cold
nitric acid some time, whilst others of a smaller and more delicate
character should, when possible, be treated with distilled water alone.

We will now consider the mode of mounting the prepared diatoms, which,
if used dry (as described in this chapter), should be carefully washed
two or three times with the purest distilled water. In this branch,
as in every other, each collector gives preference to that method in
which he is an adept. Thus the diatoms may be placed on the under side
of the cover, to be as near to the object-glass as possible, or upon
the slide itself; and each plan has its advocates. Whichsoever of these
is used, nothing seems more simple to the novice than a tolerably
equal dispersion of the objects upon the slide or cover; but this is
by no means so readily accomplished, consequently I give Mr. Rylands’
method, as his slides are perfect in this respect also. He always
places the diatoms upon the thin glass cover. It is not sufficient,
as is frequently thought, to take a drop of liquid containing the
cleansed material and spread it upon the cover or slide, as without
some additional precaution that uniform and regular distribution of the
specimens is not obtained which is desirable. In order to effect this,
let a drop of the cleansed gathering be diluted sufficiently for the
purpose--how much must be determined in each case by experiment--and
let the covers to be mounted be cleaned and laid upon the brass plate.
(See Chapter I.) By means of a glass tube, about one-twelfth of an
inch in diameter, stopped by the wetted finger at the upper end, take
up as much of the diluted material as will form a moderately convex
drop extending over the whole cover. When all the covers required are
thus prepared, apply a lamp below the brass plate, and raise the
temperature to a point just short of boiling. By this means the covers
will be dried in a few minutes, and the specimens equally distributed
over the whole area. The spread of the fluid upon the covers is
facilitated by breathing upon them; and, to insure uniformity, care
must be taken to avoid shaking them whilst drying. The best plan is to
mount at least half a dozen at once.

Before mounting, Mr. Rylands always burns the diatoms upon the glass
at a dull red heat, whether they are used with balsam or dry. This
burning, he says, is not only an additional cleaning process, but
it effectually fixes the diatoms, and prevents them floating out if
mounted with balsam. The thinnest covers may be burnt without damage
if they are placed upon a small piece of platinum foil of the size
required, which should be about one-hundredth of an inch thick,
perfectly flat, and having three of its edges slightly bent over, so as
to prevent its warping with the heat. The small flame of a spirit-lamp,
or, where there is gas, a Bunsen’s burner, may be employed. The cover
should be shaded from direct daylight, that the action of the flame
may be observed more perfectly. Care must then be taken to raise the
temperature only to the dull red heat before mentioned. The cover will
then be in a fit state for mounting as required.

It has been stated in another place that it is assumed the operator is
not mounting diatoms simply as microscopic objects, but as instructive
specimens. It is not, therefore, sufficient to take a single slide as
all that is required, but to have the same diatom prepared in as many
ways as possible. The following are the principal:--

    1. Mounted crude in fluid (see Chapter IV.).

    2. Burnt crude upon the cover, and mounted dry or in balsam (as
         before mentioned).

    3. Mounted dry or in balsam (see Chapter III.), after the
         cleansing process already described.

I will here give Mr. Rylands’ method of mounting them _dry_, the fluid
and balsam preparations being noticed in their respective chapters.
The slide with the ring of asphalt, or black varnish, should have been
prepared some weeks previously, in order to allow it to dry thoroughly.
When required, it must be held over the spirit-lamp or Bunsen’s burner
until the ring of varnish is softened. The burnt cover, having been
heated at the same time, must then be taken in the forceps and pressed
upon the softened varnish until it adheres all round. When cold, an
outer ring of asphalt completes the slide.

Such is the method which my friend Mr. T. G. Rylands employs in
the preparation of diatoms for the microscope. I have said enough
concerning his results. It is to be feared, however, that to some these
several modes of operation may appear lengthy and complicated; but if
read carefully, and the experiments tried, they will be found simple
enough in practice, and to occupy much less time than an intelligible
description would lead the novice to believe necessary.

One of the most fertile as well as the most curious magazines of
Diatomaceæ is _guano_. The siliceous forms contained therein have been
devoured by sea-birds and passed through the stomach uninjured, and
after lying for ages may be cleaned and classified. Many of these are
not elsewhere met with, so that the student who is desirous to enter
into the study of Diatomaceæ must be instructed as to the best mode
of obtaining them from this source. The particulars to be observed
so closely resemble those before mentioned in the treatment of the
ordinary diatoms, that it will be sufficiently explicit to give the
outlines of the process. The guano must be first washed in pure water,
allowed to subside perfectly, and the liquid then poured off. This must
be repeated until the top fluid is clear, and care taken not to decant
the liquid until perfect subsidence has taken place. The deposit must
then be treated with hydrochloric acid with a gentle heat for an hour
or two, adding a little fresh acid at intervals as long as it excites
any effervescence After this nitric acid must be substituted for the
hydrochloric, and the heat kept up to almost boiling-point for another
hour at least, adding a little fresh acid as before. When this ceases
to act, the deposit must be allowed to settle perfectly and the acid
poured off. All traces of the acid must now be washed away with pure
water, when the remains will be Diatomaceæ, the sand contained in the
guano, and a few other forms. Some of these may be mounted dry, as
before mentioned, but the greater portion should be put up in Canada
balsam as described in Chapter III.

Such is the ordinary method for the treatment of guano; but Mr.
Rylands’ mode of proceeding with ordinary Diatomaceæ (before given)
will be found equally successful with these deposits.

The fossil Infusoria (as they were formerly called) are now termed
Diatomaceæ, and are found in various parts of the world--“Bermuda
earth,” “Berg-mehl” from Norway, deposit from Mourne Mountain in
Ireland, &c. They are found in immense quantities, and afford the
microscopist innumerable objects. The same treatment as that usually
employed for the Diatomaceæ must be followed with these deposits, but
as they are sometimes obtained in hard masses, disintegration is first
necessary. To effect this, they are usually boiled for a short time in
diluted _liquor potassæ_, which will soon cause the mass to fall into a
mud-like deposit. Water must then be immediately added, in order that
all further action of the _liquor potassæ_ may be stopped, otherwise
the objects searched for will be dissolved. For this reason it is
necessary to understand what substance is being dealt with, because
some deposits are much finer and acted upon more readily than others.

In mounting these objects, some are so delicate that they are almost
invisible when balsam is used with them; they are therefore usually
mounted _dry_. Others, however, are much coarser, and may be mounted in
balsam like the Diatomaceæ mentioned in Chapter III.

The common Infusoria cannot be mounted dry with any great success,
though a few may be placed upon the glass slide and allowed to dry
naturally, when their characters will be very well shown. To obtain
anything like a natural appearance, they must be put up in fluid as in
Chapter IV.

Next to the Diatomaceæ, no class of microscopic objects has been more
looked into of late than the Foraminifera. These animals are almost
all marine, having a jelly-like body enclosed in one or more chambers
of shell generally composed of carbonate of lime. The shells are made
with minute orifices, through which the pseudopodia (false feet) are
extended by which the animal is enabled to lay hold of anything and
draw itself along. From the possession of these orifices they derive
their name, as _foramen_ means a door or opening. They have been found
in every depth of sea hitherto sounded, each depth being abundant in
certain species; the lowest beds containing the greatest number of
specimens, though with less variation of kinds. In chalk they are found
in a fossil state, and may readily be shown (see Chapter III.); in
limestone and other hard stones they are abundant, and some mountains
are composed principally of these shells.

The methods of obtaining Foraminifera are various. Many may be found
upon seaweeds, which should always be examined as soon as possible
after gathering. They are found in masses upon some coasts where the
waves have carried and left them; but they are to be found the most
abundantly in sand or mud dredged from the bottom of the sea. They
must, however, be cleansed and separated from the mass of impurity
with which they are usually mixed. This may be done in various ways,
according to the nature of the accompanying matter. If sand alone, as
is frequently the case, the whole mass must be _thoroughly_ dried, and
then stirred up in clean water. The sand will soon subside by its own
weight, but the chambers of the Foraminifera, being filled with air,
will float upon the surface, and may be skimmed off. There is, however,
one objection to this mode of proceeding--some of these objects are
so minute, the chambers containing comparatively so small a quantity
of air, that they sink and are cast away with the refuse sand. On this
account it is preferable to take the trouble of searching certain
soundings under the microscope, using the camel-hair pencil, or some
other contrivance before mentioned, to extract those objects which
are required. To clean the Foraminifera, Professor Williamson advises
to transfer the specimens to an evaporating dish containing a weak
solution of caustic potash. This must be boiled for “some moments,”
when the organic matter will be entirely dissolved, and the calcareous
shells left free from impurity. They must now be well washed in water,
so that all alkaline matter may be entirely got rid of.

If the specimens are in _mud_, we must proceed in a different
way:--Stir up the whole mass in water, and allow it to stand until the
heavier portion has sunk to the bottom; the water may then be poured
off and examined to see if there are any objects contained in it.
This process must be repeated until the water comes off quite clear,
when (if the search is for Foraminifera only) the solution of caustic
potash may be used as before mentioned. However the soundings, &c., are
cleaned, it is necessary to assort them under the microscope with the
camel-hair pencil or other contrivance, as it is impossible to obtain
them fit for mounting without undergoing this process.

The sea soundings taken by order of Government are drawn from the
bottom in a kind of apparatus ingeniously made for the purpose, and
the sand, mud, &c., are brought up in their original state. Common
soundings, however, are taken by lowering a heavy piece of lead coated
with tallow, which consequently brings up a small portion of the
matter from the bottom. Mr. George Mosley, the late Secretary of the
Manchester Microscopic Society, obtained numbers of the “scrapings”
from the sounding leads. To make any use of these it is, of course,
necessary to free them from all traces of the tallow. Mr. Dancer
places the sounding in a basin and pours boiling water upon it, which
causes the melted grease to rise to the surface. When cold, this may
be removed, and the water carefully decanted. The operation may be
repeated until no grease appears, when the water may be withdrawn
and _liquor ammoniæ_ used, which will form a soapy solution with any
remaining grease. This must be treated with hot water for the final
washing. Care must be taken lest the finer forms be carried away in
decanting the washing liquid. Should it be wished to make certain as
to this point, each washing should be examined under the microscope.
In some cases the process of Mr. Dancer will prove sufficient. Mr.
Dale, however, gives a method of accomplishing the same result, which
is much more readily completed; and as the results cannot be found
fault with, I will here give it in full:--It is now well known that one
of the products obtained from the naphtha of coal-tar is a volatile,
oily substance, termed _benzole_ (or, by French chemists, _benzine_),
whose boiling-point, when pure, is about 180° Fahrenheit, which is a
perfect solvent for fatty substances. In a capsule, previously warmed
on a sand-bath, Mr. Dale mixes with the tallow soundings benzole,
whose boiling-point may be about 200°, until sufficiently diluted so
as to run freely, pressing the lumps with a glass rod until thoroughly
mingled; the solution and its contents are then poured into a paper
filter, placed in a glass funnel; the capsule is again washed with
benzole, until the whole of the gritty particles are removed into
the filter. A washing-bottle is then supplied with benzole, and the
contents of the filter washed to the bottom until that liquid passes
off pure, which may be tested by placing a drop from the point of the
funnel on a warm slip of glass or bright platinum, when, if pure,
the benzole will evaporate without residue or tarnish; if grease be
present, the washings must be continued until they are free from it.
After rinsing through _weak_ acid, or alcohol, for final purification,
the calcareous forms will be ready for mounting.

The filter and its contents may be left to dry spontaneously, when the
latter can be examined by the microscope. Should time be an object,
rapid drying may be effected by any of the usual methods; one of
which, recommended by Mr. Dale, is to blow a stream of hot air through
a glass tube held in the flame of a Bunsen’s burner. The lower the
boiling-point of the benzole, the more readily can the specimens be
freed from it. A commoner quality may be used, but it is more difficult
to dry afterwards.

Pure benzole being costly, this may appear an expensive process; but,
with the exception of a trifling loss by evaporation, the whole may be
recovered by simple distillation. The mixture of tallow and benzole
being placed in a retort in a hot-water, a steam, or a sand bath, the
benzole will pass into the receiver, and the tallow or other impurities
will remain in the retort. When the whole of the benzole has distilled
over, which is ascertained by its ceasing to drop from the condenser,
the heat is withdrawn and the retort allowed to cool before the
addition of fresh material. Half a dozen to a dozen filters, each with
its specimen, can be in process at the same time; and the distillation
of the recovered benzole progresses as quickly as the filtration, which
was practically proved on the occasion named. Great caution in the use
of benzole is to be taken in the approach of lights to the inflammable
vapour.

After the Foraminifera and calcareous forms have been removed, the
residue may be treated with acids and levigation in the usual manner,
to obtain siliceous forms and discs, if there are any present; but to
facilitate their deposition, and to avoid the loss of any minute atoms
suspended in the washings, I would suggest the use of filtration. The
conical filter is unsuitable, as the particles would spread over too
great a surface of paper; but glass tubes open at both ends (such as
broken test-tubes) will be found to answer, the broad end covered
with filtering paper, and over that a slip of muslin tied on with a
thread to facilitate the passage of the water and prevent the risk of
breaking the paper. Suspend the tube over a suitable vessel through a
hole cut in thin wood or cardboard, pour in the washings which can be
thus filtered and then dried. The cloth must be carefully removed, the
paper cut round the edges of the tube, and the diatoms on the paper
disc may be removed by a camel-hair pencil or otherwise, ready for
mounting. Thus many objects may be preserved which would be either
washed away or only be obtained by a more tedious process.

Such is Mr. Dale’s method of cleaning the soundings from the tallow,
and as it thoroughly accomplishes its end, and is alike effective
and not injurious to Foraminifera and diatoms, it may be safely
recommended. The weak solution of caustic potash before advised for
Foraminifera, must not be used where it is desired to preserve the
diatoms, as they would certainly be injured, or destroyed altogether,
if this agent were employed.

In fixing the Foraminifera upon the slide, no better plan can be
followed than the “dry cells” and gum recommended in the early parts
of this chapter. Owing to their thickness and composition, most of
them are opaque objects only; but they are exquisitely beautiful,
and require no particular care, except in allowing the cell, &c., to
be perfectly dry, when the cover is placed upon it, or the damp will
certainly become condensed upon the inner side, and the examination
seriously interfered with.

Many of the Foraminifera require cutting into sections if it is
wished to examine the internal structure, &c.,--“decalcifying” is
also desirable in some cases;--both of these processes will be found
described at length in the chapter on Sections and Dissection.

When more than one specimen of some particular shell is obtained, it is
better to place them upon the slide in different positions, so as to
show as much of the structure as possible. I will conclude this subject
by quoting a passage from T. Rymer Jones:--“It is, therefore, by no
means sufficient to treat these shells as ordinary objects by simply
laying them on a glass slide, so as to see them only from one or two
points of view; they must be carefully examined in every direction,
for such is the diversity of form that nothing short of this will be
at all satisfactory. For this purpose, they should be attached to the
point of a fine needle, so that they may be turned in any direction,
and examined by reflected light condensed upon them by means of a
lens or side reflector. In many of the thick-shelled species it will
be necessary to grind them down on a hone [see Chapter V.] before the
number and arrangement of the internal chambers is discernible; and in
order to investigate satisfactorily the minutiæ of their structure, a
variety of sections, made in various ways, is indispensable.”

Plants afford an almost inexhaustible treasury for the microscope,
and many of them show their beauties best when mounted dry. When any
of these also are to be mounted, care must be taken that they are
thoroughly dry, otherwise the damp will certainly arise in the cell,
and injure the object; and it may be here mentioned that long after a
leaf has every appearance of dryness, the interior is still damp, and
no way can be recommended of getting rid of this by any quicker process
than that effected by keeping them in a warm room, as many leaves,
&c., are utterly spoiled by using a hot iron or other contrivance. The
safest way is to press them gently betwixt blotting-paper, which may
be removed and dried at short intervals; and though this may appear a
tedious operation, it is a _safe_ one.

On the surface of the leaves, hairs and scales of various and very
beautiful forms are found, most of which display their beauties best
when removed from the leaf, and used with the polarizer. These will be
noticed in another place; but a portion of the leaf should always be
prepared in its natural form, to show the arrangement of the hair or
scales upon it; which must almost invariably be mounted dry when used
for this purpose. Many of them require very delicate handling. The
_epidermis_, or, as it is by some termed, the _cuticle_, is the outer
skin which lies upon the surface of the leaves and other parts of most
plants. This is composed of cells closely connected, often bearing
the appearance of a rude network. In many plants, by scraping up the
surface of the leaf, a thin coating is detached, which may be torn off
by taking hold of it with the forceps. The piece may then be washed and
floated upon a glass slide, where, on drying, it will be firmly fixed,
and may usually be mounted dry. Amongst the most beautiful and easily
prepared of these may be mentioned the petal of the geranium, the cells
of which are well defined and amongst the most interesting.

Closely connected with the leaves are the ANTHERS and POLLEN, of
which a great number are beautiful and interesting subjects for the
microscopist.

The mallow tribe will furnish some exquisite objects, bearing the
appearance of masses of costly jewels. These are usually dried with
pressure, but the natural form may be more accurately preserved by
allowing them to dry as they are taken from the flower, with no
interference except thoroughly protecting them from all dust. Sometimes
the anther is divided, so that the cell required to receive them may
be of as little depth as possible. The common mallow is a beautiful
object, but I think the lavatera is a better, as it shows the pollen
chambers well, when dried unpressed. The pollen is often set alone, and
is well worth the trouble, as it then admits of more close examination.
Often it is convenient to have the _anther and pollen_ as seen in
nature on one slide, and the _pollen_ alone upon another. The former
should be taken from the flowers before their full development is
attained, as if overgrown they lose much of their beauty. Some pollens
are naturally so dark that it is necessary to mount them in Canada
balsam or fluid, as described in other places; but they are better
mounted dry when they are not too opaque.

Here, too, we may also mention the SEEDS of many plants as most
interesting, and some of them very beautiful, objects, requiring for
the greater part but a low power to show them. Most of these are to be
mounted dry, as opaque objects, in cells suited to them, but some are
best seen in balsam, and will be mentioned in Chapter III.

The CORALLINES, many of which are found on almost every coast, afford
some very valuable objects for the microscope. They must be well washed
when first procured, to get rid of all the salts in the sea-water,
dried and mounted in cells deep enough to protect them from all danger
of pressure, as some of them are exceedingly fragile. The white ivory
appearance which some of them present is given to them by an even
covering of carbonate of lime; and should it be desired to examine the
structure of these more closely, it may be accomplished by keeping them
for some time in vinegar or dilute muriatic acid, which will remove the
lime and allow of the substance being sliced in the same way as other
Algæ. (“Micrographic Dictionary,” p. 183.)

THE SCALES OF INSECTS.--The fine dust upon the wings of moths and
butterflies, which is so readily removed when handled carelessly,
is what is generally called _scales_. To these the wing owes the
magnificent colours which so often are seen upon it; every particle
being what may be termed a distinct flat feather. How these are placed
(somewhat like tiles upon a roof) may be easily seen in the wing of
any butterfly, a few being removed to aid the investigation. The form
of them is usually that of the “battledore” with which the common game
is played, but the handle or base of the scale is often short, and the
broad part varies in proportionate length and breadth in different
specimens. The markings upon these also vary, some being mostly
composed of lines running from the base to the apex, others reminding
us of network--bead-like spots only are seen in some--indeed, almost
endless changes are found amongst them. These scales are not confined
to butterflies and moths, nor indeed to the _wings_ of insects. The
different gnats supply some most beautiful specimens, not only from
the wings, but also from the proboscis, &c.; whilst from still more
minute insects, as the podura, scales are taken which were at one time
esteemed as a most delicate test. The gorgeous colours which the
diamond beetles also show when under the microscope are produced by
light reflected from minute scales with which the insects are covered.

In mounting these objects for the microscope it is well to have the
part of the insect from which the scales are usually taken as a
separate slide, so that the natural arrangement of them may be seen.
This is easily accomplished with the wings of butterflies, gnats, &c.;
as they require no extraordinary care. In mounting the _scales_ they
may be placed upon slides, by passing the wings over the surface, or
by gently scraping the wing upon the slide, when they must be covered
with the thin glass. Of course, the extreme tenuity of these objects
does away with the necessity of any cell excepting that formed by the
gold-size or other cement used to attach the cover. The scales of the
podura should be placed upon the slide in a somewhat different manner.
This insect is without wings, and is no longer than the common flea. It
is often found amongst the sawdust in wine-cellars, continually leaping
about by the aid of its tail, which is bent underneath its body. Dr.
Carpenter says:--“Poduræ may be obtained by sprinkling a little oatmeal
on a piece of black paper near their haunts; and after leaving it there
for a few hours, removing it carefully to a large glazed basin, so
that, when they leap from the paper (as they will when brought to the
light), they may fall into the basin, and may thus separate themselves
from the meal. The best way of obtaining their scales, is to confine
several of them together beneath a wine glass inverted upon a piece of
fine smooth paper; for the scales will become detached by their leaps
against the glass, and will fall upon the paper.” These scales are
removed to the slide, and mounted as those from the gnats, &c. When the
podura has been caught without the aid of the meal, it may be placed
upon the slide, under a test-tube, or by any other mode of confinement,
and thus save the trouble of transfer from the paper before mentioned.
Another method is to seize the insect by the leg with the forceps and
drag it across the slide, when a sufficient quantity of scales will
probably be left upon it.

These scales are usually mounted “dry;” but Hogg recommends the use of
Canada balsam (Chapter III.) as rendering their structure more definite
when illuminated with Wenham’s parabolic reflector. Some advise other
methods, which will be mentioned in Chapter IV. As most _insects_
when undissected are mounted in Canada balsam, the different modes of
treatment which they require will be stated in another place.

In mounting blood of any kind to show the corpuscles, or, as they are
often called, _globules_, which are round or oval discs, it is but
necessary to cover the slide on the spot required with a coating as
thin as possible and allow it to dry before covering with the thin
glass. There is a slight contraction in the globules when dried, but
not enough to injure them for the microscope. The shape of these varies
in different classes of animals, but the size varies much more, some
being many times as large as others.--Some of the _larvæ skins_ are
beautiful objects; but, like many sections of animal and other fragile
matter, are difficult to extend upon the slide. This difficulty is
easily overcome by floating the thin object in clear water, immersing
the slide and when the object is evenly spread gently lifting it. Allow
it then to dry by slightly raising one end of the slide to aid the
drainage, and cover with the thin glass as other objects. The tails and
fins of many small fish may be mounted in a similar manner, and are
well worth the trouble.

A few objects which are best shown by mounting _dry_ may be here
mentioned as a slight guide to the beginner, though some of them have
been before noticed;--many of the Foraminifera as elsewhere described.
Some _crystals_ are soluble in almost any fluid or balsam, and should
be mounted _dry_; a few, however, deliquesce or effloresce, which
renders them worthless as microscopic objects.

The wings of butterflies and gnats, as before noticed, afford many
specimens wherewith to supply the cabinet of the young student. A
great variety also may be found amongst the ferns; indeed, these alone
will afford the student occupation for a long time. On the under-side
of the leaves are the reservoirs for the “spores,” which in many
instances somewhat resemble green velvet, and are arranged in stripes,
round masses, and other forms. The spores are usually covered with a
thin skin termed the _indusium_, which is curiously marked in some
specimens, often very like pollen-grains. The manner in which these
spores with all their accompaniments are arranged, their changes and
developments afford almost endless subjects for study; different ferns
presenting us with many variations in this respect totally invisible
without the aid of the microscope. The hymenophyllums (of which two
only belong to England) are particularly interesting, and the structure
of the leaves when dried makes them beautiful objects, often requiring
no balsam to aid their transparency. Portions of the _fronds_ of ferns
should be mounted as opaque objects, after having been dried between
blotting paper, when they are not injured by pressure; but care must
be taken to gather them at the right time, as they do not show their
beauty before they are ripe, and if over-ripe the arrangement of the
spores, &c., is altered. The spores may be mounted as separate objects
in the same manner as pollen, before-mentioned, and are exquisitely
beautiful when viewed with a tolerably high power. The number of
foreign ferns now cultivated in this country has greatly widened the
field for research in this direction; and it may also be mentioned
that the under-sides of many are found to be covered with “scales” of
very beautiful forms. A small piece of the frond of one of these may
be mounted in its natural state, but the removal of the “scales” for
examination by polarized light will be described in another place. The
mosses also are quite a little world, requiring but a low power to show
their beauties. The leaves are of various forms, some of which resemble
beautiful net-work; the “urns” or reservoirs for the spores, however,
are perhaps the most interesting parts of these objects, as also of
the “liverworts” which are closely allied to the mosses. These “urns”
are generally covered by lids, which fall off when the fruit is ripe.
At this period they are well fitted for the microscope. The common
screw-moss may be found in great abundance, and shows this denudation
of the spores very perfectly. Many of these may be easily dried without
much injury, but they should also be examined in their natural state.

The student should not omit from his cabinet a leaf of the nettle and
the allied foreign species, the mystery of which the microscope will
make plain. The hairs or stings may also be removed, and viewed with a
higher power than when on the leaf, being so transparent as to require
no balsam or other preservative.

There are few more interesting objects than the _raphides_ or
_plant-crystals_. These are far from being rare, but in some plants
they are very minute, and consequently require care in the mounting,
as well as a high magnifying power to render them visible; in others
they are so large that about twenty-five of them placed point to point
would reach one inch. Some of these crystals are long and comparatively
very thin, which suggested the name (_raphis_, a needle); others are
star-like, with long and slender rays; while others again are of a
somewhat similar form, each ray being solid and short. If the stem
of rhubarb, or almost any of the hyacinth tribe, be bruised, so that
the “juice” may flow upon the slide, in all probability some of these
crystals will be found in the fluid. To obtain them clean, they must be
freed from all vegetable matter by maceration. After this they must be
thoroughly washed and mounted “dry.” They are also good _polarizing_
objects, giving brilliant colours; but when used for this purpose
they must be mounted as described in Chapter III. A few plants which
contain them may be mentioned here. The Cactaceæ are very prolific; the
orchids, geraniums, tulips, and the outer coating of the onion, furnish
the more unusual forms.

The Fungi are generally looked upon as a very difficult class of
objects to deal with, but amongst them some of the most available may
be found. The forms of many are very beautiful, but are so minute
as to require a high magnifying power to show them. The mould which
forms on many substances is a fungus, and in some cases may be dried
and preserved in its natural state. A friend of mine brought me a
rose-bush completely covered with a white blight. This was found to be
a fungus, which required a high magnifying power to show it. Being a
very interesting object, it was desirable to preserve it, and this was
perfectly effected without injury to the form by simply drying the leaf
in a room usually occupied. Amongst the fungi are many objects well
worth looking for, one of which is the _Diachæa elegans_. This, the
only species, says the “Micrographic Dictionary,” is found in England
upon the living leaves of the lily-of-the-valley, &c. These little
plants grow in masses, reminding one of mould, to a height of a quarter
of an inch, and each “stem” is covered with a sheath, in shape somewhat
like an elongated thimble. When ripe the sheath falls off and reveals
the same shaped column, made up of beautifully fine net-work, with the
spores lying here and there. This dries well, and is a good object for
the middle powers. Amongst the fungi the blights of wheat and of other
articles of food may be included. Many of them may be mounted “dry;”
others, however, cannot be well preserved except in liquids, and will
be referred to in Chapter IV. Amongst the zoophytes and sea-mats,
commonly called “sea-weeds,” may be found many very interesting objects
to be mounted “dry.” When this mode of preservation is used, it is
necessary that all the sea-salt be thoroughly washed from them. As they
are, however, most frequently mounted in balsam or liquid, they will be
more fully noticed in other places.

The _scales of fishes_ are generally mounted “dry” when used as
ordinary objects; but for polarized light, balsam or liquid must be
used, as noticed in Chapter III. The variety and beauty of these are
quite surprising to the novice. It is also very interesting to procure
the skin of the fish when possible, and mount it on a separate slide to
show how the scales are arranged. The sole is one of the most unusual
forms, the projecting end of each scale being covered with spines,
which radiate from a common centre, while those at the extremity are
carried out somewhat resembling the rays of a star. One of the skates
has a spine projecting from the centre of each scale, which is a very
curious opaque object, especially when the skin is mounted in the
manner described. The perch, roach, minnow, and others of the common
fishes give the student good objects for his cabinet, and may be
procured without difficulty.

Insects which are very transparent, or have the “metallic lustre”
with which any medium would interfere, are mounted “dry.” The
diamond-beetle, before mentioned, is a splendid example of this;
the back is generally used, but the legs, showing the curious feet,
are very interesting objects. Indeed, amongst the legs and feet of
insects there is a wide field of interest. When they are of a “horny”
nature, it is best to dry them in any form preferred, but to use no
pressure; when, however, they are wanted flat, so as to show the feet,
&c., extended, they must be dried with a gentle pressure betwixt
blotting-paper if possible. But this will be treated more fully in
Chapter III.

The _eyes of insects_ are sometimes allowed to dry in their natural
shape, and mounted as opaque objects; but generally they are used as
transparencies in balsam or liquid, so the description of the treatment
which they require will be deferred to Chapter III.

Hairs, when not too dark, are sometimes transparent enough when mounted
dry, but are usually mounted in balsam. These also will be more fully
noticed in another place.

These are a few of the objects which are often mounted dry, but some
of them should be shown in balsam or liquid also, and there is much
difference of opinion as to the best way of preserving others. This,
however, is explained by the transparency which the balsam gives
interfering with one property of the object and yet developing another
which would have remained invisible if preserved dry. The only method
of overcoming this difficulty is to keep the object mounted in both
ways, which is comparatively little trouble.

I may here mention that many prefer the lieberkuhn for the illumination
of opaque objects; and a good background is gained by putting upon the
under side of the slide, immediately beneath the object, a spot of
black varnish, which does not interfere materially with the light.




CHAPTER III.

MOUNTING IN CANADA BALSAM.


The nature and use of this substance has been before spoken of, so the
method of working with it may be at once described.

Perfect dryness of the objects is, if possible, more necessary in this
mode of mounting than any other, as dampness remaining in the object
will assuredly cause a cloudiness to make its appearance in a short
time after it is fixed. Where pressure does not injure the specimens,
they are most successfully treated when first dried betwixt the leaves
of a book, or in any other way which may prove most convenient, as
noticed in Chapter II.

Before describing the methods of proceeding with any particular
objects, general rules may be given which should be observed in order
to succeed in this branch of mounting.

As the object is to be thoroughly immersed in the balsam, it is evident
that when it has once been covered, so it must remain, unless we again
free it by a process hereafter mentioned, which is very troublesome;
and on this account there must be nothing whatever in the balsam except
the object. The inexperienced may think this an unnecessary caution;
but the greatest difficulty he will meet with is to get rid of minute
bubbles of air, perhaps invisible to the naked eye, which appear
like small globules when under the microscope, and render the slide
unsightly, or even worthless. Ten objects out of eleven contain air, or
at least are full of minute holes which are necessarily filled with it;
so that if they should be immersed in any liquid of thick consistency,
these cells of air would be imprisoned, and become _bubbles_. The air,
then, must be got rid of, and this is usually accomplished by soaking
for some time in turpentine, the period required differing according
to the nature of the object. In some cases, the turpentine acts upon
the colour, or even removes it altogether, so that it must be watched
carefully. Often, however, this is an advantage, as where the structure
alone is wanted, the removal of the colouring matter renders it more
transparent. There are objects, however, which retain the air with such
tenacity that soaking alone will not remove it. If these will bear heat
without being injured, they _may_ be boiled in turpentine, or even in
balsam, when the air will be partly or totally expelled. But where
heat is objectionable, they must be immersed in the turpentine, and so
submitted to the action of the air-pump. Even with this aid, sometimes
days are required to accomplish it perfectly, during which time the air
should be exhausted at intervals of five or six hours, if convenient,
and the objects turned over now and then.

Sometimes the objects are so minute that it is impossible to submit
them to any soaking, and in this case they must be laid upon the slide
at once, and the turpentine applied to them there. But it must not be
forgotten that there are some few which are much better mounted in such
a way that the balsam may thoroughly surround, and yet not _penetrate_,
the substance more than necessary. Sections of teeth are amongst these,
which will be noticed in another place, and some insects (see Dr.
Carpenter) when required to show the “ramifications of the tracheæ.”

Having freed the object, then, from these two enemies--dampness and
air--we must proceed to mount it.

The slide must first be cleaned; then on the centre a quantity of
balsam must be placed with a bluntly-pointed glass rod, according to
the size of the object about to be mounted. To this a slight heat must
be applied, which will cause any bubbles to rise from the surface
of the slide, so that they may be readily removed with a needle.
The object should be freed from all air by steeping in turpentine,
as before described, and then from superfluous liquid by a short
drainage, and carefully laid _upon_, or where it is practicable thrust
_into_, the balsam, prepared on the slide as above. In the former
case, or where the balsam has not totally covered the object, a small
quantity must be taken, warmed, and dropped upon it, and any bubbles
removed by the needle as before. To cover this, the thin glass must be
warmed, and beginning at one side, allowed to fall upon the balsam,
driving a small “wave” before it, and thus expelling any bubbles which
may remain. This is quite as safely performed (if not more so) by
making a solution of balsam in turpentine of the consistency of thick
varnish. The thin glass cover may be slightly coated with this, and
will then be much less liable to imprison any air, which frequently
happens when the cover is dry. Bubbles, however, will sometimes
make their appearance in spite of all care; but when the object is
comparatively strong, they may be removed by keeping the slide rather
warm, and _working_ the cover a little, so as to press them to one
side, when they should be immediately removed with a needle point,
otherwise they are again drawn under.

Where the slide requires keeping warm for any length of time, a
_hot-water bath_ is sometimes made use of, which is simply a flat
tin, or other metal case, with a mouth at the side, that when the hot
water is introduced it may be closed up, and so retain its warmth for
a long time. In working, the slide is laid upon it, and so admits of
longer operations, when required, without growing cold. Sometimes
a spirit-lamp is placed under it to keep up an equal heat through
excessively long processes. Where the time required, however, is but
short, a thick brass plate is sometimes used (see Chapter I.), which is
heated to any degree that is required, and the slide placed upon it.

Some objects, which are so thin that they are usually _floated_ upon
the slide, as before stated, require no steeping in turpentine or other
liquid. These are best mounted by covering with a little _diluted_
balsam, and after this has had time to penetrate the substance,
ordinary balsam is laid upon it, and the slide finished in the usual
manner.

I have stated that the balsam is usually applied to the slide and
objects with a “bluntly-pointed glass rod;” but for the purpose of
drawing the balsam from the bottle, and conveying it to the desired
place, Dr. Carpenter uses a glass syringe with a _free_ opening. These
are his instructions:--“This (the syringe) is most readily filled with
balsam, in the first instance, by drawing out the piston, and pouring
in balsam previously rendered more liquid by gentle warmth; and nothing
else is required to enable the operator at any time to expel precisely
the amount of balsam he may require, than to warm the point of the
syringe, if the balsam should have hardened in it, and to apply a very
gentle heat to the syringe generally, if the piston should not then be
readily pressed down. When a number of balsam objects are being mounted
at one time, the advantage of this plan in regard to facility and
cleanliness (no superfluous balsam being deposited on the slide) will
make itself sensibly felt.”

When the “mounting” is thus far accomplished, the outer “wall” of
balsam may be roughly removed after a few hours have elapsed; but
great care is necessary lest the cover be moved or interfered with in
any way. In this state it may be left for the final cleansing until
the balsam becomes hard, which takes place sooner or later, according
to the degree of warmth it has been subjected to. A mantel-piece, or
some place about equal to it in temperature, is the best suited to
this purpose; and when the requisite hardness is attained, it may be
proceeded with as follows:--With a pointed knife the balsam must be
scraped away, taking care that the thin glass be not cracked by the
point getting _under_ it. If used carefully, the knife will render the
slide almost clean; but any minute portions which still adhere to the
glass must be rubbed with linen dipped in turpentine or spirit. If the
balsam is not very hard, these small fragments are readily removed by
folding a piece of paper tightly in a triangular form with many folds,
and damping the point with which the glass is rubbed. As the paper
becomes worn with the friction, the balsam will be carried off with it.
In some cases I have found this simple expedient very useful.

Sometimes the object to be mounted is of such a thickness as to
require a cell. For this purpose glass rings are used (as described in
Chapter IV.), and filled with balsam. The best mode of doing this is
thus described by Mr. T. S. Ralph in the _Microscopic Journal_:--“The
question was asked me when I was in England, if I knew how to fill a
cell with Canada balsam and leave behind no air-bubbles? I replied in
the negative; but now I can state how to accomplish this. Fill the cell
with clear spirit of turpentine, place the specimen in it, have ready
some balsam just fluid enough to flow out of the bottle when warmed by
the hand; pour this on the object at one end, and, gradually inclining
the slide, allow the spirit of turpentine to flow out on the opposite
side of the cell till it is full of balsam; then take up the cover, and
carefully place upon it a small streak of Canada balsam from one end
to the other. This, if laid on the cell with one edge first, and then
gradually lowered until it lies flat, will drive all the air before
it, and prevent any bubbles from being included in the cell. It can
be easily put on so neatly as to require no cleaning when dry. If the
cover is pressed down too rapidly, the balsam will flow over it, and
require to be cleaned off when hardened, for it cannot be done safely
while fluid at the edges.”

Sometimes with every care bubbles are enclosed in the balsam, injuring
objects which are perhaps rare and valuable. The whole slide must then
be immersed in turpentine until the cover is removed by the solution of
the balsam; and the object must be cleansed by a similar steeping. It
may then be remounted as if new in the manner before described.

The balsam and chloroform described in Chapter I. is thus used; and
where the object is thin, the mounting is very easily accomplished.
When the object is laid upon the slide with a piece of glass upon it,
and the balsam and chloroform placed at the edge of the cover, the
mixture will gradually flow into the space betwixt the glasses until
the object is surrounded by it, and the unoccupied portion filled. The
chloroform will evaporate so quickly that the outer edge will become
hard in a very short time, when it may be cleaned in the ordinary way.
Sometimes the balsam is dissolved in the chloroform without being first
hardened; but this is only to render it more fluid, and so give the
operator less chance of leaving bubbles in the finished slide, as the
thicker the medium is, the more difficult is it to get rid of these
intruders.

It has been before mentioned that some have objected to chloroform
and balsam, believing that it became _clouded_ after a certain time.
Perhaps this may be accounted for in part by the fact that almost all
objects have a certain amount of dampness in them. Others are kept in
some preservative liquid until the time of mounting, and these liquids
generally contain certain salts (Chapter IV.). If this dampness, as
well as all traces of these salts, however small, are not totally
removed--the former by drying, the latter by repeated washings--the
addition of chloroform will render the balsam much more liable to the
cloudiness than when balsam alone was used, as before mentioned.

This mode of employing the balsam, however, will not be always
applicable, as _chloroform_ acts upon some substances which balsam
_alone_ does not. Some salts are even soluble in it, the crystals
disappearing after a few days or weeks, whereas in the balsam alone
they are quite permanent. Experience is the only guide in some cases,
whilst in others a little forethought will be all that is required.

The particular methods used for certain objects may be now entered
upon. Many of the Diatomaceæ and fossil Infusoria, as they are
sometimes termed, are mounted dry, and cleaned in the way described in
Chapter II. Others are almost always placed in balsam, except where
they are intended to be used with the lieberkuhn and dark background,
by which means some of them are rendered exquisitely beautiful. The
usual way of mounting them in balsam is as follows:--Take a drop of
the water containing them, place it upon the slide, and evaporate over
the lamp, whilst with a needle they may be dispersed over any space
desired. When they are thoroughly dry, drop a little balsam on one
side, and exclude the bubbles. The slide may then be warmed to such
a degree that the balsam, by lifting the glass at one end, will be
carried over the specimens, which may then be covered with thin glass,
made warm as before described. Where the objects are quite dry, and
loose upon the glass, it requires great care in placing the cover upon
them, otherwise they are forced to one edge, or altogether from under
it, in the wave of the balsam. For this reason, Professor Williamson
adds a few drops of gum-water to the last washing, which causes them to
adhere sufficiently to the glass to prevent any such mishap.

Mr. T. G. Rylands’ method differs in some degree from the above, and
is, to use his own words, as follows:--Thick balsam is preferable,
and the burnt covers (see Chapter II.) to be mounted are laid in a
convenient position with the diatoms upwards. The slides required
having been carefully cleaned and marked on the under side with a ring
of ink, by the aid of a turntable about half an inch in diameter to
point out the centre, a drop of benzole is applied by a large pin to
the diatoms on the cover, so as to exclude the air from the valves
and frustules. The slide is then held over the lamp, and when warm,
a sufficiently large drop of balsam is put upon it, and heated until
it begins to steam. If small bubbles appear, a puff of breath removes
them. The slide being held slightly inclined from the operator, and the
drop of balsam becoming convex at its lower edge, the cover is brought
in contact with it at that point, gradually laid down, pressed with the
forceps, and brought to its central position. When cool the superfluous
balsam (if any) is removed with a heated knife-blade, the slide cleaned
with a little turpentine, and finished by washing in a hand-basin with
soap and water. In this process there is no delay if the balsam be
sufficiently thick, as the slide may be cleaned off almost before it is
cold.

It is now well known that from common chalk it is an easy matter to
obtain interesting specimens of Foraminifera. Scrape a small quantity
of chalk from the mass and shake it in water; leave this a few minutes,
pour the water away and add a fresh quantity, shake up as before, and
repeat two or three times. Take a little of the residue, and spread it
upon the slide, and when quite dry add a little turpentine. When viewed
with a power of two hundred and fifty diameters this will generally
show the organisms very well. If it is desired to preserve the slides,
they may be then mounted in Canada balsam. Mr. Guyon, in “Recreative
Science,” observes that the accumulation of the powder, by the action
of the rain or exposure to the atmospheric action, at the foot or any
projection of the chalk cliffs, will afford us better specimens than
that which is “scraped,” as the organisms are less broken in the former.

When the Foraminifera are of a larger size, though transparent enough
to be mounted in balsam, the air must be first expelled from the
interior, otherwise the objects will be altogether unsatisfactory. To
accomplish this they must be immersed in turpentine and submitted to
the action of the air-pump. So difficult is it to get rid of this enemy
that it is often necessary to employ three or four exhaustions, leaving
them for some time under each. When all air has given place to the
turpentine, they must be mounted in the ordinary way.

Of all objects which are commonly met with, few are such general
favourites as the POLYCYSTINÆ, and deservedly so. Their forms are
most beautiful, and often peculiar--stars varying in design, others
closely resembling crowns; the _Astromma Aristotelis_ like a cross,
and many whose shapes no words could describe. The greater part,
perhaps, of those which are usually sold, is from the rocky parts of
Bermuda; but they are also found in Sicily, some parts of Africa and
America. They are usually mounted in balsam, but are equally beautiful
mounted “dry” and used with the lieberkuhn. They require as much care
in cleaning as the Diatomaceæ, but the process is a different one.
Sometimes this is effected by simply washing until they are freed from
all extraneous matter, but this is seldom as effectual as it should be.
In the _Microscopic Journal_ Mr. Furlong gives the following method of
treatment as the best he knew:--

Procure--

    A large glass vessel with 3 or 4 quarts of water.

    New tin saucepan holding 1 pint.

    2 thin precipitating glasses holding 10 oz. each.

Take 3 oz. of dry “Barbadoes earth” (lumps are best), and break into
rather small fragments. Put 3 or 4 oz. of common washing soda into the
tin and half fill it with water. Boil strongly, and having thrown in
the earth, boil it for half an hour. Pour nine-tenths of this into the
large glass vessel, and gently crush the remaining lumps with a soft
bristle brush. Add soda and water as before, and boil again; then pour
off the liquid into the large vessel, and repeat until nothing of value
remains. Stir the large vessel with an ivory spatula, let it stand for
three minutes, and pour gently off nine-tenths of the contents, when
the shells will be left, partially freed only, like sand.

2ND PROCESS.--Put common washing soda and water into the tin as before,
and having placed the shells therein, boil for an hour. Transfer to the
large vessel as before, and after allowing it to stand for one minute
pour off. Each washing brings off a kind of “flock,” which seems to be
skins.

3RD PROCESS.--Put the shells in precipitating glass and drain off the
water until not more than ½ oz. remains. Add half a teaspoonful of
bicarbonate of soda, dissolve, and then pour in gently 1 oz. of strong
sulphuric acid. This liberates the “flock,” &c., and leaves the shells
beautifully transparent. Wash well now with water to get rid of all
salts and other soluble matter.

Some of the large shells are destroyed by this method, but none that
are fit for microscopic use. An oblique light shows these objects best.

These are sometimes treated in the manner described in Chapter II.
where the diatoms are spoken of, but many forms are liable to be
injured by this severe process.

It has been before stated that some of the zoophytes may be mounted
dry, and others examined as opaque or transparent objects according to
their substance. They are very interesting when examined in the trough
whilst living, but to preserve many of them for future examination they
must be mounted in some preservative medium. Sometimes this may be one
of the liquids mentioned in Chapter IV., but if possible they should
be kept in balsam, as there is less danger of injury by accident to
this kind of slide. This method of mounting presents some difficulties,
but I think that all agree as to the trustworthiness of Dr. Golding
Bird’s information on the subject, which appeared in the _Microscopic
Journal_. Of this, space forbids me to give more than a condensed
account, but I hope to omit nothing of moment to the reader for whom
these pages are written.

After stating that there are few who are not familiar with these
exquisite forms, and have not regretted the great loss of beauty
they sustain in dying, he informs us that from their so obstinately
retaining air in the cells and tubes when dried, it is hardly
practicable to get rid of it; and they also shrivel up very seriously
in the process of drying. The following plan, however, he has found
almost faultless in their preparation.

To preserve them with extended tentacles, they should be plunged in
cold fresh water, which kills them so quickly that these are not often
retracted. The specimens should be preserved in spirit until there is
leisure to prepare them; if, however, they have been _dried_, they
should be soaked in cold water for a day or two before being submitted
to the following processes:--

1. After selecting perfect specimens of suitable size, immerse them
in water heated to about 120° and place them under the receiver of an
air-pump. Slowly exhaust the air, when bubbles will rise and the water
appear to be in a state of active ebullition. After a few minutes
re-admit the air and again exhaust, repeating the process three or four
times. This will displace the air from most, if not all, of the class.

2. Remove the specimens and allow them to drain upon blotting-paper
for a few seconds; then place them in an earthen vessel fitted with a
cover, and previously heated to about 200°. This heat may be easily
got by placing the vessel for a short time in boiling water, wiping
it immediately before use with a thick cloth. The specimens are then
dropped into this, covered with the lid, and immediately placed under
the receiver of the air-pump, and the air rapidly exhausted. By this
means they are dried completely, and so quickly that the cells have no
time to wrinkle.

3. In an hour or two remove them from the air-pump and drop them into
a vessel of perfectly transparent camphine. This may be quite cold
when the horny, tubular polypidoms, as those of the Sertulariæ, are
used; but should be previously heated to 100° when the calcareous,
cellular Polyzoa are the objects to be preserved. The vessel should be
covered with a watch-glass and placed under the receiver, the air being
exhausted and re-admitted two or three times.

4. The slide which is to receive the specimen should be well cleaned
and warmed so as to allow the balsam to flow freely over it. This
must be applied in good quantity, and air-bubbles removed with the
needle-point. Take the polypidom from the camphine, drain it a little,
and with the forceps immerse it fully in the balsam. The glass to be
laid upon it should be warmed and its surface covered with a thin
layer of balsam, and then lowered gradually upon it, when no bubbles
should be imprisoned. A narrow piece of card-board at each end of the
object for the cover to rest upon, prevents any danger of crushing the
specimen.

This mode of mounting polypidoms, &c., seems to give almost the
complete beauty of the fresh specimens. They are very beautiful objects
when viewed with common light, but much more so when the polarizer is
used (in the manner described a little farther on).

To the above instructions there can be little to add; but I may here
mention that some young students may not be possessed of the air-pump,
and on this account put aside all search for those specimens which need
little looking for at the seaside. Many of these, however, though they
lose some beauty by the ordinary mode of drying, will by steeping for
some time in turpentine not only be freed from the air-bubbles, but
suffer so little contraction that they are a worthy addition to the
cabinet.

Another class of objects is the _spicula_ met with in sponges, &c.
These are often glass-like in appearance and of various shapes; many
are found resembling needles (whence their name); some from the synapta
are anchor-like, whilst others are star-like and of complex and almost
indescribable combinations. As some of these are composed of silex and
are consequently not injured by the use of nitric acid, the animal
substance may be got rid of by boiling them in it. Those, however,
which are calcareous must be treated with a strong solution of potash
instead; but whichever way is used, of course they must afterwards be
freed from every trace of residue by careful washing.

These spicules may be often found amongst the sand which generally
accumulates at the bottom of the jars in which sponges are kept by
those who deal in them, and must be picked out with a camel-hair
pencil. The specimens obtained by this means will seldom if ever
require any cleaning process, as they are quite free from animal
matter, &c.

In the former chapter was noticed those insects or parts of them which
are usually mounted dry. When they are large and too opaque to admit of
the dry treatment, they must be preserved in Canada balsam or fluid.
The first of these may now be considered.

It may be here mentioned, that with these objects much heat must not be
employed, as it would in some instances give rise to a cloudiness, and
almost invariably injure them.

In killing the insect it is necessary not to rub or break any part of
it. This may be performed by placing it in a small box half filled with
fragments of fresh laurel leaves, by immersion in turpentine or strong
spirit, as also in solutions of various poisonous salts. After which
it may be preserved for sometime in turpentine or other preservative
liquid (Chapter IV.) until required. As an assistance to the student,
I believe that I can do no better than give him the plan pursued by my
friend Mr. Hepworth, whose specimens are in every way satisfactory;
but when his method is used, the insects must not have been placed in
turpentine for preservation:--

“After destroying the insects in chloroform or sulphuric ether
(methylated being cheaper), wash them thoroughly in a wide-necked
bottle, half-filled, with two or three waters; the delicate ones
requiring great care. Then immerse them in liquid potash (or Brandish’s
solution, which is stronger than the usual preparation), and let them
remain a longer or shorter time according to their texture. When ready
to remove, put one by one into a small saucer of clear water, and with
a camel-hair pencil in each hand press them flat to the bottom, holding
the head and thorax with the left-hand brush, and apply pressure with
the other from above, downwards, giving the brush a rolling motion,
which generally expels the contents of the abdomen from the thorax.
A minute roller of pith or cork might be used instead of the brush.
In larger objects, use the end of the finger to flatten them. Large
objects require more frequent washing, as it is desirable to remove the
potash thoroughly, or crystals are apt to form after mounting. Having
placed them on the slides with thin glass covers, tied down with
thread,[C] dry and immerse them in rectified spirits of turpentine;
place the vessel under the receiver of an air-pump, and keep it
exhausted until the turpentine has taken the place of the air-bubbles:
they are then ready for the application of the balsam. Larger objects
may often with advantage be transferred to a clean slide, as during the
drying there is considerable contraction, and an outline often remains
beyond the margin showing this. When closely corked they may remain in
the spirits two or three months. As you take them from the bottle, wipe
as much turpentine off as possible before removing the thread, and when
untied carefully wipe again, placing the finger on one end of the cover
whilst you wipe the other, and vice versâ. By this means you remove
as much turpentine from under the cover as is necessary; then drop
the balsam, thinned with chloroform (see Chapter I.), upon the slide,
letting the fluid touch the cover, when it will be taken in between the
surfaces by capillary attraction; and after pressing the cover down it
may be left to dry, or you may hold the slide over a spirit-lamp for
a few seconds before pressing down the cover. If heat is not applied,
they are much longer in drying but are more transparent. If made too
hot the boiling disarranges the objects, and if carried too far will
leave only the resin of the balsam, rendering it so brittle that the
cover is apt to fly off by a fall or any jar producing sufficient
percussion. Never lift the cover up, if possible, during the operation,
as there is danger of admitting air. A few bubbles may appear
immediately after mounting, but generally subside after a few hours,
being only the chloroform or turpentine in a state of vapour, which
becomes condensed.”

    [C] This applies to the more delicate ones, which will not bear
        transferring after being once spread out and dried.

This method of preparing and mounting insects I can strongly recommend
as giving first-rate results; but where the specimens are small they
seldom need the soaking in caustic potash which larger ones must have.
It is only necessary to leave them awhile in turpentine, especially
when they have been first dried with gentle pressure between two
glasses, and then mount with balsam in the ordinary way.

Amongst the insect tribes there is abundant employment, especially
for the lower powers of the microscope. But if the deeper wonders and
beauties of the animal economy are to be sought out and studied, it is
desirable that the various parts should be set separately, in order
that they may receive a more undivided attention, as well as to render
them capable of being dealt with under the higher powers. We will,
therefore, briefly consider the treatment which the different portions
require.

The eyes of the butterflies, and indeed of almost all insects, afford
materials for a study which is complete in itself. When examined with
a tolerably high power, instead of finding each eye with an unbroken
spherical surface, it is seen that many are composed of thousands of
hexagonal divisions, each being the outer surface of a separate portion
termed the _ocellus_. In others these divisions are square; but in
all there is a layer of dark pigment surrounding their lower parts.
The ocelli may be partly removed from the eye, which will show how
their tapering forms are arranged. But here we have to consider how to
place them in balsam for preservation. The eye being removed from the
insect, and the dark pigment removed by the use of a camel-hair pencil,
must be allowed to remain in turpentine at least for some days. The
turpentine should then be renewed and the eye well washed in it just
before it is to be mounted. It may then be set in balsam in the same
way as any other object;--but here a difficulty is met with. The eye
being spherical upon the surface required, must necessarily be “folded”
or broken in attempting to flatten it. This difficulty may be often
overcome by cutting a number of slits round the edges; but some object
to this mode of treatment, and where it is practicable it is much more
satisfactory to mount one in the natural rounded form and another
flat. Instead, however, of mounting the organ _whole_, four or five
slides may be procured from each of the larger ones, as those of the
dragon-fly, &c.

The _antennæ_ also are often mounted on separate slides, as being
better suited for higher powers and more minute examination than when
connected with the insect. These two projecting organs, issuing from
the head, are jointed, and moveable at will. They differ very much in
form amongst the various species, and are well worth the attention of
the microscopist. They are usually mounted with the head attached, and
perhaps they are more interesting when thus seen. Some few are very
opaque, to prepare which the following method has been advised:--

Bleach the antennæ by soaking in the following solution for a day or
two:--

  Hydrochloric acid, 10 drops.
  Chlorate of potash, ½ drachm.
  Water, 1 oz.

This will render them transparent. Wash well, dry, and mount in Canada
balsam. Instead of the above, a weak solution of chloride of lime may
be used, by which means the nerves will be well shown. Many, however,
are rendered transparent enough by simply soaking in turpentine for
a longer or shorter time. Where the antennæ, however, are “plumose,”
or feather-like, extreme care is required in mounting, though the
difficulty is not so great as some seem to think. If they are first
dried with gentle pressure, and then subjected to the action of the
air-pump in a small quantity of turpentine until the air is thoroughly
expelled, they can be easily finished upon the slide, especially when
balsam and chloroform are used.

Insects supply us with another series of beautiful objects, viz.,
the _feet_.[D] These are sometimes simply dried and mounted without
any medium, as before mentioned; but most of them are rendered much
more fit for examination by using balsam in their preservation, as it
greatly increases their transparency. The smaller kinds may be dried
with gentle pressure betwixt blotting-paper, and then immersed for
some days in turpentine, without requiring the treatment with liquor
potassæ. This immersion will render them beautifully transparent, when
they may be mounted in balsam in the usual manner.

    [D] See Mr. Hepworth’s interesting articles on the fly’s
        foot in the second and third volumes of the _Microscopic
        Journal_.

It is, however, sometimes found difficult to fix the feet when
_expanded_, in which state the interest of the object is greatly
increased. Mr. Ralph recommends the following mode:--“First wash
the feet, while the insect is yet alive, with spirits of wine; then
holding it by a pair of forceps close to the edge of a clean piece of
glass, the insect will lay hold of the upper surface by its foot, then
suddenly drop another small piece of glass over it, so as to retain
the foot expanded, and cut it off with a pair of scissors, tie up and
soak to get rid of air.” Mr. Hepworth says that he never found any
difficulty in expanding the foot on a drop of water or well-wetted
slide, and laying a thin glass cover over it, tying with thread,
drying, and immersing in turpentine.

The mouth, also, with its organs, is an interesting object in
many insects. That of the common fly is often made use of, and is
comparatively easy to prepare. By pressing the head, the tongue (as it
is commonly termed) will be forced to protrude, when it must be secured
by the same means as the foot, and may be subjected to the soaking
in turpentine, and mounted as usual. The honey-bee is, however, very
different in formation, and is well worth another slide; indeed, even
in insects of the same class, the differences are many and interesting.

Another worthy object of study is the _respiration_ of insects, which
is effected by tracheæ or hollow tubes, which generally run through the
body in one or more large trunks, branching out on every side. These
terminate at the surface in openings, which are termed _spiracles_, or
breathing organs. The _tracheæ_ often present the appearance of tubes
constructed by a twisted thread, somewhat resembling the spiral fibres
of some plants. These are very beautiful objects, and are generally
mounted in balsam, for which reason they are mentioned here; but as
they evidently belong to the “dissecting portion,” they will be fully
treated of in another place.

Amongst the parasitic insects a great variety of microscopic subjects
will be found. As these are usually small, they may be killed by
immersion in spirits of turpentine; and if at all opaque, may be
allowed to remain in the liquid until transparent enough, and then
mounted in Canada balsam.

The acarida, or _mites_ and _ticks_, are well known; none, perhaps,
better than those which are so often found upon cheese. Flour,
sugar, figs, and other eatables, are much infested by them; whilst
the diseases called the _itch_ in man, and the _mange_ in animals,
are produced by creatures belonging to this tribe. These animals
are sometimes mounted by simply steeping them in turpentine, and
proceeding as with other insects. The “Micrographic Dictionary” gives
the following directions as to mounting _parts_ of these:--“The parts
of the mouth and the legs, upon which the characters are usually
founded, may be best made out by crushing the animals upon a slide with
a thin glass cover, and washing away the exuding substance with water;
sometimes hot solution of potash is requisite, with the subsequent
addition of acetic acid, and further washing. When afterwards dried
and immersed in Canada balsam, the various parts become beautifully
distinct, and may be permanently preserved.”

Feathers of different kinds of birds are usually mounted in balsam
when required to show much of the structure. This is particularly
interesting when the feathers are small, as they then show the
inner substance, or _pith_, as it may be termed, with the cells,
&c. The “pinnæ,” or soft branches of the feathers, will be found
of various constructions; some possessing hooks along one side,
whereby they fasten themselves to their neighbours; others branching
out, with straight points somewhat resembling the hairs from certain
caterpillars. But, of course, when the metallic-looking gorgeous
colours are all that is required to be shown, and reflected light used
(as with the feathers of the hummingbird, peacock, &c.), it is much
better that they should be mounted dry, as in Chapter II.

The _seeds_ and _pollen_ of plants are most frequently mounted dry,
as mentioned in Chapter II.; but the more transparent of the former,
and the darker kinds of the latter, are perhaps better seen in Canada
balsam. There is nothing particular to be observed in the manipulation,
except that the glass cover must be applied lightly, otherwise the
grains may be crushed. There are some objects which cannot be shown
in a perfect manner when mounted _dry_, but when immersed in balsam
become so very transparent that they are almost useless. To avoid this,
it has been recommended to stain the objects any colour that may be
convenient, and afterwards mount in balsam in the ordinary manner.

Most objects intended for the polariscope may be mounted in Canada
balsam; but there are some exceptions to this. Many of the salts are
soluble in this medium, or their forms so injured by it, that glycerine
or oil has to be used (see Chapter IV.); others must be left in the dry
form, as before mentioned; and some few it is impossible to preserve
unchanged for any length of time. _Crystals_, however, are amongst the
most beautiful and interesting subjects for polarisation; and it is
very probable that, by the aid of the polariscope, new and valuable
facts are yet to be made known. For one who finds pleasure in form
and colour, there is a field here which will only open wider upon him
as he advances; and instead of being in anywise a merely mechanical
occupation, it requires deep and careful study. The little here said on
the subject will show this in some degree.

With almost every salt the method of _crystallization_ must be modified
to obtain the best forms; I may even go further than this, and say that
it is possible to change these forms to such a degree that the eye
can perceive no relationship to exist betwixt them. If a solution of
sulphate of iron is made, a small quantity spread evenly upon a slide,
and then suffered to dry whilst in a flat position, the crystals often
resemble the fronds of the common fern in shape. But if, whilst the
liquid is evaporating, it is kept in motion by stirring with a thin
glass rod, the crystals form separately, each rhombic prism having its
angles well defined, and giving beautiful colours with the polarized
light. Again, pyro-gallic acid, when allowed to flow evenly over the
slide in a saturated solution, covers the surface in long “needles,”
which are richly coloured by polarized light; but if any small portion
of dust or other matter should form a nucleus around which these
“needles” may gather, the beauty is wonderfully increased. A form very
closely resembling the “eye” of the peacock’s tail, both in form and
colour, is then produced, which to one uninitiated in crystallography
bears very little resemblance to the original crystal. From these
simple facts it will be clearly seen that in this, as in every other
department, study and experience are needful to give the best results.

To obtain anything like uniformity in the formation of crystals upon
the glass slide, every trace of grease must be removed by cleaning with
liquor potassæ or ammonia immediately before using, care also being
taken that none of the agent is left upon the slide, otherwise it may
interrupt and change their relative position, and even their form.

Amongst those which are generally esteemed, the most beautiful are the
crystals of oxalurate of ammonia. The preparation of this salt from
uric acid and ammonia is a rather difficult process, and will not on
that account be described here; but when possessed, a small quantity
of a strong solution in water must be made, and a little placed
on the slide, and evaporated slowly. Part of the salt will then be
deposited in circles with the needle-like crystals extending from
common centres. They should then be mounted in pure Canada balsam;
and, when the best colours are wanted, used with the selenite plate.
Of this class of crystal salicine is a universal favourite, and can be
easily procured of most chemists. The crystals may be produced in two
ways:--A small portion of the salt must be placed upon the slide, and a
strong heat applied underneath until fusion ensues; the matter should
then be evenly and thinly spread over the surface. In a short time
the crystals will form, and are generally larger than those procured
by the following process; but the uncertainty is increased a little
when fusion is used, which, however, is desirable with many salts.
Secondly, make a saturated solution of salicine, which in cold water
is effected by adding one part of the salt to eighteen parts of water.
Lay a little upon the slide, and allow it to evaporate spontaneously,
or with the aid of gentle heat. The crystals are generally uniform, and
with ordinary powers quite large enough to afford a beautiful object.
The circular shape and gorgeous colours of this crystal have made it so
great a favourite that there are few cabinets without it.

Many new forms may be procured by uniting two totally different salts
in solution in certain proportions. This is a field affording new
facts and beauties; but requires some chemical knowledge and much
perseverance to obtain very valuable results. One of the most beautiful
I have met with has been composed of sulphate of copper and sulphate of
magnesia. The flower-like forms and uniformity of crystallization when
successful make it well worth a few failures at first; and as I became
acquainted with some new facts in my frequent trials, I will give the
preparation of the double salt from the beginning.

Make a saturated solution of the two sulphates, combined in the
proportion of three parts copper to one part magnesia, and then add
to the solution one-tenth of pure water. Dust or other impurities
should be guarded against, and the slide made free from all trace
of grease by cleaning immediately before use with liquor potassæ or
ammonia. A drop of the solution should then be placed upon the slide,
and by a thin glass rod spread evenly upon the surface. Heat this
whilst in a horizontal position until the salt remains as a viscous
transparent substance, which will not be effected until it is raised
to a high degree. The slide may now be allowed to cool, and when this
is accomplished, the flower-like crystals will be perceived forming
here and there upon the plate. When these are at any stage in which it
is wished to preserve them, a few seconds’ exposure to the fire, as
warm as the hand can comfortably bear, will stop the expansion, when
the portion which we wish to mount should be cut off from the mass of
salt by simply scratching the film around, and pure Canada balsam with
the thin glass used. Breathing upon the film, or allowing the slide to
become cold and attract the moisture from the atmosphere, will cause
the crystallization to extend, and sometimes greatly rob the effect; so
it is necessary to mount quickly when the desired forms are obtained.
As the crystals are very uncertain as to the place of their formation,
I may here mention that they may be got in _any_ part of the slide
by piercing the film with a needle-point; but in some degree this
necessarily interferes with the centre. As the cause of this has no
need to be entered into here, and has been elsewhere discussed, I shall
only give the above directions, and say that there is a great field in
this branch of study which the microscope alone has opened.

It would be useless to enter into particulars respecting the various
salts and treatment they require, as a great difference is effected
even by the strength of the solution. There are some crystals, also,
which are called forth in insulated portions, showing no formation
upon the ground; but even when mounted in any preserving fluid, and
unchanged for a year, a new action seems to arise, and a groundwork
is produced which bears little resemblance to the original crystal.
Sometimes this new formation adds to the beauty of the slide; in other
cases the reverse is the result, the slide being rendered almost
worthless. This action, I believe, frequently arises from some liquid
being contained in the balsam or other mounting medium used; and this
is rendered the more probable by the crystallization being called
forth in an hour after the balsam diluted with chloroform is employed,
whereas no change would have taken place for months (if at all) had
pure balsam been used.

Sections of some of the salts are very interesting objects, but the
method of procuring these and their nature will be described in Chapter
V.

The scales of various fish have been before mentioned as mounted “dry;”
when, however, they are required for polarising objects they are
generally mounted in balsam, and some few in liquid. The former method
will be considered here.

The eel affords a beautiful object for this purpose. The scales are
covered by a thin “_skin_,” which may be slightly raised with a knife
and then torn off, in the same manner as the covering of the geranium
and other petals, described in Chapter II. The required portion may
then be removed; or if a piece of skin can be procured as stripped off
in cooking, the scales may be easily taken from the inner surface. They
must then be washed and thoroughly cleaned. After drying, soak for a
day in turpentine, and mount in the ordinary manner with balsam. This
is a good polarising object; but the interest, and I think the beauty,
is increased by procuring a piece of eel’s skin with the scales _in
sitû_, washing and drying under pressure, and mounting in balsam as
before. The arrangement of the scales produces beautiful “waves” of
colour, which are quite soothing to the eye after examining some of the
very gorgeous salts, &c.

There are many scales of fish which are good subjects for the
polariscope when mounted in balsam; but as they require no particular
treatment, they need no mention by name.

Among hairs we find some which are beautiful when mounted in balsam
and examined by polarized light. Some, when wanted as common objects,
are always used dry, as before mentioned; but if they are intended to
be shown as _polarizing_ objects, they must be placed in some medium.
The “Micrographic Dictionary” mentions a mode of making an interesting
object by plaiting two series of white horse-hairs at an angle,
mounting in balsam, and using with the polariscope. All hairs, however,
must be steeped in turpentine for a short time before mounting, as they
will thus be rendered cleaner and more transparent. When this is done,
there is no difficulty in mounting them.

Many of the “tongues” of fresh-water and marine mollusca are deeply
interesting and most beautiful objects when examined by polarized
light. As these are usually mounted in balsam, I mention them in this
place; but as they must be removed from the animals by dissection,
particulars respecting them will not be entered into until we come to
the part in which that operation is described (Chapter V.).

The manner of preparing and mounting many of the Polyzoa and Zoophytes
has been before described; but any notice of _polarizing_ objects would
be incomplete without some allusion to them. A small piece of the
_Flustra avicularis_, well prepared, is beautiful when examined in this
manner. No selenite is needed, and yet the colours are truly gorgeous.
It is often met with upon shells and zoophytes of a large size, and
will well repay the trouble of searching for. Many of the Sertularidæ
are very beautiful with polarized light, and, indeed, no ramble upon
the seaside need be fruitless in this direction.

The different _starches_ are quite a study in themselves, and
are peculiarly connected with polarized light. They are found in
the cellular tissue of almost every plant in small white grains
which vary considerably in size; that from the potato averages
one-three-hundredth of an inch in diameter, and that from arrow-root
about one-six-hundredth. To procure starch from any plant, the texture
must first be broken up or ground coarsely; the mass of matter must be
then well washed in gently-flowing water, and, as all starch is totally
insoluble in cold water, the grains are carried off by the current
and deposited where this is stayed. In procuring it from the potato,
as well as many other vegetables, it is but necessary to reduce the
substance to a coarse pulp by the aid of a culinary “grater;” the pulp
should then be well agitated in water, and allowed to rest a short
time, when the starch will be found at the bottom, its lighter colour
rendering it easily distinguishable from the pulp. It should, however,
be washed through two or three waters to render it perfectly clean.

These grains have no crystalline structure, but present a very peculiar
appearance when examined with polarized light. Each grain shows a dark
cross whose lines meet at the point where it was attached to the plant,
called the _hilum_. Round the grain, also, a series of lines are seen,
as though it were put together in plates. This is more distinctly
visible in some kinds than others.

As to the mounting of these starches there is little to be said. If the
grains are laid upon the slide, and as small a portion as possible of
the balsam diluted with turpentine, as before mentioned, be applied,
they will cling to the glass and allow the pure balsam to flow readily
over them without being so liable to imprison air-bubbles when the thin
glass is put upon them.

The raphides, which were fully described in Chapter II., when required
for use with polarized light, must be mounted in balsam, and many are
found which give beautiful colours. They require no peculiar treatment,
but must be washed quite clean before putting up.

There is one class of objects for the polariscope which differs in
preparation from any we have yet considered, and affords very beautiful
specimens. Some of the plants, including many of the grasses and
the Equisetaceæ (_i. e._ horsetails), contain so large a quantity
of silica, that when the vegetable and other perishable parts are
removed, a skeleton of wonderful perfection remains. This skeleton
must be mounted in balsam, the method of performing which will now be
considered.

Sometimes the cuticle of the equisetum is removed from the plant,
others dry the stem under pressure, whilst the grasses, of course,
require no preparation. The vegetable should be immersed in strong
nitric acid and boiled for a short time; an effervescence will go on
as the alkalies are being removed, and when this has ceased more acid
should be added. At this point the modes of treatment differ; some
remove the object from the acid and wash, and having dried, burn it
upon thin glass until all appears _white_, when it must be carefully
mounted in balsam. I think, however, it is better to leave it in strong
acid until all the substance, except the required portion, is removed;
but this will take a length of time, varying according to the mass,
&c., of the plant. Of course, when this latter method is used, the
skeleton must be washed from the acid, &c., before being mounted in
balsam.

These _siliceous cuticles_ are readily found. The _straws_ of most of
the cereals, wheat, oat, &c.; the _husks_, also, of some of these; many
_canes_; the equisetum, as before described; and some of the grasses.
Many of these are everywhere procurable, so that the student can never
want material for a splendid object for the polariscope.

In Chapter II. the _scales_ (or hairs) which are often found upon the
leaves of plants were mentioned as beautiful objects when mounted dry;
but some of these when detached from the leaf--which is easily done
by gently scraping it, when dried, with a knife--present brilliant
starlike and other forms, if mounted in balsam and used with the
polariscope. There is a little danger, when placing the thin glass
upon the balsam, of forcing out the scales in the wave of matter which
is always ejected; this may be overcome by applying to the slide,
previously to placing the objects upon it, an extremely thin covering
of the balsam diluted with turpentine as before mentioned, and thus
giving them every chance of adherence; or by using the balsam with
chloroform, as before noticed. These scales are much more abundant than
was formerly supposed, and daily new specimens are discovered; so that
the student should always be on the lookout for them in his researches
in the vegetable world.

Most classes of objects, and the treatment they require when mounting
them in balsam, have now been considered. The next chapter will be
devoted to the preservative liquids, and the best methods of using
them.




CHAPTER IV.

PRESERVATIVE LIQUIDS, ETC., PARTICULARLY WHERE CELLS ARE USED.


There are many objects which would lose all their distinctive
peculiarities if allowed to become dry, especially those belonging
to the fresh-water Algæ, many animal tissues, and most of the very
delicate animal and vegetable substances in which structure is to be
shown. These must be preserved by immersion in some fluid; but it is
evident that the fluid must be suited to the kind of matter which it
is intended to preserve. As it often requires much study and trouble
to _obtain_ microscopic objects of this class, it is well that their
_preservation_ should be rendered as perfect as possible; and for this
reason the CELLS, or receptacles of the fluids, should be so closed
that all possibility of escape should be prevented. The accomplishment
of this is not so easy a matter as it might appear to the inexperienced.

Before giving any directions as to the manipulation required in
mounting the objects, we must consider the different _liquids_ and
_cells_ which are requisite for their preservation. Of the former there
are a great number, of which the principal may be mentioned.

DISTILLED WATER is strongly recommended by many for Diatomaceæ and
other Protophytes. It has been, however, stated that confervoid growths
often disturb the clearness of the liquid, and on this account various
additions are made to it. A lump of camphor is often left in the
bottle, so that the water may dissolve as much as possible. One grain
of bay-salt and one of alum are added to each ounce of water; or a drop
or two of creosote shaken up with the ounce of water, which should be
afterwards filtered. These additions are often made; perhaps each of
them good for certain objects.

GLYCERINE.--Some affirm this to be one of the best preservative
liquids, especially for vegetable objects; but others think that it is
much better when diluted with two parts of camphor-water, prepared as
above.[E]

    [E] Dr. Carpenter says:--“Glycerine has a solvent power for
        carbonate of lime, and should not be employed when the
        object contains any calcareous structure. In ignorance
        of this fact, the author (Dr. C.) employed glycerine to
        preserve a number of remarkably fine specimens of the
        pentacrinoid larva of the Comatula, whose colours he was
        anxious to retain; and was extremely vexed to find, when
        about to mount them, that their calcareous skeletons had so
        entirely disappeared, that the specimens were completely
        ruined.”

GLYCERINE AND GUM.--This is also believed to be a very good liquid for
vegetable tissues, and is thus prepared:--

  Pure gum-arabic       1 oz.
  Glycerine             1  ”
  Water (distilled)     1  ”
  Arsenious acid        1½ grains.

Dissolve the arsenious acid in the cold water, then the gum, add the
glycerine, and mix without bubbles.

DEANE’S COMPOUND.--This is usually deemed about the best medium for
preserving Algæ, mosses, &c., and is thus prepared:--Soak 1 oz. of best
gelatine in 4 oz. of water until the gelatine becomes soft, when 5 oz.
of honey heated to boiling-point are added; boil the mixture, and when
it has cooled, but not enough to become stiff, add ½ oz. rectified
spirit with which 5 or 6 drops of creosote have been well mixed, and
filter the whole through fine flannel. This compound when cold forms a
stiff jelly, the use of which will be described elsewhere.

GLYCERINE JELLY.--This mixture closely resembles the above, but as the
composition differs a little it may be mentioned here. It is strongly
recommended by Mr. Lawrance in the _Microscopic Journal_, where he
states “that the beautiful green of some mosses mounted two years
ago, is still as fresh as the day they were gathered;” and that this
is the only medium he knows which will preserve the natural colour of
vegetable substances. He takes a quantity of Nelson’s gelatine, soaks
it for two or three hours in cold water, pours off the superfluous
water, and heats the soaked gelatine until melted. To each fluid ounce
of the gelatine, _whilst it is fluid but cool_, he adds a fluid drachm
of the white of an egg. He then boils this until the albumen coagulates
and the gelatine is quite clear, when it is to be filtered through fine
flannel, and to each ounce of the clarified solution add 6 drachms of a
mixture composed one part of glycerine to two parts of camphor-water.

GOADBY’S FLUID.--This is much used in the preservation of animal
objects; and seldom, if ever, acts upon the colours. It is thus
prepared--Bay salt, 4 oz.; alum, 2 oz.; corrosive-sublimate, 4 grains.
Dissolve these in two quarts of boiling water and filter. For delicate
preparations some recommend that this mixture be reduced by the
addition of an equal quantity of water; but where there is bone or
shell in the object the above acts injuriously upon it, in which case
this fluid may be used:--Bay-salt 8 oz., corrosive sublimate 2 grains,
water 1 quart.

THWAITES’ LIQUID.--This is recommended for the preservation of
Algæ, &c., as having little or no action on the colour, and is thus
prepared:--Take one part of rectified spirit, add drops of creosote
enough to saturate it; to this add sixteen parts of distilled water and
a little prepared chalk, and filter. When filtered, mix with an equal
quantity of camphor-water (as before mentioned), and strain through
fine muslin before using.

CHLORIDE OF ZINC SOLUTION.--In the “Micrographic Dictionary” this is
stated to be “perhaps the best preservative known for animal tissues.”
Persons of great experience, however, have given a very different
opinion; but it is certainly very useful in many cases where a small
degree of coagulating action is not injurious. It is used of strengths
varying according to the softness of the parts to be preserved; the
average being 20 grains of the fused chloride to 1 oz. of distilled
water. To keep this liquid a lump of camphor may be left floating in
the bottle. I have heard complaints that this mixture becomes turbid
with keeping, but I think this must only be the case when some impurity
has got into the bottle.

CARBOLIC ACID.--This substance has not been known long enough to
warrant any decided opinion upon its merits. A solution of 1 part of
acid to 20 of water has been recommended on account of its antiseptic
properties.

CASTOR OIL.--This is a very useful preservative for crystals and
other objects. Many salts are quite destroyed when Canada balsam is
used with them; but very few are acted upon by this oil. To use it,
it must be dropped in a sufficient quantity to cover the crystal or
object to be preserved with a thin coating of oil. It may be necessary
sometimes to spread it with a needle or other instrument. The thin
glass should then be carefully placed upon it, so that all air may
be excluded; and should any oil be forced out, owing to the quantity
used being too great, it must be removed with blotting paper. When
the edge of the thin glass cover and the surrounding parts of the
slide are as clean as possible, a coating of sealing-wax varnish or
liquid glue must be applied and allowed to dry. A second or even a
third coating may be required, but not before the previous cover is
quite dry. These varnishes, however, are very brittle, and it is much
safer, as a finish, to use one of the tougher cements--gold-size, for
instance--which will render it doubly secure.

The above are the principal liquids, &c., used for preserving objects
in cells. The different cells may be here mentioned; and it is
recommended that these should always be kept some time before use in
order that the cement may become perfectly dry; and care must be taken
that no cement be used on which the preservative liquid employed has
any action whatever.

CEMENT CELLS.--Where the object is not very thick, this kind of cell
is generally used. They are easily made with the turntable before
described; but when the objects to be preserved are _very_ minute,
these cells need not be much deeper than the _ordinary_ circle of
cement on the slide. When, however, a comparatively great depth is
required, it is sometimes necessary to make the wall of the cell as
deep as possible, then allow it to dry and make another addition. Of
these cements gold-size is one of the most trustworthy, and may be
readily used for the shallow cells. The asphaltum and india-rubber,
before noticed, I have found very durable when well baked, and
exceedingly pleasant to work with. It may be used of such a thickness
as to give space for tolerably large objects. Black japan also is much
used. Many cements, however, which are recommended by some writers,
are worse than useless, owing to the brittleness which renders their
durability uncertain, as sealing-wax varnish, liquid-glue, &c.

The student may feel himself at a loss in choosing the cement which
will give him the _safest_ cells, many of them becoming partially
or wholly dry in a year or two, as stated in another place. I can
only give him a few general directions, and he must then use his own
judgment. Of course it would be lost labour to employ any cement upon
which the preservative liquid has _any action whatever_. It is also a
good rule to avoid those in whose composition there are any particles
which do not become a thorough and intimate portion, as these unreduced
fragments will almost certainly, sooner or later, prepare a road by
which the liquid will escape; and, lastly, whatever cement he uses, the
cells are always better when they have been kept a short time before
use, as already stated.

GUTTA-PERCHA RINGS have been recommended by some, as affording every
facility for the manufacture of cells for liquids; but they cannot be
recommended, as, after a certain length of time, they become so brittle
as to afford no safeguard against ordinary accidents.

Often the cells must necessarily be of a large size, and for this
reason are made by taking four strips of glass of the thickness and
depth required, and grinding the places where these are to meet with
emery, so as to form a slightly roughened but flat edge. The glass
strip must also be ground on the side where it meets the plate, and
each piece cemented with the marine glue mentioned in Chapter I. in the
following manner:--On that part of the glass to which another piece
is to be attached should be laid thin strips of the glue; both pieces
must then be heated upon a small brass table, with the aid of the
spirit-lamp, until the strips become melted; the small piece is then
to be taken up and placed upon the spot to which it is to be attached,
and so on until the cell is completed. It will be found necessary to
spread the glue over the surface required with a needle or some other
instrument, so that an unbroken line may be presented to the wall of
the cell, and no bubbles formed. Too great a heat will “burn” the
marine-glue, and render it brittle; care must be therefore taken to
avoid this.

When shallow cells are required, those which are made by grinding
a concavity in the middle of an ordinary slide will be found very
convenient. The concavities are cut both circular and oblong; and the
surface being flat, the cover is easily fastened upon it. These are now
cheap, and are very safe as to leakage.

Circular cells with a flat bottom used to be made by drilling a hole
through glass of the required thickness, and fixing this upon an
ordinary slide with marine-glue; but the danger of breakage and the
labour were so great that this method is seldom used now, and, indeed,
the rings about to be mentioned do away with all necessity of it.

GLASS RINGS.--Where any depth is required, no method of making a cell
for liquids is so convenient as the use of glass rings, which are now
easily and cheaply procurable. They are made of almost every size and
depth, and, except in very extraordinary cases, the necessity for
building cells is completely done away with. These rings have both
edges left roughened, and consequently adhere very well to the slide,
this adherence being generally accomplished by the aid of marine glue,
as before noticed with the glass cells. Gold-size has been occasionally
used for this purpose; and the adherence, even with liquid in the
cell, I have always found to be perfect. This method has the advantage
of requiring no heat, but the gold-size must be perfectly dry, and
the ring must have been fixed upon the slide some time before use.
Canada balsam has also been used for the same purpose, but cannot be
recommended, as when it is perfectly dry it becomes so brittle as to
bear no shock to which the slide may be ordinarily exposed.

These are the cells which are mostly used in this branch of microscopic
mounting. The mode of using them, and the different treatment
which certain objects require when intended to be preserved in the
before-mentioned liquids, may now be inquired into.

I may mention, however, that this class of objects is looked upon by
many with great mistrust, owing to the danger there is of bubbles
arising in the cells after the mounting has been completed, even for
years. I know some excellent microscopists who exclude all objects
in cells and preservative liquids from their cabinets, because they
say that eventually almost all become dry and worthless; and this is
no matter of surprise, for many of them do really become so. Perhaps
this is owing to the slides being sold before they could possibly be
thoroughly dry. As to the air-bubbles, I shall have something to say
presently.

We will now suppose the cell employed is made by placing a glass
ring upon the slide with marine glue or gold-size, and is quite dry.
Around the edge of the cleaned thin glass which is to cover it, I
trace with a camel-hair pencil a ring of gold-size, and also around
the edge of the cell to which it is to adhere. Dr. Carpenter objects
to this, as rendering the later applications of the gold-size liable
to “run in.” All danger of this, however, is completely done away with
by leaving the slide and cover for awhile until the cement becomes
partially “fixed,” but still adhesive enough to perform its function
(Chapter II.). With many slides this is not accomplished in less than
twenty-four hours, even if left two or three days no injury whatever
ensues; but with other kinds an hour is too long to leave the exposed
cement, so that the operator must use his own discretion. The liquid
required may be drawn up by the mouth into the pointed tube mentioned
in Chapter I., and then transferred to the cell. In the various books
of instruction, the object is now to be placed in the cell; this,
however, I think a great mistake, as another process is absolutely
necessary before we advance so far. The cell, full of liquid, must
be placed under the receiver of an air-pump, and the air withdrawn.
Almost immediately it will be perceived that the bottom and sides of
the cell are covered with minute bubbles, which are formed by the air
that is held in suspension by the liquid. The slide may now be removed,
and the bubbles may require the aid of a needle or other point to
displace them, so obstinately do they adhere to the surface of the
glass. This process may be then repeated, and one cause, at least, of
the appearance of bubbles in cells of liquid will be got rid of. The
object to be mounted should also be soaked in one or two changes of the
preservative liquid employed, and, during the soaking, be placed under
the air-pump and exhausted. It may then be transferred to the cell,
which will probably cause the liquid to overflow a little. The cover
with the gold size applied to the edge must then be carefully laid upon
the cell, and slightly pressed down, so that all air-bubbles may be
displaced. The two portions of gold-size will now be found to adhere
wherever the liquid does not remain, although the whole ring may have
been previously wet. The outer edge of the thin glass and cell must
now be perfectly dried, and a coating of gold-size applied. When this
is dry, the process must be repeated until the cement has body enough
to protect the cell from all danger of leakage. When some preservative
liquids are used, a scum is frequently found upon the surface when
placed in the cell, which must be removed immediately before the cover
is laid upon it.

I believe this method to be perfectly secure against leakage when
carefully performed; and some of my friends have told me that their
experience (some years) has been equally satisfactory.

In using some of the particular kinds of preservative liquids, it will
be found necessary to make slight change in the manipulation. This will
be best explained by mentioning a few objects, and the treatment they
require.

For the preservation of the Mosses, Algæ, &c., Deane’s compound is
much used, and considered one of the best media. The specimen to be
mounted should be immersed in the compound, which must be kept fluid
by the vessel containing it being placed in hot water. In this state
the whole should be submitted to the action of the air-pump, as it is
not an easy matter to get rid of the bubbles which form in and around
the objects. The cell and slide must be warmed; and heat will also
be necessary to render the gelatine, &c., fluid enough to flow from
the stock-bottle. The cell may then be filled with the compound, and
the specimen immersed in it. A thin glass cover must then be warmed,
or gently breathed upon, and gradually lowered upon the cell, taking
care, as with all liquids, that no bubbles are formed by the operation.
The cover may be fixed by the aid of gold-size, Japan, or any of the
usual varnishes, care being taken, as before, that all the compound is
removed from the parts to which the varnish is intended to adhere.

The glycerine jelly of Mr. Lawrance, before mentioned, requires almost
a similar treatment. “The objects to be mounted in this medium should
be immersed for some time in a mixture of equal parts of glycerine
and dilute alcohol (six of water to one of alcohol). The bottle of
glycerine jelly must be placed in a cup of hot water until liquefied,
when it must be used like Canada balsam, except that it requires less
heat. A ring of asphaltum varnish round the thin glass cover completes
the mounting.”

The Infusoria (see Chapter III.) are sometimes preserved in liquid;
but present many difficulties to the student. Different kinds require
different treatment, and consequently it is well, when practicable, to
mount similar objects in two or more liquids. Some are best preserved
in a strong solution of chloride of calcium, others in Thwaites’
liquid, whilst a few keep their colour most perfectly when in glycerine
alone. Many of them, however, are so very transparent that they present
but faint objects for ordinary observance. The Desmidiaceæ require
somewhat similar treatment, and may be mentioned here. The solution of
chloride of calcium has been strongly recommended; but no preservative
liquid seems to be without some action upon them. Both of the above
classes of objects should be mounted in shallow cells, so as to allow
as high a microscopic power as possible to be used with them.

Many of the ZOOPHYTES which are obtained on our sea-coasts are well
preserved by mounting in cells, in the manner before mentioned, with
Goadby’s fluid, or distilled water with one of the additions noticed
amongst the preservative liquids. For examination by polarized light,
however, they are usually mounted in balsam (see Chapter III.), whilst
those in cells present a more natural appearance as to position, &c.,
for common study.

As to the use of preservative liquids with the Diatomaceæ there are
various opinions. Some experienced microscopists say that there is
little or no satisfaction in mounting them in this way. Dr. Carpenter,
however, explains this difference by his instructions as to what method
should be used when certain ends are desired. He says: “If they can
be obtained quite fresh, and it be desired that they should exhibit
as closely as possible the appearance presented by the living plants,
they should be put up in distilled water within cement-cells; but if
they are not thus mounted within a short time after they have been
gathered, about a sixth part of alcohol should be added to the water.
If it be desired to exhibit the stipitate forms in their natural
parasitism upon other aquatic plants, the entire mass may be mounted
in Deane’s gelatine in a deeper cell; and such a preparation is a very
beautiful object for the black-ground illumination. If, on the other
hand, the minute structure of the siliceous envelopes is the feature
to be brought into view, the fresh diatoms must be boiled in nitric or
hydrochloric acid” (which process is fully described in Chapter II.).
It is very convenient to have many of these objects mounted by two
or more of the above methods; and if they are to be studied, this is
indispensable. Mr. Hepworth once showed me about one hundred slides
which he had mounted in various ways, for no other purpose than the
study of the fly’s foot.

My friend, Mr. Rylands, successfully mounts the diatoms in the state
in which he finds them, and gave me the following method as that which
he always employs. He says that he has had no failures, and hitherto
has found his specimens unchanged. Take a shallow ring cell of asphalt
or black varnish (which must be at least three weeks old), and on the
cell, whilst revolving, add a ring of benzole and gold-size mixed in
equal proportions. In a minute or two pure distilled water is put in
the cell until the surface is slightly convex. The object having been
already floated on to the cover (the vessel used for this purpose being
an ordinary indian-ink palette), is now inverted and laid carefully
upon the water in the cell. By these means the object may be laid down
without being removed. The superfluous moisture must not be ejected by
pressure, but a wetted camel-hair pencil, the size made in an ordinary
quill, being partially dried by drawing through the lips, must be
used repeatedly to absorb it, which the pencil will draw by capillary
attraction as it is very slowly turned round. When the cover comes in
contact with the benzole and gold-size ring, there is no longer any
fear of the object being removed, and a slight pressure with the end
of the cedar stick of the pencil will render the adhesion complete,
and cement the cover closely and firmly to the cell. When dry, an outer
ring of asphalt makes the mounting neat and complete.

The _Fungi_ have been before mentioned; but it may be here stated that
some few of the minute forms are best preserved in a very shallow cell
of liquid. For this purpose creosote-water may be advantageously used.

The _antennæ_ of insects have been before noticed as being very
beautiful when mounted in balsam. This is readily accomplished when
they are large; but those of the most minute insects are much more
difficult to deal with, and are less liable to injury when put up in
fluid. _Goadby’s Fluid_ serves this purpose very well; but, of course,
the object must be thoroughly steeped in the liquid before it is
mounted, for a longer or shorter time according to the thickness.

The _eggs of insects_ afford some worthy objects for the microscope,
amongst which may be mentioned those of the common cabbage butterflies
(small and great), the meadow-brown, the puss-moth, the tortoiseshell
butterfly, the bug, the cow-dung fly, &c. These, however, shrivel up on
becoming dry, and must, therefore, be preserved in some of the fluids
before mentioned. To accomplish this no particular directions are
required; but the soaking in the liquid about to be employed, &c., must
be attended to as with other objects.

_Glycerine_ may be advantageously used for the preservation of various
insects. These should first be cleaned with alcohol to get rid of all
extraneous matter, and then, after soaking in glycerine, be mounted
with it like other objects. This liquid may also be used for the
Entomostraca, which offer a wide field for study. They are to be
found abundantly in ponds of stagnant water, &c. Some insects, such
as May-flies, &c., are, however, often preserved by immersion in a
solution of one part of chloride of calcium in three or four parts of
water; but this has not been recommended amongst the “preservative
liquids,” as the colour, which is often an attractive quality of this
class of objects, is thereby destroyed.

We have now noticed the treatment which must be applied to those
objects which are to be preserved in liquids and cells. We may here
state that all slides of this kind should be examined at short
intervals, as they will be found now and then to require another
coating of varnish round the edge of the thin glass cover to prevent
all danger of leakage. The use of the air-pump, in the first instance
(as before recommended), and this precaution as to the varnish, will
render the slides less liable to leakage and air-bubbles, which so very
frequently render them almost worthless.




CHAPTER V.

SECTIONS AND HOW TO CUT THEM, WITH SOME REMARKS ON DISSECTION.


Many objects are almost worthless to the microscopist until the
extraneous matter is removed from them; and this is frequently
difficult in the extreme to perform satisfactorily. As an instance,
certain Foraminifera may be mentioned in which the cells are placed
one upon another, consequently the object must be reduced to a certain
degree of _thinness_ before a single uniform layer of these cells can
be obtained to show something of the internal arrangements.

Most animal and vegetable forms require an examination of the separate
parts before much can be known about them. The mass must be divided
into separate portions, each part intended to be preserved being
cleaned from the useless matter with which it is surrounded. It will
frequently be found necessary to make thin sections, which from a very
tender substance is no easy matter; and much patience will be necessary
to attain anything like proficiency.

This making of sections was not until very recently entered into by
many except those belonging to the medical profession, but I do not see
why this should be so, as much may be accomplished by a persevering
and interested mind where there is time for entering into the subject.
I will therefore make an attempt to give some instructions on this
subject also. We will first consider the cutting of sections from hard
substances, in which the ordinary knife, chisel, &c., are of no avail.
Most of these require no particular care in mounting, but are placed in
balsam like the other objects noticed in Chapter III.: where, however,
any special treatment is necessary it will be commented upon as we
proceed.

SHELLS, &C.--It is seldom, if ever, necessary to possess apparatus
for this process except a small thin saw made with a steel blade, for
which a piece of watch-spring serves very well; a fine stone such as
is used for sharpening pen-knives; and two smooth leather strops,
one of which is to be used with putty-powder to polish the section
after grinding, and the other dry, to give the final surface. It is,
however, very convenient to have three or four files of different
degrees of fineness. The shell, if very thick, may be divided by
using the watch-spring saw; and this section may then with ordinary
care be rubbed down with water on the stone until one side of it is
perfectly flat. When this is accomplished it must be again rubbed with
putty-powder upon the strop, and finally upon the other strop without
the powder. This surface will then be finished and must be firmly
united to the slide in the position it is intended to occupy. To do
this a small quantity of Canada balsam may be dropped upon the middle
of the slide and heated over the lamp until on cooling it becomes hard;
but this must be stopped before it is rendered brittle. Upon this the
polished surface must be laid, and sufficient heat applied to allow
the object to fall closely upon the slide, when slight pressure may be
used to force aside all bubbles, &c. On cooling, the adherence will be
complete enough to allow the same grinding and polishing upon the upper
surface which the lower received. Whilst undergoing this, the section
must be examined from time to time to ascertain whether the necessary
degree of thinness has been reached. When this is the case the section
should be washed thoroughly and dried. It must then be covered, which
is best done by using the ordinary Canada balsam, as recommended in
Chapter III.

Sections of some exquisitely beautiful objects are cut with much less
trouble than the above. The Orbitolite, for instance, may be prepared
in this manner. Take the object and by pressure with the finger rub
the side upon a flat and smooth sharpening stone with water until the
portion is reached which it is wished to show. The strength of the
object will easily allow this to be accomplished with ordinary care.
This side may then be attached to the glass slide with heated balsam,
as above described, and the object may then be gently rubbed down to
the degree of thinness required to show it to the best advantage.
After removing all disengaged matter from the object by washing and
thoroughly drying, it may be mounted in balsam in the usual manner,
when it is equally beautiful as a transparent or opaque object. From
this it will be seen that in many instances where a smooth stone is
found sufficient for the work (which is often the case when the section
is mounted in balsam) the final process of polishing advised above
may be dispensed with, as in the Orbitolite, Nummulite, &c., &c. It
is quite necessary that the stones on which the objects are rubbed
be _perfectly flat_, otherwise one side must be acted upon before
the other, and it will be found impossible to attain anything like
uniformity. Where it is not practicable to cut a section and the object
is very thick, a coarse stone may be first used to reduce it and the
smoother afterwards.

The consideration of the cutting of sections from shells would scarcely
be deemed complete without some mention of what Dr. Carpenter terms the
decalcifying process. Muriatic acid is diluted with twenty times its
volume of water, and in this the shell is immersed. After a period,
differing according to the thickness of the shell, the carbonate of
lime will be dissolved away, and a peculiar membrane left, showing
the structure of the shell very perfectly. This may be mounted dry,
in balsam, or sometimes in liquid, according to the appearance of
the object; but no rule can be given. The discretion of the student,
however, will enable him to choose the most suitable method.

From some shells it is easy to divide thin plates, or “laminæ,” which
require nothing but mounting in Canada balsam to show the texture very
well. In working, however, with those which are “pearly,” it will
be found that experience and patience are needed, as they are very
brittle and peculiarly hard; but a little practice will overcome these
difficulties.

Amongst the Echinodermata, which include the star-fishes,
sea-hedgehogs, &c., there are many whose outer surface is covered with
“spines,” or thin projections. Some of these are sharp and thorn-like,
others blunt, longer or shorter, and, indeed, of endless variety. In
many of these, when a section is made, rings are seen which have a
common centre, with radiating supports, resembling sections of some of
the woods. These are very beautiful objects, and methods of procuring
them may now be considered. It is the best to cut as thin a section
as can safely be got with the watch-spring saw first, when the smooth
“sharpening stone” may be used to polish one side, which is easily
accomplished with water only. When this is effected, it must be washed
clean, and _thoroughly_ dried, and then may be united to the slide in
the same manner as before recommended for the Orbitolite, &c. If it is
ever necessary to displace it on account of inequalities, bubbles, or
other remediable fault, this may be done by warming the slide; though
too much heat must be avoided, otherwise fresh bubbles will certainly
be produced. The covering with thin glass, balsam, &c., will present no
difficulty to the student; but he must remember that the transparency
is somewhat increased by this last operation.

_Corals_ are often treated in this way, in order to reveal their
structure. Except, however, the student has had much practice, he will
often find this a most difficult task, as many of them are exceedingly
brittle. He will find the method before described equally applicable
here, and should take both horizontal and vertical sections.

COAL.--This substance is one of the most interesting objects to the
microscopist. It is, of course, of a vegetable origin; and though this
is in many cases of such minute separate portions as to have lost
all appearance of vegetation, yet it is very frequently met with
in masses, bearing the form, even to the minute markings, of wood,
in various directions. To see this and prepare it for microscopic
research, a suitable piece of coal must be obtained; but in every case
the cutting and preparation of these sections require great care and
skill. Sometimes the coal is first made smooth on one side, fastened to
the glass, reduced to the requisite degree of thinness, and finished
in the method before described. This mode of treating it is sometimes,
however, very tantalizing, as, at the last moment, when the section
is about thin enough, it often breaks up, and so renders the trouble
bestowed upon it fruitless. The dark colour and opacity of coal render
an extraordinary thinness necessary, and so increase the liability to
this accident.

Perhaps the best method which can be pursued is that recommended in the
“Micrographic Dictionary,” which is as follows:--“The coal is macerated
for about a week in a solution of carbonate of potash; at the end of
that time it is possible to cut tolerably thin slices with a razor.
These slices are then placed in a watch-glass with strong nitric acid,
covered and gently heated; they soon turn brownish, then yellow, when
the process must be arrested by dropping the whole into a saucer of
cold water, else the coal would be dissolved. The slices thus treated
appear of a darkish amber colour, very transparent, and exhibit the
structure, when existing, most clearly. We have obtained longitudinal
and transverse sections of coniferous wood from various coals in this
way. The specimens are best preserved in _glycerine_ in cells; we find
that spirit renders them opaque, and even Canada balsam has the same
defect. Schulz states that he has brought out the cellulose reaction
with iodine, in coal treated with nitric acid and chlorate of potash.”

_Cannel-coal_ is so close and firm in its structure as to be much used
instead of jet in the manufacture of ornaments: it takes a beautiful
polish, and consequently presents the student with none but ordinary
difficulties in getting sections of it. Its formation is somewhat
different from that of coal, sometimes showing the transition very
clearly.

In _flint_ there are often found remains of sponges, shells,
Diatomaceæ, &c.; but to show these well, sections must be cut and
polished by the lathe and wheel of the lapidary, which the microscopic
student seldom possesses. Thin chippings may, however, be made, which
when steeped in turpentine and mounted in balsam, will frequently show
these remains very well.

_Teeth_ are very interesting objects to all microscopists, more
especially to those who give much study to them; as the class of animal
may very frequently be known from one solitary remaining tooth. To
examine them thoroughly, it is necessary to cut sections of them; but
this is rather difficult to perform well, and needs some experience.
Some instructions, however, will at least lessen these difficulties,
and we will now endeavour to give them.

It is generally thought that Canada balsam injures the finer markings
of these sections, consequently, they are almost invariably mounted
_dry_. A thin piece is first cut from the tooth with the saw of
watch-spring before mentioned, if possible; but should the substance be
too hard for this, the wheel and lathe must be used with diamond dust.
If this cannot be procured, there is no alternative but to rub down the
whole substance as thin as practicable on some coarse stone or file.
The surface will then be rough; but this may be much reduced by rubbing
upon a flat sharpening stone with the finger, or a small piece of
gutta-percha, upon the object to keep it in contact. The scratches may
be much lessened by this, but not so thoroughly removed as microscopic
examination requires in dry sections. It must, therefore, be polished
with the putty-powder and dry strop, as recommended in the working of
the shell-sections. The other side of the section of the tooth may then
be rubbed down to the requisite thinness, and polished in the same
manner, when the dust and other impurities must be removed by washing,
after which the section must be carefully dried and mounted. Some of
these sections are equally interesting as opaque or transparent objects.

The dentine of the teeth may be decalcified by submersion of the
section in dilute muriatic acid; after drying and mounting in Canada
balsam it presents a new and interesting appearance, showing the enamel
fibres very beautifully when magnified about three hundred diameters. A
friend tells me that after submersion of the _whole tooth_ in the acid
he has been able to cut sections with a razor.

SECTIONS OF BONE.--With the aid of the microscope few fragmentary
remains have proved so useful to the geologist and students of the
fossil kingdom as these. From a single specimen many of our naturalists
can tell with certainty to what _class_ of animal it has once belonged.
To arrive at this point of knowledge much study is necessary, and
sections of various kinds should be cut in such a manner as will best
exhibit the peculiarities of formation. The methods of accomplishing
this will now be considered. It may, however, be first mentioned that
the chippings of some bones will be found useful now and then, as
before stated with flint, though this is by no means a satisfactory
way of proceeding. Sometimes the bones may be procured naturally so
thin that they may be examined without any cutting; and only require
mounting _dry_, or in _fluid_, as may be found the best.

When commencing operations we must provide the same apparatus as is
needed in cutting sections of teeth, before described. A fine saw, like
those used for cutting brass, &c.; two or three flat files of different
degrees of coarseness; two flat “sharpening” stones; and a leather
strop with putty-powder for polishing. As thin a section as possible
should first be cut from the part required by the aid of the fine saw;
and it is better when in this state to soak it for some short time
in camphine, ether, or some other spirit to free it from all grease.
With the aid of a file we may now reduce it almost to the necessary
degree of thinness, and proceed as before recommended with teeth. The
“sharpening” stone will remove all scratches and marks sufficiently
to allow it to be examined with the microscope to see if it is ground
thin enough; and if it is to be mounted _dry_ we must polish it with
putty-powder and water upon the strop to as high a degree as possible,
and having washed all remains of polishing powder, &c., from the
section we must place it upon the slide and finish it as described in
Chapter II.

If the bone is not sufficiently hard in its nature to bear the above
method of handling whilst grinding and polishing--as some are far more
brittle than others--as thin a section as possible must first be cut
with the saw, and one surface ground and polished. The piece must then
be dried and united to the glass by heated balsam in the same manner as
shells, &c. After which the superabundance of balsam must be removed
from the glass; then rub down upon the stone and strop as before.
When the polishing is completed the whole slide must be immersed in
chloroform, ether, or some other spirit, to release and cleanse the
section, when it may be mounted as the one above mentioned.

Some have recommended a strong solution of isinglass to affix the
half-ground teeth or bones to the glass as causing them to adhere very
firmly and requiring no heat, and also being readily detached when
finished.

The reason why the sections of bone are usually mounted _dry_ is
that the “_lacunæ_,” bone cells, and _canaliculi_ (resembling minute
canals) show their forms, &c., very perfectly in this state, as
they are hollow and contain air, whereas if they become filled with
liquid or balsam--which does sometimes occur--they become almost
indistinguishable. There are some dark specimens, however, where the
cells are already filled with other matter, and it is well to mount
these with balsam and so gain a greater degree of transparency.

To gain a true knowledge of the structure of bone, sections must be cut
as in wood, both transversely and longitudinally; but with _fossil_
bones, without the lapidary’s wheel, &c., it is a laborious task, and
indeed can seldom be properly accomplished. In this place, also, it may
be mentioned that by submitting bone to the action of muriatic acid
diluted ten or fifteen times with water, the lime, &c., is dissolved
away and the cartilage is left, which may be cut into sections: in
_caustic potash_ the animal matter is got rid of. Both of these
preparations may be mounted in fluid.

The method of cutting thin sections of bone may be also employed with
the stones of fruit, vegetable ivory and such like substances; many of
which show a most interesting arrangement of cells, especially when
the sections are transverse. Most of these objects present a different
appearance when mounted _dry_ to that which they bear when _in balsam_,
owing to the cells becoming filled; and to arrive at a true knowledge
of them we must have a specimen mounted in both ways.

To those who study polarized light, few objects are more beautiful than
the sections of the different kinds of horn. We will briefly inquire
into the best method of cutting these. There are three kinds of horn,
the first of which is hard, as the stag’s, and must be cut in the same
manner as bone. The second is somewhat softer, as the cow’s. The third
is another and still softer formation, as the “horn” (as it is termed)
of the rhinoceros. In cutting sections of the two last we should
succeed best by using the machine invented for these purposes which I
shall shortly describe when the method of cutting wood is considered.
To aid us in this when the horn is hard it must be boiled for a short
time in water, when the cutting will be more easily accomplished. The
sections should be both transverse and longitudinal, those of the
former often showing cells with beautiful crosses, the colours with
the selenite plate being truly splendid. Of this class the rhinoceros
horn is one of the best; but the buffalo also affords a very handsome
object. The cow’s, and indeed almost every different kind of horn, well
deserves the trouble of mounting. Whalebone, when cut transversely,
strongly resembles those of the third and softer formation. All these
are best seen when mounted in Canada balsam, but care must be taken
that they have been thoroughly dried after cutting, and then steeped in
turpentine.

An interesting object may also be procured from whalebone by cutting
long sections of the hairs of which it is composed. Down the centre
of each hair we shall find a line of cells divided from one another
very distinctly. And (as recommended in the “Micrographic Dictionary”)
if whalebone be macerated twenty-four hours in a solution of caustic
potash it will be softened, and by afterwards digesting in water, the
outer part will be resolved into numerous transparent cells, which will
show more plainly the structure of this curious substance.

In a former chapter, hairs were mentioned, their many and interesting
forms, and their beauty when used with polarized light. The sections of
them, however, are no less a matter of study, as this mode of treatment
opens to sight the outer “casing,” and the inner substance somewhat
resembling the pith of plants.

It would be out of place to enter into the description of the different
forms met with; but the ways in which sections are to be procured
may be glanced at. If transverse sections are required, some place a
quantity of hairs betwixt two flat pieces of cork, which by pressure
hold them firmly enough together to allow the required portions to be
cut with a razor. Others take a bundle of the hairs and dip it into gum
or glue, which gives it when dry a solidity equal to wood. Sections of
this are then cut with the machine mentioned a little further on, and
these may be mounted in balsam. The human hair is easily procured in
the desired sections by shaving as closely as possible a second time
and cleansing from the lather, &c., by carefully washing. Most hairs,
however, should be examined both transversely and longitudinally. It
is not difficult to procure the latter, as we may generally split them
with the aid of a sharp razor. In a great number of hairs there is a
quantity of greasy matter which must be got rid of by soaking in ether
or some other solvent before mounting.

We may next consider the best method of procuring _sections of wood_,
which must be cut of such a degree of thinness as to form transparent
objects, and so display all the secrets of their structure. There
is no monotony in this study, as the forms are so various, and the
arrangement of the cells and woody fibre so different, that the
microscopist may find endless amusement or study in it. From a single
section the _class_ of trees to which it has belonged may be known,
often even when the wood is _fossil_. The apparatus best adapted for
cutting these sections is made as follows:--A flat piece of hard wood,
about six inches long, four wide, and one thick, is chosen, to which
another of the same size is firmly fixed, so as to form, in a side
view, the letter =T=. On one end of the upper surface is fastened a
brass plate, perfectly flat, in the centre of which a circular opening
is cut about half an inch in diameter. Coinciding with this opening
is a brass tube, fixed in the under side of the table (if it may be
termed so). This tube is so cut at the bottom as to take a fine screw.
Another screw is also placed at the same end of the “table,” which
works at right angles to this, so that any substance in the tube may be
wedged firmly by working this last screw. To use this instrument, the
piece of wood or other object of which a section is required must be
placed in the tube, when, by turning the screw underneath, the wood is
raised above the brass plate more or less as wished, and by using the
screw at the end, it is held firmly in the same position. With a flat
chisel the portion of the object which projects above the surface of
the brass plate may now be cut off, and by means of the bottom screw
another portion may be raised and treated in the same manner. As to
the thickness of which objects should be cut, no proper directions can
be given, as this differs so greatly that nothing but experience can
be any guide. The same thickness can be obtained by working the screw
underneath in uniform degrees, the head being marked for this purpose;
and when the substance to be cut is _very_ much smaller than the hole
in the brass plate, it may be wedged with cork.

As this instrument is peculiarly adapted for cutting wood (though
used for other substances, as before mentioned), I shall notice a
few particulars concerning this branch of sections. It may here be
remarked, that to obtain anything like a true knowledge of the nature
of wood, it should be cut and examined in at least two directions,
_across_ and _along_. The piece of wood is often placed in spirits
for a day or two, so that all resinous matter may be dissolved out of
it; it must then be soaked in water for the same length of time, so
as to soften and render it easy to cut. Sections are obtained in the
manner described above, but often curl to such a degree as to make it
necessary to immerse them in water, from which they may be taken and
dried under slight pressure. They are often mounted _dry_, and require
no care beyond other objects, as in Chapter II. Some, however, are
best mounted in balsam, particularly the long sections when used for
the polariscope; these must be soaked in turpentine, and the greatest
care taken that all air bubbles are got rid of. Others are thought
to be most useful when mounted in shallow cells with some of the
_preservative liquids_ mentioned in Chapter IV.--weak spirit and water,
chloride of calcium solution of the strength of one part of the salt to
three parts of distilled water, &c.

The above “_section-cutter_” may not be within the reach of every
student, nor is it absolutely necessary; though where any _great
number_ of specimens is required it is very useful, and insures greater
uniformity in the thickness. Many employ a razor for the purpose, which
must always be kept sharp by frequent stropping. Sections of leaves
also may be procured by the same means, though, as before mentioned,
they are sometimes easily divided by stripping the coatings off with
the fingers. The cells which come to sight by cutting some of the
orchideous plants are most interesting. To cut these leaves they
may be laid upon a flat piece of cork, thus exposing the razor to no
danger of injury by coming in contact with the support. It may be
mentioned here that the _razor_ may also be used in cutting sections
of the rush, than which a more beautiful object can scarcely be found
when viewed transversely, as it shows the stellate arrangements of the
ducts to convey the liquids to the different parts of the plant very
clearly. This should be mounted _dry_. In the same way sections of the
leaf-stalks of ferns may also be cut, some of which, as Dr. Carpenter
states, show the curious ducts very beautifully, especially when cut
rather obliquely.

When sections of the softer substances are required, no instrument
can be compared with “Valentin’s knife,” which consists of two steel
blades lying parallel with one another and attached at the lower end.
The distance of separation may be regulated at will by a small screw
near the handle. When, therefore, a section is wanted, the substance
must be cut through, and betwixt the blades a thin strip will be found,
which may be made of any thickness, according to the distance of their
separation. By loosening the screw the blades may be extended, and the
section may be floated out in water if the damp will not injure it.
The knife cuts much better if dipped in water immediately before use
and also when the substance to be operated upon is wet, or even under
water altogether; but care must be taken, after use, to clean the
blades thoroughly and oil them before laying by, if the place is at all
damp. This instrument is most useful in such subjects as anatomical
preparations where the sections are required to show the position of
the different vessels, &c.; but, as before stated, is very valuable for
all soft substances. As an instance of this, it may be mentioned, that
it is frequently used in cutting sections of sponges; but as these are
often very full of spicula, &c., it is much better to press the sponge
flat until dry, and then cut off thin shavings with a very sharp knife;
these shavings will expand when placed in water. After this they may
be laid betwixt two flat surfaces and dried, when they may be mounted
as other dry objects, or, when desirable, in balsam.

_Valentin’s knife_ is very much used in taking sections of skin, which
are afterwards treated with potash solution, acids, &c., to bring out
in the best way the different portions. Dr. Lister’s mode, however,
of getting these is thus given in the _Microscopic Journal_:--“But
I afterwards found that much better sections could be obtained from
dried specimens. A portion of shaved scalp being placed between two
thin slips of deal, a piece of string is tied round them so as to
exercise a slight degree of compression; the preparation is now laid
aside for twenty-four hours, when it is found to be dried to an almost
horny condition. It then adheres firmly by its lower surface to one
of the slips, and thus it can be held securely, while extremely thin
and equable sections are cut with great facility in any plane that may
be desired. These sections, when moistened with a drop of water and
treated with acetic acid, are as well suited for the investigation of
the muscular tissue as if they had not been dried.”

There are many who almost confine their attention to polarized light
and its beautiful effects. Such would not deem these efforts to aid
the student in cutting sections complete without some notice of those
which are taken from various crystals, in order to display that curious
and beautiful phenomenon, _the rings with a cross_. The arrangement of
these is somewhat changed by the crystal which affords the section; but
nitrate of potash gives two sets of rings with a cross, the long line
of which passes through both, the short line dividing it in the middle.

The process of cutting these sections is rather difficult, but a
little care and perseverance will conquer all this. The following is
extracted from the _Encyclopædia Metropolitana_:--“Nitre crystallizes
in long six-sided prisms whose section, perpendicular to their sides,
is the regular hexagon. They are generally very much interrupted in
their structure; but by turning over a considerable quantity of the
ordinary saltpetre[F] of the shops specimens are readily found which
have perfectly transparent portions of some extent. Selecting one
of these, cut it with a knife into a plate _above_ a quarter of an
inch thick, directly across the axis of the prism, and then grind it
down on a broad wet file till it is reduced to about one quarter or a
sixth of an inch thick, smooth the surface on a wet piece of emeried
glass, and polish on a piece of silk strained very tight over a strip
of plate-glass, and rubbed with a mixture of tallow and colcothar
of vitriol. This operation requires practice. It cannot be effected
unless the nitre be applied wet and rubbed till quite dry, increasing
the rapidity of the friction as the moisture evaporates. It must be
performed in gloves, as the vapour from the fingers, as well as the
slightest breath, dims the polished surface effectually. With these
precautions a perfect vitreous polish is easily obtained. We may here
remark, that hardly any two salts can be polished by the same process.
Thus, Rochelle-salt must be finished wet on the silk, and instantly
transferred to soft bibulous linen and rapidly rubbed dry. Experience
alone can teach these peculiarities, and it is necessary to resort to
contrivances (sometimes very strange ones) for the purpose of obtaining
good polished sections of soft crystals, especially of those easily
soluble in water.

“The nitre is thus polished on both its surfaces, which should be
brought as near as possible to parallelism.”

    [F] Sometimes the saltpetre of the shops is nitrate of _soda_,
        and as this is slightly deliquescent, it is well to be
        certain that we have the nitrate of _potash_, which is free
        from this defect.

Some sections of the naturally formed crystals also show the “rings”
very well,--as Iceland Spar, which gives a single ring and cross; but
the difficulty of cutting and polishing them is almost too great for
the amateur, and must be left to the lapidary. This curious phenomenon,
however, may be seen by using a plate of ice uninterruptedly formed of
about one inch in thickness.

Before concluding these remarks on sections, I must mention a few
difficulties which may be met with, and their remedies. The foremost of
these is the softness of some objects, which have not resistance enough
in themselves to bear cutting even with the sharpest instruments.
This may often be removed by soaking in a solution of gum, and then
drying, which will render the substance firm enough to be cut, when the
sections must be steeped in water, and the gum thus got rid of. Small
seeds, &c., may be placed in wax when warmed, and will be held firmly
enough when it is again cold to allow of them being cut into sections,
&c. And, lastly, where a substitute for a microscopist’s hand-vice is
required, a cork which fits any tube large enough may be taken and
split, the object being then placed between the two parts, and the cork
thrust into the tube, a sufficient degree of firmness will be obtained
to resist any necessary cutting, &c.

DISSECTION.--As I stated at the commencement of this chapter, no
written instructions can enable any student to become an adept in this
branch without much experience and no little study. I will, however,
describe the necessary apparatus, and afterwards mention the mode of
treatment which certain objects require.

A different microscope is manufactured for the purpose of dissection,
most first-rate makers having their own model. The object-glasses of
many of these are simple, and consequently not expensive; but one of
the great requisites is a stage large enough to hold the trough, in
which the operation is often performed. Where this is the case it would
scarcely be worth the expense of getting a dissecting microscope if the
student were pursuing no particular study, but merely made use of the
instrument when an object to be operated upon turned up accidentally.
The ordinary form is much improved for this purpose, by having two
wooden rests placed at the sides of the microscope, upon which the
hands may be supported when working upon the stage. These should be
weighty enough to be free from danger of moving. These supports will
also be found to be a remedy against much of the weariness which
inevitably arises from having to sustain the hands as well as work
with them. The erector, as I before observed, is necessary to a young
student; but with a little practice he may work very well without it.

We will now notice some of the instruments which are most useful in
dissection. Two or three different sizes of ordinary scissors should
be possessed, but the shapes must be modified in others for many
purposes, as those used by surgeons; a pair with the cutting parts
bent in a horizontal direction, and another pair slightly curved in
a perpendicular; so that parts of the substance operated upon may be
reached, which it would be impossible to touch with straight scissors.
One point of these is sometimes blunt, and the other acute, being
thus made very useful in opening tubular formations. Another form of
these is made, where the blades of the scissors are kept open by a
spring, the handles being pressed together by the fingers. Where it is
desirable, one or both of these handles may be lengthened to any degree
by the addition of small pieces of wood.

THE KNIVES which are most useful are those of the smallest kind which
surgeons employ in very delicate operations. These are made about the
length of an ordinary pen-knife, and are fixed in rather long flattish
handles; some are curved inwards, like the blade of a scythe, others
backwards; some taper to a point, whilst others again are broad and
very much rounded. Complete boxes are now fitted up by the cutlers, of
excellent quality and surprisingly cheap.

NEEDLES.--These are very useful and should be firmly fixed in handles
as recommended in Chapter I. It is convenient to have them of various
lengths and thicknesses. If curved by heating and bending to any
required shape they may be re-hardened by putting them whilst hot into
cold water. Dr. Carpenter also makes edged instruments by rubbing down
needles upon a hone. They are more pleasant to work with when _short_,
as the spring they have whilst _long_ robs them of much of their
firmness.

A _glass syringe_ is also useful in many operations, serving not
only to cleanse the objects but to add or withdraw liquids from the
_dissecting-trough_. This trough will now be described, as many
substances are so changed by becoming dry that it is impossible to
dissect them unless they are immersed in water during the operation.
If the object is opaque and must be worked by reflected light, a small
square trough may be made to the required size of gutta-percha, which
substance will not injure the edge of the knives, &c.; but where
transparency is necessary, a piece of thin plate-glass must be taken,
and by the aid of marine glue (as explained in Chapter IV.) the sides
affixed of the required depth. As pins, &c., cannot be used with the
glass troughs and the substance must be kept extended, a thin sheet of
cork loaded with lead in order to keep it under water may be used; but
this, of course, renders the bottom opaque. When working with many thin
substances, a plate of glass three or four inches long and two wide
will serve every purpose, and be more pleasant to use than the trough.
A drop or two of water will be as much liquid as is needed, and this
will lie very well upon the flat surface. As these are the principal
apparatus and arrangements which are requisite in dissection, the
method of proceeding in a few cases may now be noticed.

VEGETABLES.--The dissection of vegetable matter is much less
complicated than that of animal; maceration in water being a great
assistant, and in many cases removing all necessity for the use of the
knife. This maceration may be assisted by needles, and portions of
the matter which are not required may be removed by them. When, for
instance, the spiral vessels which are found in rhubarb are wanted,
some parts containing these are chosen and left in a small quantity of
water until the mass becomes soft, and this is more quickly effected
when the water is not changed. The mass must be then placed upon a
glass plate when practicable, or in the trough when large, and with
the aid of two needles the matter may be removed from the spiral
vessels, which are plainly seen with a comparatively low power; and by
conveying these to a clean slip of glass, repeating the process, and
at last washing well, good specimens may be procured. Most of these
should be mounted in some of the preservative liquids in the manner
described in Chapter IV. Many, however, may be dried on the slide,
immersed in turpentine, and then mounted in balsam; but liquid is
preferable, as it best preserves the natural appearance. Certain kinds
of vegetables require a different treatment to separate these spiral
vessels. Asparagus is composed of very hard vegetable matter, and some
have recommended the stems to be first boiled, which will soften them
to such a degree that they may easily be separated. Dilute acids are
also occasionally used to effect this; and in some instances to obtain
the _raphides_ caustic potash may be employed; but after _any_ of these
agents have been made use of, the objects must be thoroughly cleansed
with water, else the dissecting instruments (and perhaps the cell) will
be injured by the action of the remaining portion of the softening
agent.

For the dissection of _animal tissues_ it is necessary that the
instruments be in the best order as to sharpness, &c.; and as the rules
to be observed must necessarily be somewhat alike in many instances,
the treatment required by some of the objects most frequently mounted
will now be described. We may here remark that _cartilage_ can be best
examined by taking sections, which will show the arrangement of the
cells very perfectly. This, however, is plainly seen in the mouse’s
ear without any section being necessary. Glycerine, the preservative
liquids before mentioned, and Canada balsam are all used to mount it;
but perhaps the first named may be preferred in many cases.

MUSCLE.--This is what is commonly called the “flesh” of animals. If a
piece be laid upon the slide under the microscope, bundles of “fibres”
will be perceived, which with needles and a little patience may be
separated into portions, some of these being “striated,” or marked
with alternate spaces of dark and light. Some of the _non-striated_
or _smooth_ class of muscle, such as is found in intestines, may be
prepared for the microscope by immersing for a day or two in nitric
acid diluted with three or four parts of water, and then separating
with needles and mounting as soon as possible. Sometimes _boiling_ is
resorted to to facilitate the separation, and occasions little or no
alteration in the material. Specimens are often taken from the _frog_
and the _pig_, as being amongst the best, _Goadby’s solution_ being
generally used in mounting them. The muscle of insects also shows the
striæ very perfectly.

NERVE-TISSUE.--This is seldom mounted; as Dr. Carpenter observes, “no
method of preserving the nerve-tissue has been devised which makes it
worth while to mount preparations for the sake of displaying its minute
characters,” but we will mention a few particulars to be observed in
its treatment. The nerve should be taken from the animal as soon as
possible after death, and laid upon a glass slide, with a drop or two
of serum if possible. The needles may be used to clean it, but extreme
delicacy is necessary. It will be found that the nerve is tubular
and filled with a substance which is readily ejected by very slight
pressure. When the nerve is submitted to the action of acetic acid,
the outer covering, which is very thin, is considerably contracted,
whilst the inner tube is left projecting; and thus is most distinctly
shown the nature of the arrangement. Mr. Lockhart Clarke, who has made
great researches into the structure of the spinal cord, gives a part
of his experience as follows:--He takes a perfectly fresh spinal cord
and submits it to the action of strong spirits of wine. This gives the
substance such a degree of hardness that thin sections may be readily
cut from it, which should be placed upon a glass in a liquid consisting
of three parts of spirit and one of acetic acid, which renders them
very distinct. To mount these sections, they must now be steeped in
pure spirit for two hours and afterwards in oil of turpentine, and
lastly must be mounted in Canada balsam.

TRACHEÆ OF INSECTS, &C.--The nature of these was described in Chapter
III., but the method of procuring them was not explained, as this
clearly belongs to “dissection.” The larger tubes are readily separated
by placing the insect in water, and fixing as firmly as possible,
when the body must be opened and the viscera removed. The tracheæ may
then be cleaned by the aid of a camel-hair pencil, and floated upon a
glass, where they must first be allowed to dry, and then be mounted
in balsam. Mr. Quekett gives the following method of removing the
tracheæ from the larva of an insect:--“Make a small opening in its
body, and then place it in strong acetic acid. This will soften or
decompose all the viscera, and the tracheæ may then be well washed with
the syringe, and removed from the body with the greatest facility,
by cutting away the connections of the main tubes with the spiracles
by means of fine-pointed scissors. In order to get them upon the
slide, it must be put into the fluid, and the tracheæ floated upon
it; after which they may be laid out in their proper position, then
dried and mounted in balsam.” If we wish them to bear their _natural_
appearance, they must be mounted in a cell with Goadby’s fluid; but
the structure is _sometimes_ well shown in specimens mounted dry. As
before mentioned, these tracheæ terminate on the outside in openings
termed spiracles, which are round, oblong, and of various shapes. Over
these are generally a quantity of minute hairs, forming a guard against
the entrance of dust, &c. The forms of these are seldom alike in two
different kinds of insects, so that there is here a wide field for the
student. The dissection, moreover, is very easy, as they may be cut
from the body with a sharp knife or scissors, and mounted in balsam or
fluid. Many of the larvæ afford good specimens, as do also some of the
common Coleopterous insects.

TONGUES, OR PALATES, OF MOLLUSCS.--Of the nature of these, Dr.
Carpenter gives the following description:--“The organ which is
commonly known under this designation is one of a very singular nature;
and we should be altogether wrong in conceiving of it as having any
likeness to that on which our ordinary ideas of such an organ are
founded. For, instead of being a projecting body, lying in the cavity
of the mouth, it is a tube that passes backwards and downwards beneath
the mouth, its higher end being closed, whilst in front it opens
obliquely upon the floor of the mouth, being, as it were, slit up and
spread out so as to form a nearly flat surface. On the interior of
the tube, as well as on the flat expansion of it, we find numerous
transverse rows of minute teeth, which are set upon flattened plates;
each principal tooth sometimes having a basal plate of its own, whilst
in other instances one plate carries several teeth.” These palates, or
tongues, differ much amongst the Gasteropods in form and size, some
of them being comparatively of an immense length. Many are amongst
the most beautiful objects when examined with polarized light. They
must, however, be procured by dissection, which is usually performed
as follows:--The animal is placed on the cork in the dissecting-trough
before mentioned, and the head and forepart cut open, spread out,
and firmly pinned down. With the aid of fine scissors or knife, the
tongue must be then detached from its fastenings, and placed in water
for a day or two, when all foreign matter may with a little care be
removed. In what way it should be mounted will depend on the purpose
for which it is intended. If for examination as an ordinary object, it
may be laid upon the slide and allowed to dry, which arrangement will
show the teeth very well. If we wish to see it as it is naturally, it
must be mounted in a cell with Goadby’s fluid; but if it is wanted as
a polarizing object, it must be floated upon a slide, allowed to dry
thoroughly, and then Canada balsam added in the usual manner.

In the stomach, also, of some of these molluscs teeth are found, which
are very interesting objects to examine, and must be dissected out in
the same manner as the “tongues.”

Since writing the above. Dr. Alcock (whose very beautiful specimens
prove him to be a great authority in this branch) has published some
of his experience in the second volume of the third series of “Memoirs
of the Literary and Philosophical Society of Manchester.” By his
permission I make the following extract:--

“This closes my present communication on the tongues of mollusca; but
as some members may possibly feel inclined to enter upon the inquiry
themselves, I think it will not be amiss to add a few remarks on the
manner in which they are to be obtained.

“First, as to the kinds best worth the trouble of preparation. Whelks,
Limpets, and Trochuses should be taken first. Land and fresh-water
snails can scarcely be recommended, except as a special study,--their
tongues being rather more difficult to find, and the teeth so small
that they require a high power to show them properly. It would appear,
from Spallanzani’s description of the anatomy of the head of the snail,
that even he did not make out this part, although, in his curious
observations on the reproduction of lost parts, he must have carefully
dissected more snails than any other man.

“As to preserving the animals till wanted, they should simply be
dropped alive into glycerine or alcohol. Glycerine is perhaps best
where only the tongues are wanted; but it leaves the animals very soft;
and as it does not harden their mucus at all, they are very slippery
and difficult to work upon when so preserved.

“Then as to the apparatus required for dissection. In the first
place, all the work is to be done under water, and a common saucer is
generally the most convenient vessel to use. No kind of fastening down
or pinning out of the animal is needed; and, in fact, it is much better
to have it quite free, that you may turn it about any way you wish.
The necessary instruments are a needle-point, a pair of fine-pointed
scissors, and small forceps; the forceps should have their points
slightly turned in towards each other.

“A word or two on the lingual apparatus generally, and on its special
characters in a few different animals, will conclude what I have to say.

“The mode of using the tongue can be easily seen in any of the common
water-snails, when they are crawling on the glass sides of an aquarium;
it may then be observed that from between the fleshy lips a thick
mass is protruded, with a motion forwards and upwards, and afterwards
withdrawn, these movements being almost continually repeated. The
action has the appearance of licking; but when the light falls suitably
on the protruded structure, it is seen to be armed with a number of
bright points, which are the lingual teeth, so arranged as to give the
organ the character and action of a rasp.

“If you proceed to dissection, and open the head of one of these
mollusca (say, for instance, a common limpet), you will find the cavity
of the mouth almost filled with the thick fleshy mass, the front of
which is protruded in the act of feeding; and on its upper surface,
extending along the middle line from back to front, is seen the strong
membranous band upon which the teeth are set. The mass itself consists
of a cartilaginous frame, surrounded by strong muscles; and these
structures constitute the whole of the active part of the lingual
apparatus....

“But the peculiarity of the toothed membrane, which makes its name of
‘_ribbon_’ so appropriate, is, that there is always a considerable
length of it behind the mouth, perfectly formed, and ready to come
forward and supply the place of that at the front, which is continually
wearing away by use.

“In the limpet this reserve-ribbon is of great length, being nearly
twice as long as the body, and the whole of it is exposed to view on
simply removing the foot of the animal; nothing, then, can be easier
than to extract the tongue of the common limpet. But, unfortunately,
what you find in one kind of mollusc is not at all what you find in
another. In the Acmæas, for instance, which are very closely related
to the limpets, and have shells which cannot be distinguished, the
reserve portion of the ribbon has to be dug out from the substance
of the liver, in which it is imbedded, that organ being, as it were,
stitched completely through by a long loop of it.... It might be
thought a comfortable reflection that, at all events, one end of the
ribbon can always be found in the mouth; but in many cases this is
about the worst place to look for it. Perhaps it may appear strange
that in some of the smaller species, with a retractile trunk, a
beginner may very likely fail altogether in his attempt to find the
mouth; if, however, the skin of the back is removed, commencing just
behind the tentacles, there will be very little difficulty in making
out the trunk, which either contains the whole of the ribbon, as in
the whelk, or the front part of it, as in _Purpura_ and _Murex_, where
a free coil is also seen to hang from its hinder extremity.... In the
periwinkles the same plan of proceeding, by at once opening the back of
the animal, is best; and on doing so, the long ribbon, coiled up like a
watch-spring, cannot fail to be found.

“In the Trochuses, and indeed in all the _Scutibranchiata_, one point
of the scissors should be introduced into the mouth of the animal, and
an incision made directly backwards in the middle line above to some
distance behind the tentacles; the tongue is then immediately brought
into view, lying along the floor of the mouth.”

Dr. Alcock’s method of dissection will be found to differ in some
degree from the general rules before given; and when the tongue is
dissected out he washes it for one hour (shaking it now and then) in a
weak solution of potash. After cleaning thoroughly in water, it must be
mounted by one of the methods before mentioned.

Amongst insects, especially the grasshopper tribe, are found many which
possess a gizzard, armed with strong teeth, somewhat similar to those
of the molluscs. It requires great nicety of manipulation to obtain
these for the microscope; but it would be useless to attempt any
description of the process here, as the strident can only be successful
by experience in dissecting objects less difficult to manage, and by
using the most delicate instruments.

We have now considered most of those objects which require any
_peculiar_ treatment in section-cutting, &c.; but in no branch of
microscopic manipulation is experience more necessary than in this.




CHAPTER VI.

INJECTION.


1. Injection is the filling of the arteries, veins, or other vessels
of animals with some coloured substance, in order that their natural
arrangement may be made visible. This is, of course, a delicate
operation, and needs special apparatus, which I will now attempt to
describe.

2. _Syringe._--This is usually made to contain about two ounces.
On each side of the part next to the handle is a ring, so that the
finger may be thrust through it, and the thumb may work the piston
as in an ordinary syringe. The plug of the piston must be “packed”
with wash-leather, and fit so closely as to be perfectly air-tight;
and if, when it has been used awhile, it is found that some of the
liquid escapes past the plug into the back part of the body, it must
be _repacked_, which operation will be best understood by examining
the part. These syringes are made of various sizes, but in ordinary
operations the above will be all that is needed. The _nozzle_ is about
an inch long, and polished so accurately that there is no escape when
the _pipes_ are tightly placed upon it _dry_.

3. The _pipes_ are usually about an inch long, to the end of which are
affixed thicker tubes so as to fit the nozzle, as before mentioned,
whilst a short arm projects from each side of these, so that the silk
or thread which is used to tie this artery, &c., upon the thin pipe may
be carried round these arms, and all danger of slipping off prevented.
The _pipes_ are made of different sizes, from that which will admit
only a very fine needle (and this will need now and then to be cleaned,
or to be freed from any chance obstruction), to that which will take a
large pin. These sizes must always be at hand, as the vessels of some
subjects are exceedingly minute.

4. _Stopcock._--This is a short pipe like a small _straight_ tap,
which fits accurately upon the end of the syringe like the pipes, and
also takes the pipes in the same manner. The use of this is absolutely
necessary when the object is so large that one syringe full of liquid
will not fill it. If no preventive were used, some part of the liquid
would return whilst the syringe was being replenished, but the
stopcock is then turned as in an ordinary tap, and all danger of this
effectually removed.

5. _Curved needles._--These are easily made by heating common needles
at the end where the eye is situated, and bending them with a small
pair of “pliers” into a segment of a circle half an inch in diameter.
They are, perhaps, more convenient when the bent part is thrown
slightly back where it commences. The pointed end is then thrust into
a common penholder, and the needle needs no re-tempering, as the work
for which it is wanted is simply to convey the thread or silk _under_
any artery or vessel where it would be impossible to reach with the
unassisted fingers.

6. A kind of _forceps_, commonly know by the name of “bullnose
forceps,” will be constantly required during the process of injecting.
These are short, usually very strong, but not heavy, and close very
tightly by their own spring, which may be easily overcome and so
released by the pressure of the fingers. When any vessel has not been
tied by the operator, and he finds the injected fluid escaping, one
of these “bulldogs” may be taken up and closed upon the opening. This
will cause very little interruption, and the stoppage will be almost as
effectual as if it were tied.

7. When the ordinary mode of injection is employed, it is necessary
that the preparations be kept warm during the time they are used,
otherwise the gelatine or size which they contain becomes stiff, and
will not allow of being worked by the syringe. For this purpose we
must procure small earthenware or tin pots of the size required, which
will differ according to the kind of work to be done; and to each of
these a loose lid should be adapted to protect it from dust, &c. These
pots must be allowed to stand in a tin bath of water, under which a
lamp or gas flame may be placed to keep the temperature sufficiently
high to insure the perfect fluidity of the mixture. The tin bath is,
perhaps, most convenient when made like a small shallow cistern; but
some close it on the top to place the pots upon it, and alter the shape
to their own convenience.

8. We will now inquire into some of the materials which are needed in
this operation; the first of which is _size_. This substance is often
used in the form of _glue_, but it must be of the very best and most
transparent kind. To make the liquid which is to receive the colours
for the usual mode of injecting, take of this glue seven ounces, and
pour upon it one quart of clean water; allow this to stand a few hours,
and then boil gently until it is thoroughly dissolved, stirring with a
wooden or glass rod during the process. Take all impurities from the
surface, and strain through flannel or other fine medium. The weather
affects this a little as to its stiffness when cold, but this must be
counteracted by adding a little more glue if found too liquid.

9. Instead of glue, gelatine is generally used, especially when the
work to be accomplished is of the finer kind. The proportions are very
different in this case, one ounce of gelatine to about fourteen ounces
of water being sufficient. This, like glue, must be soaked a few hours
in a small part of the cold water, the remainder being boiled and
added, when it must be stirred until dissolved. A good size may be made
by boiling clean strips of parchment for awhile, and then straining
the liquid whilst hot through flannel; but when the injections are
to be _transparent_, it is of the greatest importance that the size
be as colourless as possible. For this purpose good gelatine must be
employed, as Nelson’s or Cox’s: some persons of experience prefer the
latter.

10. _Colours._--The size-solution above mentioned will need some
colouring matter to render it visible when injected into the vessels
of any animal, and different colours are used when two or more kinds
of vessels are so treated, in order that each “set” may be easily
distinguished by sight. The proportion in which these colours are added
to the size-solution may be given as follows:--

11. For--

  Red     8 parts of size-solution
                  (by weight) to 1 part of vermilion.
  Yellow  6      ”      ”        1   ”     chrome yellow.
  White   5      ”      ”        1   ”     flake-white.
  Blue    3      ”      ”        1   ”     blue-smalt, fine.
  Black  12      ”      ”        1   ”     lamp-black.

Whichever of these colours is made use of must be levigated in a mortar
with the addition of a very small quantity of water until every lump
of colour or foreign matter is reduced to the finest state possible,
otherwise in the process of injecting it will most likely be found
that some of the small channels have been closed and the progress
of the liquid stopped. When this fineness of particles is attained,
warmth sufficient to render the size quite fluid must be used, and the
colour added gradually, stirring all the time with a rod. It may be
here mentioned that where one colour only is required, vermilion is,
perhaps, the best; and blue is seldom used for opaque objects, as it
reflects very little more light than black.

12. When it is wished to fill the capillaries (the minute vessels
connecting the arteries with the veins), the “Micrographic Dictionary”
recommends the colouring matter to be made by double decomposition. As
a professed handbook would be, perhaps, deemed incomplete without some
directions as to the mode of getting these colours, I will here make
use of those given in that work. For red, however, vermilion, as above
stated, may be used; but it must be carefully examined by reflected
light to see whether it be free from all colourless crystals or not.
It must first be worked in a mortar, and then the whole thrown into a
quantity of water and stirred about; after leaving it not longer than
a quarter of a minute, the larger portions will settle to the bottom,
and the liquid being poured off will contain the finer powder. This may
then be dried slowly, or added to the size whilst wet in the manner
before advised.

13. _Yellow injection._--To prepare this, take--

  Acetate (sugar) of lead         380 grains.
  Bichromate of potash            152   ”
  Size                              8 ounces.

Dissolve the lead salt in the warm size, then add the bichromate of
potash finely powdered.

Some of the chromic acid remains free, and is wasted in this solution,
so the following is given:--

  Acetate of lead                 190 grains.
  Chromate of potash (neutral)    100   ”
  Size                              4 ounces.

The first of these has the deepest colour, and is the most generally
used.

14. _White injection._--This is a carbonate of lead:--

  Acetate of lead                 190 grains.
  Carbonate of potash              83   ”
  Size                              4 ounces.

Dissolve the acetate of lead in the warm size, and filter through
flannel; dissolve the carbonate of potash in the smallest quantity of
water, and add to the size: 143 grains of carbonate of soda may be
substituted for the carbonate of potash.

15. For blue injection, which is not, however, much used with reflected
light, as before stated, take--

  Prussian blue    73 grains.
  Oxalic acid      73   ”
  Size              4 ounces.

The oxalic acid is first finely powdered in a mortar, the Prussian blue
and a little water added, and the whole then thoroughly mixed with the
size.

16. It may here be repeated, that it is only when the capillaries are
to be filled that there is any need to be at the trouble to prepare
the colours by this double decomposition; and, indeed, colours ground
so finely may be procured that the above instructions would have been
omitted, had it not been supposed that some students might find a
double pleasure in performing as much of the work as possible by their
own unaided labours.

17. The process of injection may now be considered; but it is
impossible for written instructions to supply the place of experience.
I will do my best, however, to set the novice at least in the right
way. There are two kinds of injection--one where the object and colours
are opaque, and consequently fit for examination by _reflected_ light
only; the other, where the vessels are filled with transparent colours,
and must be viewed by _transmitted_ light. The first of these is most
frequently employed, so we will begin with it. In the object which
is to be injected a vessel of the kind which we wish to be filled
must be found; an opening must then be made in it to allow one of the
small pipes before mentioned to be thrust some distance within it.
When this is accomplished, thread the curved needle with a piece of
silk thread, or very fine string, which some operators rub well with
beeswax. This thread must not be too thin, else there is danger of
cutting the vessel. The cord is then carried under the inserted pipe,
and the vessel bound tightly upon it, the ends being brought up round
the transverse arms, and there tied; so that all danger of accidentally
withdrawing the pipe is obviated. Care must now be used in closing
all the vessels which communicate with that where the pipe is placed
lest the injecting fluid escape; and this must be done by tieing them
with silk. Should, however, any of these be left open by accident, the
bullnose forceps must be made use of, as before recommended.

18. The part to be injected must now be immersed in warm water,
not, however, above 100° Fahrenheit, and be left until the whole is
thoroughly warmed. Whilst this is being done, the coloured size must
be made ready by the pot being placed in the tin bath of warm water,
which must be of sufficient temperature (about 110° Fahrenheit) to
keep it perfectly liquid. For the same purpose, the syringe is often
tightly covered with two or three folds of flannel; and, indeed,
there is no part of the process which requires more attention. If the
substance to be injected is too hot, it is injured; whilst, if any of
the articles are too cold, the gelatine, or size, loses a part of its
fluidity, and consequently cannot enter the minute parts. When all is
prepared, the syringe, with the stopcock attached, should be warmed,
and then filled and emptied with the injecting fluid two or three
times, care being taken that the end of the syringe be kept beneath any
bubbles which form upon the surface. The syringe may then be filled,
and closely attached to the pipe which is tied in the vessel. With a
firm and steady pressure the piston must be forced downwards, when the
substance will be perceived to swell, and the colour show itself in
places where the covering is thin. When the syringe is _almost_ emptied
of its contents, the stopcock must be turned to prevent any escape
of the injection from the subject. It must then be refilled, as in
the first instance, and the process repeated. I say _almost emptied_,
because it is well not to force the piston of the syringe quite to the
bottom, lest the small quantity of air which frequently remains be
driven into some of the vessels, and the object be injured or quite
ruined. As the injection is proceeded with, it will be found that the
force required grows greater, yet care must be taken not to use too
much, or the vessels will burst, and render all the labour fruitless.
The movement of the piston must be occasionally so slow as to be almost
imperceptible, and for this reason it is sometimes marked with lines
about one-eighth of an inch apart.

19. Of course, during the whole process the injecting fluid and subject
must be kept at a temperature high enough to allow the liquid to flow
freely; and the escape of a little of it need cause no fears to the
student, as it is almost impossible to fill any subject without some
loss. When the injected object has received sufficient fluid, it should
have a plump appearance, owing to all the vessels being well filled.
The vessel must then be tied up where the pipe was inserted, and the
whole left in cold water two or three hours, after which time it may be
mounted; but it may be well to notice a few things which the beginner
ought to know before entering into that part of the process; and he
may be here informed that it is not necessary to mount the objects
immediately, otherwise it would be impossible for one person to make
use of half of any large subject, as it would be in a state of decay
long before each part could have been examined and separated. Large
pieces should be therefore immersed in equal parts of spirits of wine
and water, or glycerine, which some think better still, and thus
preserved in bottles until time can be given to a closer examination.

20. In operating upon large subjects, entire animals, &c., the constant
pressure required by the piston of the syringe grows wearisome, besides
occupying both hands, which is sometimes inconvenient when working
without assistance. To obviate this, another way of driving the syringe
was published in the “Micrographic Dictionary” which I will quote
here:--“We have therefore contrived a very simple piece of apparatus,
which any one can prepare for himself, and which effects the object
by mechanical means. It consists of a rectangular piece of board, two
feet long and ten inches wide, to one end of which is fastened an
inclined piece of wood (equal in width to the long board, and one foot
high). The inclined portion is pierced with three holes, one above the
other, into either of which the syringe may be placed--the uppermost
being used for the larger, the lowermost for the smaller syringe; and
these holes are of such size as freely to admit the syringe covered
with flannel, but not to allow the rings to pass through them. The
lower part of the syringe is supported upon a semiannular piece of
wood, fastened to the upper end of an upright rod, which slides in
a hollow cylinder fixed at its base to a small rectangular piece of
wood; and by means of a horizontal wooden screw, the rod may be made to
support the syringe at any height required. The handle of the syringe
is let into a groove in a stout wooden rod connected by means of two
catgut strings with a smaller rod, to the middle of which is fastened
a string playing over a pulley, and at the end of which is a hook for
supporting weights, the catgut strings passing through a longitudinal
slit in the inclined piece of wood.” When in use the syringe is filled
with injecting fluid, and passed through one of the three holes which
is most suitable. The object being placed so that the pipe and syringe
can be best joined, the rod and strings are set in order, and a weight
placed on the hook. The stopcock must then be opened gradually, when
the operator will be able to judge whether the weight is a proper one
or not: if the piston is driven with any speed, there is danger of
injuring the subject, and less weight may be used; if, however, the
piston does not move, more must be added.

21. Such is the method recommended by the “Micrographic Dictionary,”
and perhaps it is as good as any mechanical plan could be; but where
the operator is willing to undergo the labour of performing all this
with the hand, he has a much better chance of succeeding, because the
pressure can be regulated so accurately, and changed so quickly when
requisite, that no mere machine can compete with it, however well
contrived.

22. When the beginner attempts to inject a subject, one of his
difficulties is finding the vessel from which to commence. Another
consists in distinguishing the arteries from the veins; but this is
partly removed by making a longitudinal incision in the vessel, and
with a blunt thick needle probing a little distance into the tube. The
artery will be found thicker in the coating than the vein, and the
difference is easily perceived by this mode of testing: the vein is
also of a bluer colour than the artery. I say above, a “longitudinal
incision” must be made: the reason for this is, the artery when cut
across contracts considerably, and is lost in the adjoining substance;
but where the opening is made _longitudinally_ all danger of this
contraction is obviated.

23. The different systems of vessels are often injected with various
colours, so that their relative positions, &c., may be shown most
clearly. In some specimens, the veins are injected with white, and the
arteries with red; in the kidney, the urinary tubes are often filled
with white, and the arteries with red. Then, again, the liver affords
tubes for three or four colours. But no written instructions on this
point can benefit the young student, and he must be content for a
while to employ himself with single colours until he has gained the
mechanical skill and the primary knowledge which are necessary before
he can make any advance.

24. We will now consider the best methods of _mounting_ injected
objects. They must always be well washed in water after they have been
kept in any preservative liquid, using a camel-hair pencil to clean the
surface if necessary. Many parts when injected are in masses, such as
the lungs, liver, &c., of animals, and consequently sections of these
must be cut. For this purpose Valentine’s knife is very convenient,
as the thickness can be regulated so easily; but where the injections
are opaque, there is no need to have the sections very thin. Some few
of this kind undergo comparatively little change in drying, so that
the section may be well washed and floated upon the glass slide in the
place desired, where it will dry perfectly and adhere to it. It must
be then moistened with turpentine and mounted in Canada balsam like
other objects. No great heat should be used with these preparations,
as it is very liable to injure them; and some of the colours seem to
suffer a slight contraction when any great degree of warmth is applied.
There are many objects, however, which must be seen in the mass to be
understood, and, indeed, lose all their form and beauty in drying, such
as certain parts of the intestines, &c. These must be mounted in fluid,
with the precautions noticed at length in Chapter IV., and for this
purpose either Goadby’s fluid, the chloride of zinc solution, or spirit
diluted with ten parts of distilled water, may be employed. It is a
good thing, when practicable, to mount similar objects on two separate
slides, using different preservative liquids, and taking the precaution
of marking each with the kind of liquid employed. This not only serves
as a guide to what is best for certain subjects, but if one is injured,
there will probably be a good specimen in the other.

25. It may be here mentioned that many are now mounting sections of
injected substances with the balsam and chloroform before mentioned,
instead of using balsam alone, and consider that the labour is much
lessened thereby.

26. A description of that mode of injection which is most generally
employed has now been given, but this is not the only method of
effecting our object. A most ingenious process was invented by M.
Doyers, requiring no artificial warmth, by which many beautiful objects
have been prepared. Make a solution of bichromate of potash, 524 grains
to a pint of water, and throw this into the vessels to be injected;
then take 1,000 grains of acetate of lead dissolved in half a pint
of water, and force this into the same vessels. A decomposition now
takes place in the vessels, and the yellow chromate of lead is formed.
In this decomposition, however, the acetate of potash also is formed
and as this salt has an injurious action upon the cells, Dr. Goadby
recommends nitrate of lead to be used, which preserves rather than
destroys them. He also advises the addition of two ounces of gelatine
dissolved in eight ounces of water, to eight ounces of the saturated
solution of each salt; but with this addition the hot-water bath would
be required to keep the injecting fluid liquid.

27. Many of these are best mounted in balsam, in the same manner as
those made in the ordinary way; whilst others are best shown when
preserved in liquids, for which purpose Goadby’s fluid may be employed.

28. This mode of making injections with chromate of lead is deemed by
many the best, especially where one colour only is employed. But it
must be allowed that there is a little more danger of failure where two
separate fluids are used for the same vessels.

29. We will now consider the best manner of making _transparent_
injections, which, for many purposes, possess an undoubted advantage
over the opaque ones. But it must be remembered that there are certain
subjects to which no transparent injection could be applied, as they
are too thick when in their natural state, and cutting would destroy
all that beauty which is shown by the different parts in their
relative adaptation. For those objects, however, which must be cut
into sections to display their wonders, or are naturally thin--such
as some of the finer tissues, livers, kidneys, &c.--transparency is a
great acquisition, and enables us to understand the arrangement of the
vessels more perfectly. Again, another advantage is the simplicity of
the process; no hot water is needed with some preparations, either for
the subject or the injecting fluid, which runs into the minute vessels
thoroughly and easily, whilst the cost is small.

30. For this kind of injection no colour is so commonly made use of
as Prussian blue. It is not a good one, as was before stated, for
any opaque object, as the light reflected from it appears almost
black; yet by transmitted light no colour is more useful, because
its distinctness is equally great by artificial light and ordinary
daylight. The method of preparing this, as given by Dr. Beale, is as
follows:--

  Glycerine                                     1 ounce.
  Wood naphtha, or pyroacetic acid              1½ drachm.
  Spirits of wine                               1 ounce.
  Ferrocyanide (yellow prussiate) of potash    12 grains.
  Tincture of sesquichloride of iron            1 drachm.
  Water                                         4 ounces.

Dissolve the ferrocyanide of potash in one ounce of the water;
add the tincture of sesquichloride of iron to another ounce. Mix
these solutions gradually together, shaking the bottle well which
contains them--it is best to add the iron to the potash solution.
When thoroughly mixed, these solutions should produce a dark-blue
mixture, perfectly free from any perceptible masses or flocculi. Next
mix the naphtha and spirits of wine, and add the glycerine and the
remaining two ounces of water. This must now be slowly mixed with the
blue liquid, shaking the whole well in a large bottle whilst the two
come together. The tincture of sesquichloride of iron is recommended,
because it can always be obtained of a uniform strength.

31. Dr. Turnbull used a mixture slightly different from the above,
which is made with the sulphate of iron:--

  Purified sulphate of iron                    10 grains.
  Ferrocyanide of potassium                    32 grains.
  Glycerine                                     1 ounce.
  Pyroacetic acid                               1½ drachm.
  Alcohol                                       1 ounce.
  Water                                         4 ounces.

Dissolve the sulphate of iron in one ounce of the water, gradually add
the ferrocyanide of potassium dissolved in another ounce, and proceed
as above.

32. Dr. Beale also gives us the following carmine injection to be
employed in the same way as the blue.[G] Take--

  Carmine                                               5 grains.
  Glycerine, with 8 or 10 drops of hydrochloric acid    ½ ounce.
  Glycerine (pure)                                      1 ounce.
  Alcohol                                               2 drachms.
  Water                                                 6 drachms.

Mix the carmine with a few drops of water, and when well incorporated
add about five drops of liquor ammoniæ. To this dark-red solution about
half an ounce of the glycerine is to be added, and the whole well
shaken in a bottle. Next, very gradually pour in the acid glycerine,
frequently shaking the bottle during admixture. Test the mixture with
blue litmus-paper, and if not of a very decidedly acid reaction, a few
more drops of acid may be added to the remainder of the glycerine and
mixed as before. Lastly, mix the alcohol and water very gradually,
shaking the bottle thoroughly after adding each successive portion
till the whole is mixed. This fluid may be kept ready prepared, and
injections made very rapidly with it.

    [G] When, however, it is desirable to cut very thin sections of
        the injected subject, the carmine is sometimes added to a
        solution of fine gelatine--gelatine one part to water eight
        parts. But the warm water and mode of proceeding which
        are used with the size solutions before described will be
        necessary in this case also.

33. The method of making injections with these colours is the same as
with the gelatine mixtures before described, except that no heat is
required, and consequently most of the trouble removed. The bottle of
the fluid must be well shaken immediately before use; and when the
object is injected, we must allow it to remain in a cool place for a
few hours before cutting it. Thin sections of the subject may be cut
with Valentine’s knife, as before described, and are very beautiful
transparent objects. Some of the finer tissues, also, are shown much
better by this mode of injection than by the opaque, and are easily
mounted by washing in clean water when first separated, and floating
upon a slide, where they must be allowed to dry thoroughly. They may
then be immediately mounted in balsam, or kept in the dry state until
it is convenient to finish them; but in many cases this keeping, if too
much prolonged, will injure the object. If it is desired to transfer
the section to another slide, it will be necessary to wet it thoroughly
with water by the aid of a camel-hair pencil, and then gently strip it
off with the forceps. When it is wished to preserve injected subjects
in “masses,” it must be done by immersion in spirit, and the sections
may be cut at leisure. Most of these transparent objects may be mounted
in Canada balsam; but some recommend glycerine or glycerine jelly, as
allowing the use of a higher power in their examination, and preserving
them in a more natural form.

34. A few subjects may be noticed which are very beautiful when
injected, and amongst these are the eyes of many animals. They
must be injected by the artery in the back part, and when the blue
transparent liquid is employed, nothing can exceed the delicate beauty
which some of the membrane bears. It must, however, be dissected with
care, but well repays us for the trouble. Water-newts and frogs are
not difficult subjects, and in their skin and other parts are many
interesting objects. Amongst the commoner animals--rats, rabbits, cats,
&c. &c.--almost endless employment may be found, making use either of
portions or the whole animal at once. The intestines of many of these
are very beautiful. We must divide them with a pair of scissors along
the tube, and cleanse them from all the matter; the coating may then be
laid upon a slide, and any remaining impurity removed with a camel-hair
pencil and water. When dried it may be mounted in balsam, and having
been injected with the transparent blue, its minute beauty is shown
most perfectly. In injecting a sheep’s foot, which is a good object,
the liquid should be forced into it until a slight paring of the hoof
shows the colour in the fine channels there.

35. When the lungs of small animals are injected, the finest fluid must
be used, as some of the capillaries are so small that it is not an easy
matter to fill them properly. And before entering upon these subjects,
a certain proficiency in the mode of using the syringe, &c., should be
obtained by practising upon simpler parts.

36. No subjects are more difficult to inject than fish, owing to the
extreme softness of their tissues. Dr. Hogg recommends the tail of the
fish to be cut off, and the pipe to be put into the divided vessel
which lies just beneath the spinal column; by which method beautiful
injections may be made. The gills, however, are the most interesting
part as microscopic objects.

37. These instructions may seem very imperfect to those who have had
much experience in this branch; but they will remember that their
own knowledge was not gained from any written descriptions, but was
forced upon them by frequent failures, some of which probably were
very disheartening. As I before stated, it is very difficult (if not
impossible) to accomplish much without some knowledge of anatomy.

38. I may here mention that the transparent injections sent over from
the Continent are beautifully executed by Hyrtl of Vienna (who states
that the injected fluid is composed of gelatine and carmine), Dr.
Oschatz of Berlin, the Microscopic Institute of Wabern, Schaffer and
Co. of Magdeburg, and others. Some of these will bear examining with a
high power. A friend informs me that he measured a vessel in a rat’s
tongue by Hyrtl, which was 1-7200th of an inch in diameter, and had
a clear outline with quarter-inch objective. He has also made many
experiments with the same materials, but has as yet failed in producing
perfectly distinct outlines, there being a tendency of the colouring
matter (magenta, carmine. &c.) to diffuse itself through the coats of
the vessels into the surrounding tissues, although he has varied the
pressure from one half a pound to sixty pounds. He believes the vessels
are first washed out (injected with warm water and pressure applied),
then some fluid introduced which renders the arteries impervious to the
coloured fluid afterwards injected.

39. He finds that after washing out the vessels as above, the injecting
fluid is much more easily introduced. He has used a strong solution
of gallic acid previously to injecting with the colouring matter
(in one experiment only), and the result was satisfactory. He puts
the query,--Might not carbolic acid have a similar effect? He has
often used it with injections to preserve the specimens, but not in
sufficient quantity to act in the way indicated above.

Since writing the above, Mr. J. G. Dale, F.C.S., and I have made
numerous experiments with carmine injection, and have at length
been favoured with what we deem success. Some of the vessels in a
kitten lately injected do not exceed 1-2000th of an inch in diameter,
and present a clear outline with one-fifth objective. There is no
extra-vasation, neither does the colouring matter show any grain except
when a very high power is employed. The following is our process:--

  Take 180 grains best carmine.
         ½ fluid ounce of ammonia, commercial strength, viz.,
                0·92, or 15° ammonia meter.
         3 or 4 ounces distilled water.

Put these into a small flask, and allow them to digest without
heat from twenty-four to thirty-six hours, or until the carmine is
dissolved. Then take a Winchester quart bottle, and with a diamond mark
the spot to which sixteen ounces of water extend. The coloured solution
must be filtered into the bottle, and to this pure water should be
added until the whole is equal to sixteen ounces.

Dissolve 600 grains potash alum in ten fluid ounces of water, and add
to this, under constant boiling, a solution of carbonate of soda until
a slight permanent precipitate is produced. Filter and add water up
to sixteen ounces. Boil and add the solution to the cold ammoniacal
solution of carmine in the Winchester quart, and shake vigorously for
a few minutes. A drop of this placed upon white filtering-paper should
show no coloured ring. If much colour is in solution the whole must
be rejected, because, although it is possible to precipitate all the
colouring matter by the addition of ammonia or alum, it is not well to
do so, as the physical condition of the precipitate is thereby altered.

Supposing the precipitation to be complete, or very nearly so, shake
vigorously for at least half an hour, and allow it to stand until quite
cold. The shaking must then be renewed for some time, and the bottle
filled up with pure water.

After allowing the precipitate to settle a day, draw off the clear
supernatant fluid with a syphon. Repeat the washing until the clear
liquid gives little or no precipitate with chloride of barium. So much
water must be left with the colour at last that it shall measure forty
fluid ounces.

For the injecting fluid take twenty-four ounces of the above coloured
liquid, and three ounces of good gelatine. Allow these to remain
together twelve hours, and then dissolve by the heat of a water bath;
after which it should be strained through fine muslin.

As this injecting fluid contains gelatine, the hot water, and other
contrivances mentioned in a former part of the chapter, will be
necessary here also, but no peculiar treatment will be required.




CHAPTER VII.

MISCELLANEOUS.


It must be evident to all readers that there are various objects
of interest to the microscopist which cannot be properly placed
amongst any of the forementioned classes, but must not be omitted in
such a guide as this professes to be. Of these may be mentioned the
circulation of the blood in various animals, the rotary motion of the
fluid in many plants, the best means of taking minute photographs, &c.
&c.

Perhaps the most interesting of these objects is the circulation of
the blood through the finer vessels of various parts of the animals
made use of for these purposes, which parts, it is evident, must be
very transparent to afford a perfect view of this phenomenon. The
web of the frog’s foot is very frequently made use of, but requires
a certain arrangement, which we will now describe. A piece of thin
wood (Dr. Carpenter recommends _cork_) is taken, about eight inches
long and three wide; about an inch from one end is cut a hole, half or
three-quarters of an inch in diameter. The body of the frog is then
placed in a wet bag, or wrapped in wet calico, whilst the hind-foot
projects; the whole is then laid upon the piece of wood so that the
foot, which is left free, may be extended over the hole. The web must
then be spread out, and secured either by threads to small pins on
the wood, or the pins must be driven through the web into the wood,
and so kept in position. A few bands of tape must be passed round the
body, the leg, and the wood, to prevent any disarrangement arising from
the animal starting, &c. Care must be taken that the tape is not too
tight, else the circulation will be very slow or altogether stopped.
The wood must now be fixed upon the stage, with the aperture under
the object-glass: this is sometimes done by simply binding it, or a
spring is fixed so as to accomplish the same object without so much
trouble. With a half-inch power the blood may now be seen to flow very
distinctly. The frog may be used for hours if care is taken to prevent
the web from becoming dry, by wetting it with a little water from time
to time. The piece of wood or cork upon which the frog is laid is
often made to give place to the “frog-plates,” supplied by opticians.
These are made of brass, somewhat resembling the piece of wood above
recommended, but each maker’s pattern differs according to his own
taste.

The tongue of the frog is also sometimes used for the purpose of
showing the circulation of the blood, which is done in the following
manner:--The body is wrapped with the calico, and made fast to the
plate as before, only the _mouth_ of the frog is brought to the
opening. The tongue is then gently drawn out of the mouth and pinned
down over the aperture, when the circulation will be well shown. But,
as Dr. Carpenter observes, the cruelty of this mode of treatment is so
repulsive that it is unjustifiable.

Tadpoles of the frog (which, of course, are only obtainable in their
season) are good subjects for showing the circulation of the blood.
They are best suited for the microscope when about one inch long. The
tadpoles of the newt and toad also are equally suitable. They may
be placed in a very shallow glass trough with a little water, and a
narrow band of linen bound lightly round in some part not required for
examination, to keep them from moving; or they may be laid upon a glass
plate with a drop or two of water, and a thin glass covering lightly
bound upon it. Dr. Carpenter, however, places them first in cold water,
gradually adding warm until the whole becomes about 100°, when the
tadpole becomes rigid, whilst the circulation is still maintained. I
have not, however, found this necessary, the thin glass accomplishing
all that is desired. The tail is generally the most transparent, and
shows the circulation best; but in some of the newt larvæ the blood may
be traced down to the very extremities if they are not _too old_. Mr.
Whitney places the tadpole upon its back, by which means the heart and
other internal arrangements may be seen.

Amongst fishes also may be found subjects for the same purpose, but
they seldom furnish as good examples as those before mentioned, because
the blood-vessels are not nearly so abundant, as in the foot of the
frog, &c. The stickleback is, however, procurable almost in any place
during the summer months, and may be laid in a shallow trough, loosely
bound down as the tadpole. The tail may be covered with a piece of thin
glass to prevent him curling it to the object-glass. The power needed
with this will be about the same as with the other subjects--viz., half
to quarter inch object-glass.

It is not absolutely necessary to go to reptiles or fishes for this
curious sight, as some other animals serve very well. In the wings of
the common bat may be found a good subject. These must be stretched out
on something resembling the frog-plate before described, when those
parts near to the bones will show the _largest_ vessels very clearly.
The ear of a young mouse is an illustration of the same phenomenon, but
it is very difficult to fix it in a good position, as these animals are
so very timid and restless.

Amongst insects also the same law may be observed, by placing them in
the “cage,” or “live-box,” so as to keep them still, but not to injure
them by too much pressure. In certain larvæ this is particularly well
shown, as that of the day-fly and plumed gnat; but in some of these
the blood is almost colourless. In the wings also of many insects this
circulation is well seen, as in those of the common housefly; but as
these parts become dry in a few days, the subject should not be more
than twenty-four hours old.

Somewhat approximating to the forementioned phenomenon, is the
“rotation” of fluid in the cells, or, as it is usually termed, the
_circulation of the sap_, of plants. This is shown in certain vegetable
growths as a constant stream of thick fluid, wherein small globules
are seen; which stream flows round the individual cells, or up the
leaf, turning at the extremity, and down again by a different but
parallel channel. There is little or no difficulty in showing this in
many plants; but some are, of course, better than others, and require
a different treatment; we will, therefore, notice a few of these.
Perhaps the best of all is the _Vallisneria spiralis_, which is an
aquatic plant, frequently grown in, but not really belonging to, this
country. As it somewhat resembles grass, the leaf is not used in its
natural state, but a thin section cut lengthwise with a razor or other
sharp instrument--this section, however, is much better when the outer
surface has been first removed. It should then be laid upon a slide
with a drop or two of water, and covered with a piece of thin glass.
Often the cutting of the section seems to be such a shock to the leaf
that no motion is visible for awhile, but in a short time the warmth
of an ordinary sitting-room will revive it, and with a quarter-inch
object-glass the currents will be rendered beautifully distinct.
Where the “stream” is unusually obstinate the warmth may be slightly
increased, but too great heat destroys the movement altogether. In the
summer, any of the leaves show this “circulation” very well; but in the
winter, the slightly yellow ones are said to be the best.

The Vallisneria requires to be cut in sections to show this
“circulation;” but there are many plants of which it is but necessary
to take a fragment and lay it upon a slide. The _Anacharis alsinastrum_
is one of these: it grows in water, having three leaves round the
stem, then a bare portion, again another three leaves, and so on. One
of these leaves must be plucked close to the stem, and laid upon a
slide with a drop of water. Thin glass should be placed upon it, and
along the mid-rib of the leaves the “circulation” may be seen most
beautifully when a good specimen has been chosen; but it requires
rather more power than the Vallisneria. This plant is very common in
many parts of the country, a great number of our ponds and streams
being literally choked up by it. In the _Chara vulgaris_ and two or
three of the Nitellæ, &c., this phenomenon may also be seen with no
preparation except plucking a part from the stem and laying it upon a
slide as with the Anacharis. In using the Frog-bit, the outer part of
the young leaf-buds must be taken to obtain the best specimens for this
purpose; but a section of the stem will also show the “circulation,”
though not so well. The plants before mentioned are all aquatic, but
the same movement of the globules has been observed in several kinds
of land plants, as in the hairs upon the leaf-stalks of the common
groundsel; but these do not show it so well, nor are they so easily
managed as the above.

Many microscopists who are not fortunate enough to be in the
neighbourhood of these plants (indeed the Vallisneria is a foreign one)
grow them in jars, so a few remarks as to the treatment they require
will not be out of place. The Vallisneria requires a temperature not
lower than 55° or 60°, and even a higher degree than this renders its
growth quicker; and no great change must take place: the more equable
the temperature the more healthy will the plant be. A glass jar should
be taken, having an inch or two of mould at the bottom, which must be
pressed down closely, and the plant must be set in this. Water must
then be gently poured in, so as not to disturb the mould. As this plant
flourishes best when the water is frequently changed, Mr. Quekett
recommends that the jar should be occasionally placed under a tap of
water, and a very gentle stream allowed to fall into it for several
hours, by which means much of the confervoid growth will be got rid of
and the plant invigorated. The Anacharis may be rooted in the earth
like the Vallisneria, but a small detached piece may be thrown into the
jar of water and there left until wanted. For months the “circulation”
will be well shown by it, and it will probably grow and increase. It
is also very healthy in an in-door aquarium. It is recommended that
the jars in which any of the _Chara_ are grown should be moved about
as little as possible, as the long roots are very tender, and will not
bear agitation.

An object which is interesting to the microscopist, as well as the
unscientific observer, is the _growth_ of seeds, as it is often
erroneously termed. A shaving of the outside of the seed is taken and
laid upon the glass slide; a thin glass cover is then placed upon it,
and a drop of water applied to the edge of this. The water will then
gradually flow under the glass and reach the section of the seed,
when the transparent fibres will appear to spring out and “grow” for
some minutes. This, however, is produced by the unfolding of a spiral
formation in the cells, and, therefore, has really no similarity to the
true growth. The seeds of the Salvias, Collomias, Senecio, Ruellia,
&c., are well suited for the display of this curious sight.

To watch the development of the spores of ferns, and the fertilization
and products, Dr. Carpenter recommends the following mode of
proceeding:--“Let a frond of a fern, whose fructification is mature,
be laid upon a piece of fine paper, with its spore-bearing surface
downwards; in the course of a day or two this paper will be found
to be covered with a very fine brownish dust, which consists of the
discharged spores. This must be carefully collected, and should be
spread upon the surface of a smoothed fragment of porous sandstone;
the stone being placed in a saucer, the bottom of which is covered
with water, and a glass ‘tumbler’ being inverted over it, the
requisite supply of moisture is insured, and the spores will germinate
luxuriantly. Some of the prothallia soon advance beyond the rest;
and at the time when the advanced ones have long ceased to produce
antheridia, and bear abundance of archegonia, those which have remained
behind in their growth are beginning to be covered with antheridia.
If the crop be now kept with little moisture for several weeks and
then suddenly watered, a large number of antheridia and archegonia
simultaneously open, and in a few hours afterwards the surface of
the larger prothallia will be found almost covered with moving
antherozoids. Such prothallia as exhibit freshly opened archegonia are
now to be held by one lobe between the forefinger and thumb of the
left hand, so that the upper surface of the prothallium lies upon the
thumb; and the thinnest possible sections are then to be made with a
narrow-bladed knife perpendicularly to the surface of the prothallium.
Of these sections, which after much practice may be made no more than
1-15th of a line of thickness, some will probably lay open the canals
of the archegonia, and within these, when examined with a power of 200
or 300 diameters, antherozoids may be occasionally distinguished.”

Another interesting object to the young microscopist is afforded by the
spores of the equiseta (or horsetails, as they are often called). These
may be obtained by shaking the higher portion of the stems when the
spores are ripe. They will then fall like small dust, and may be placed
under the microscope. The spores are then seen to consist of a somewhat
heart-shaped mass with bands rather intricately curled around it. As
they dry these bands expand, and are seen to be four lines at right
angles, with the ends clubbed, as it may be called. If, whilst watching
them, the spores are breathed upon, these bands immediately return to
their former state, and are closely curled around the spore; but as
they gradually dry again expand. This experiment may be repeated many
times, and is a very interesting one.

The above are the principal objects which could not possibly be
included in any of the former chapters, but would have left a most
interesting branch untouched had it been neglected. There is another
subject also which should not be passed by--viz., the production of
minute pictures which serve as objects for microscopic examination.

I may here mention that as this manual is simply to enable the
young student to prepare and mount his objects, the photography of
_magnified_ objects has evidently no place here.

Few slides caused so much astonishment as these minute photographs when
first exhibited; small spots were seen to contain large pictures, and a
page of printed matter was compressed into the one-hundredth part of a
square inch. It would be impossible in this place to give the inquirer
any instruction in the manipulation of photography, so it must be
assumed that he already knows this.

We will first consider the process performed by artificial light. The
collodion employed in photographing generally shows as much structure
when magnified as is found in linen of moderate texture; but this is
not always the case, as some samples bear much enlargement without any
of this appearance. It is evident that a structure so coarse would make
it entirely unfit for these minute pictures, as all the small markings
would be destroyed, or so interfered with that no great enlargement
would be practicable. To obtain almost structureless collodion is not
an easy matter, and a clever practitioner in this branch of photography
states that he knows of no method to accomplish this with certainty,
but he himself tries different samples until he falls upon a suitable
one, which he then lays aside for this object. A beneficial effect
is often derived from keeping the collodion awhile, but this is not
always the case. The slides should be chosen of an equal thickness,
so that when focussed upon one no re-adjustment may be necessary for
the others. The glass should, of course, be free from any roughness,
scratches, or other imperfections, and of first-rate quality and colour.

The microscope must then be placed in a horizontal position, and the
eye-piece removed, the stage having a small clip upon it to keep the
prepared plate in position. The negative must then be supported at a
distance from the end of the microscope tube from which the eye-piece
was withdrawn. This distance will, of course, vary according to the
relative sizes of the negative and desired picture. With a one-inch
object-glass, which is a very convenient focus, it will have to be
changed usually betwixt one and four feet. The negative must be lighted
by an argand gas-burner or camphine-lamp, and the rays rendered as
parallel as possible by the use of a large lens placed betwixt the
light and the negative. It is not easy to arrange the apparatus so as
to get the light _uniform_; but a little practice will soon do away
with this difficulty. Ordinary ground-glass is too coarsely grained
to focus upon, as the magnifying power used to examine the minute
reflection must be considerable. One of the slides must therefore be
coated with the collodion, submitted to the silver-bath, and after
washing with water be allowed to dry. Upon this may be focussed
the reflected image, and its minuteness examined with a powerful
hand-magnifier, or another microscope placed behind in a horizontal
position. When the utmost sharpness of definition is obtained, it
is usually required to remove the plate a little distance from the
object-glass, as object-glasses for the microscope are slightly “over
corrected,” and the chemical rays which accomplish the photography
are beyond the visual ones. The exact distance required to give a
picture to bear the greatest enlargement cannot be given by rule; but
experiments must be made at first, and it will always be the same with
the object-glass which we have tested.

The plate may now be prepared as in ordinary photography, and placed
upon the stage whilst the light is shaded. When all is ready, the
shade is removed and the process allowed to go on, usually for thirty
or forty seconds; but no certain rule can be given as to the required
time, on account of various collodions, lamps, and powers being used.
It may be here mentioned, that it is well to contrive some little frame
to receive the prepared plate, as the silver bath solution is liable to
get upon the microscope stage and so, to say the least, disfigure it.
When the exposure has been continued sufficiently long, the picture
may be developed by any of the ordinary methods, but some of the best
productions have been brought out by the aid of pyrogallic and citric
acid solution, with the addition of a little alcohol. The “fixing” may
be effected by a strong solution of hypo-sulphite of soda, and the
picture must then be very well washed with pure water. When dry, the
photograph must be mounted with Canada balsam, in the same manner as
any ordinary object; but great heat must not be used, or the picture
may be injured.

When ordinary daylight is employed for this purpose, a dark slide
will be required for the prepared plate, in the same way as for
photographing landscape, &c. These dark slides are generally made by
each individual to suit his particular arrangements of negatives, &c.;
but it may be here recommended that the operator should always focus in
the same slide which he is about to use, as so small a difference in
distance lies betwixt perfection and failure.

For an ordinary student, perhaps the above method is that which is the
most readily used, and consequently the most generally available; but
almost every one has a different arrangement of microscope, &c., by
which he procures these minute pictures. Mr. Shadbolt (one of our most
successful photographers) gives the following instructions:--“Having
removed the upper stage plate of a large compound microscope, I replace
it with one of wood, supplied with guide pins of silver wire, in order
to admit of its supporting a slip of glass coated with collodion, and
excited in the nitrate of silver bath in the usual way. If the ordinary
brass stage plate were left undisturbed, it is obvious that it and the
excited slip of glass would be mutually destructive.

“The microscope is now to be placed in a horizontal position, the
objective, intended to produce the picture, made to occupy the place
usually filled by the achromatic condenser on the _sub-stage_ of the
microscope, while _another_ objective is screwed into the lower end of
the body of the instrument, which is used not only to focus with, but
also to make the requisite allowance for actinic variation.

“The negative intended to be reduced is then arranged vertically, with
its centre in the axis of the microscopic body, at a distance of from
two to four feet from the lower object-glass, and with a convenient
screen of card, wood, or thick paper, to cut off any extraneous light
that would otherwise pass beyond the limits of the picture.

“A small camphine-lamp is employed for the purpose of illuminating the
negative, having a good bull’s-eye lens as a condenser, so arranged
with its flat side next the lamp that the refracted rays shall just
fill the whole of a double convex lens of about six inches in diameter,
the latter being placed in such a position as to refract the rays of
light in a parallel direction upon the negative. By this arrangement
the _bull’s-eye lens_ of about two inches and a half in diameter
_appears_ as the source of the light instead of the small flame of the
lamp.

“By using a bat’s-wing gas-burner of a good size, a _single_ lens,
instead of the two, may be so placed as to give the necessary
uniformity of illumination.”

This arrangement requires the same care in working as that before
mentioned, the pictures being produced, developed, and fixed by the
same treatment.

As before stated, almost every manipulator makes some small changes in
the method of producing these minute pictures; but the rules given,
though far from new, are sufficient for all purposes; and I may state
with truth, that those which I procured when these wonders were quite
new, are fully equal in every respect to the best usually met with at
the present time.

With these instructions I shall close my Handbook, as I believe that
every branch of Preparation and Mounting of Microscopic Objects has
been treated of. Not that the beginner can expect that he has nothing
to do except read this to be able to mount everything; but there are
difficulties from which he may be freed by instruction, when otherwise
he would have been compelled to learn by failure alone. I may, here,
however, repeat certain advice before given,--that, when practicable,
it is a good thing to mount each object by two or more different
methods, as very frequently one feature is best shown dry, another in
liquid, and a third in balsam. Secondly, let the _mounting_ be studied
thoroughly, as no part of the microscopic science is more worthy of
thought than this. And lastly, let no failures prevent you following
up what will assuredly one day become a source of great pleasure, and
render your daily “constitutional walk,” which is often dull in the
extreme, very delightful, as it will afford you some new wonder in
every hedge-row.




INDEX.


  Air-bubbles, 56.

  Air-pump, 13;
    use of, 57.

  Alcock, Dr., on tongues of Mollusca, 118.

  Algæ, mounting of, 91.

  Anacharis alsinastrum, rotation in, 143;
    to cultivate, 144.

  Animal tissues, dissection of, 114.

  Antennæ of insects, 71;
    in preservative liquid, 94.

  Apparatus required in mounting objects, i.

  Arteries, how to distinguish, in injection, 131.

  Asparagus, spiral vessels of, 114.

  Asphaltum, 15;
    and india-rubber, as cement and varnish, 18.


  Bat, circulation of blood in wings of, 142.

  Beale’s, Dr., Prussian blue for injecting, 134;
    carmine, 135.

  Bell-glasses, use of, 12.

  Berg-mehl, 40.

  Bermuda earth, 40.

  Bichromate of lead, injection with, 132.

  Bird, Dr. Golding, on preparation of Zoophytes, 65.

  Black-japan, as a cement, 16.

  Black varnish, 18.

  Blood, as a microscopic object, 50;
    circulation of, 140–142.

  Bone, sections of, 102;
    fossil, 103.

  Brass plate for heating glasses, &c., 13.


  Cactaceæ, raphides of, 52.

  Camel-hair pencils, use of, 10.

  Canada balsam, 14;
    with chloroform, 14, 60, 132;
    with turpentine, 14;
    air bubbles in, 56;
    Dr. Carpenter’s syringe for, 59;
    to fill cells with, 60;
    mounting of objects in, 56.

  Carbolic acid, 86.

  Carmine injecting fluid, Dr. Beale’s, 135:
    Dale & Davies’, 138.

  Cartilage, dissection of, 114.

  Castor oil, as a preservative, 86.

  Cells for dry objects, 6;
    with rings of cardboard, 7;
    with gutta-percha, 7;
    with leather, 7;
    with ivory, 8;
    with thin glass, 7;
    with varnish, 23, 26;
    for balsam, 60;
    for preservative liquids, 87–89;
    Shadbolt’s turntable useful in making, 9.

  Cements, 13–17.

  Chalk, foraminifera from, 63.

  Chara vulgaris, rotation in, 144.

  Chloride of zinc, solution of, 85;
    of calcium, solution of, 92.

  Chloroform, use of (_see_ Canada balsam).

  Circulation of blood, 140–142;
    of sap (_see_ Rotation).

  Cleanliness in microscopic work, 1.

  Coal, sections of, 99.

  Collection of diatoms, 29.

  Colours for injection, 125–127.

  Condenser, cheap, to make, 20.

  Corals, sections of, 99.

  Corallines, to mount, 48.

  Cover of objects, to remove, 60.

  Crystals, mounting of, dry, 50;
    to vary form of, and mount in balsam, 75;
    sections of, 109.

  Cuticle of equisetum, &c., 80.


  Dale & Davies’, carmine injection, 138.

  Deane’s gelatine, 84.

  Decalcifying process for shells, 98.

  Desmidiaceæ, in preservative liquid, 92.

  Diachæa elegans, 53.

  Diamond beetle, 54.

  Diatomaceæ, nature of, 28;
    collection of, 29;
    in stomachs of fish, 30;
    to prepare and mount dry, 30–40;
    Mr. Rylands on, 33;
    in guano, 39;
    fossilized, 40;
    mounting in balsam, 61;
    mounting in preservative liquids, 92, 93.

  Discs used in mounting, 22;
    supporter, Smith & Beck’s, 23.

  Dissection, 111;
    microscope for, 111;
    instruments for, 112.

  Distilled water, as preservative liquid, 83.

  Doyer’s, M., method of injection, 132.

  Dry objects, to mount, 22.


  Echinodermata spines, sections of, 99.

  Eel, scales of, 78.

  Eggs of insects, 94.

  Elastic objects, to keep flat upon the slide, 11.

  Electrical cement, 16.

  Equisetaceæ, 80;
    spores of, 146.

  Erector, not indispensable, 19.

  Eyes of insects, 70, 54;
    of animals injected, 136.


  Feathers, 73.

  Feet of insects, 54, 71.

  Ferns, 51;
    development of spores of, 145.

  Fish, fins and tails of, 50;
    scales of, 53, 78;
    injection of, 137;
    circulation of blood in, 142.

  Flint, sections of, 101.

  Fluid, mounting objects in, 83.

  Flustra avicularis, 79.

  Fly (_see_ Insects).

  Foot of sheep injected, 136.

  Foraminifera, 41;
    separation of, 41;
    to clean, 42;
    to clean from tallow soundings, 42;
    to mount, 45, 63;
    from chalk, 63.

  Forceps, ordinary and wooden, 11;
    bull-nosed, for injection, 123.

  Fossil infusoria, 40.

  Frog, injected, 136;
    to show circulation of blood, 140.

  Frog-bit, rotation in, 144.

  Fruit-stones, sections of, 104.

  Fungi, 53, 94.


  Gastric teeth of insects, 120;
    of molluscs, 117.

  Gelatine for injection, 124;
    preservative liquid, 84.

  Glass slides, 1;
    thin, 3;
    thin, to cut, 3;
    thin, to measure, 4;
    thin, to clean, 5;
    cells, to make, 88;
    rings for cells, 88;
    tubes, 10.

  Glycerine, 84, 94;
    jelly, 84, 91.

  Goadby’s fluid, 85.

  Gold-size, 15.

  Grasses, 80.

  Grasshopper, gizzard of, 120.

  Guano, containing Diatomaceæ, 39.

  Gum-water, and modifications of, 17.

  Gutta-percha cells, 7;
    for liquids, 87.


  Hairs, vegetable, 46;
    to mount dry, 54;
    to mount as polarizing objects, 79;
    sections of, 105.

  Hepworth, Mr., on mounting insects, 68.

  Horn, sections of, 104.

  Hot-water bath, use of, 58.


  Infusoria, in preservative liquid, 92;
    fossil, 40.

  Injections, vi., 122;
    apparatus for, 122;
    colours for, 125;
    directions for, 127;
    with various colours, 131;
    mounting of, 131;
    transparent, 133, 138.

  Insects, scales of, 48;
    to mount, 49;
    legs and feet of, 54, 72;
    eyes of, 54, 70;
    Mr. Hepworth on mounting, 68;
    antennæ of, 71;
    mouth of, 72;
    tracheæ and spiracles of, 72, 115;
    parasitic, 73;
    in preservative liquid, 94;
    eggs of, 94;
    gizzard of, 120;
    circulation of blood in, 142.

  Intestines, injected, 136.


  Knives for dissecting, 112;
    Valentin’s, 108.


  Labelling of objects, 19.

  Lamps, for mounting, 12.

  Larvæ, skins of, 50.

  Leaves, sections of, 107;
    scales of, 46, 81.

  Liquid-glue, 16.

  Lungs of animals injected, 137.


  Mallow, pollen of, 47.

  Marine glue, 15, 88.

  Microscope for dissection, 111.

  Miscellaneous, vii., 140.

  Mites, 73.

  Molluscs, tongues of, 116.

  Mosses, 51;
    in preservative fluids, 91.

  Mould (_see_ Fungi).

  Mounting objects, apparatus for, i.;
    dry, ii., 22;
    in Canada balsam, iii., 56;
    in cells, iv., 83.

  Mouse, ear of, 114;
    circulation of blood in, 142.

  Mouth of insects, 72.

  Muscle, dissection of, 114.


  Needles, how to mount, 10;
    for dissection, 112;
    curved, for injection, 123.

  Nervous tissue, dissection of, 115.

  Nettle leaf, 52.

  Newts, injected, 136.


  Onion, raphides of, 52.

  Orbitolite, section of, 97.

  Oxalurate of ammonia, crystals of, 75.


  Palates of Molluscs (_see_ tongues).

  Papers, ornamental, to cover slides, 8, 27.

  Photographs, microscopic, to produce, 147;
    Mr. Shadbolt on, 149.

  Pipes for injecting syringe, 122.

  Podura, scales of, 49.

  Polariscope, objects for, 74–82, 104.

  Pollen, 47, 74.

  Polycystina, preparation and mounting of, 63.

  Preservative liquids, iv., 83;
    cells suited for, 87–89.

  Prussian blue for injection, 134.


  Raphides, vegetable, 52, 80.

  Rhinoceros, horn of, 104.

  Rhubarb, spiral vessels of, 113.

  Rings and cross of crystals, 109.

  Rotation of fluid in cells of plants, 143–145.

  Rush, section of, 108.

  Rylands, Mr. T. G., on Diatomaceæ, 33, 93.


  Salicine, crystals of, 76.

  Saw of watch-spring, 97.

  Scales of fishes, 53, 78;
    of leaves, 46, 81;
    of insects, 48.

  Scissors, 10;
    for dissection, 112.

  Sea-mats, 53.

  Sea-soundings, to cleanse, 42.

  Sealing-wax varnish, 17.

  Sections, 96;
    of shells, 97;
    of orbitolite, 97;
    of spines of Echinodermata, 99;
    of corals, 99;
    of coal, 99;
    of flint, 101;
    of teeth, 101;
    of bone, 102;
    of fruit-stones, 104;
    of horn, 104;
    of whalebone, 105;
    of hairs, 105;
    of wood, 106;
    of leaves, 107;
    of sponges, 108;
    of skin, 109;
    of crystals, 109;
    of seeds, 111.

  Seeds, 47, 74;
    sections of, 111;
    growth of, 145.

  Shadbolt’s turntable, 9.

  Shells, sections of, 97;
    decalcifying, 98;
    laminæ of, 98.

  Siliceous cuticles, 80.

  Size for injection, 124.

  Skins of larvæ, 50;
    sections of, 109;
    sole, 54.

  Slides, glass, for mounting objects, 1;
    glass, to clean, 2;
    wood, &c., 6;
    most useful, 8;
    to cover and varnish, 27.

  Spicula, from sponges, &c., 67.

  Spines of Echinus, 99.

  Spiracles of insects, 72, 116.

  Spiral vessels of vegetables, 113.

  Split bristles, use of, 10.

  Sponges, sections of, 108.

  Spores of ferns, development of, 145;
    equisetum, 146.

  Starch, preparation and mounting of, 79.

  Sulphate of copper and magnesia, crystals of, 76.

  Syringe for Canada balsam, 59;
    for dissection, 113;
    for injection, 122.


  Tadpole, to show circulation of blood of, 141.

  Teeth, sections of, 101.

  Thin glass, to cut, 3;
    to measure thickness of, 4;
    to clean, 5.

  Thwaites’ preservative liquid, 85.

  Ticks, 73.

  Tissues, animal and vegetable (_see_ Dissection).

  Tongues or palates of Molluscs, 116.

  Tracheæ of insects, 72, 115.

  Transfer of objects, 10.

  Trough for dissection, 113.

  Tubes, glass, 10.

  Turnbull’s, Dr., Prussian blue for injection, 134.

  Turpentine, use of, 57.


  Universal stand, to make, 20.


  Valentin’s knife, 108.

  Vallisneria spiralis, rotation in, 143;
    to cultivate, 144.

  Varnishes, 17.

  Vegetable objects, to mount dry, 46;
    to mount in jelly, 91;
    dissection of, 113.


  Watch-glasses, 12.

  Whalebone, sections of, 105.

  Wood, sections of, 106.


  Zoophytes, to mount dry, 53;
    Dr. Golding Bird on mounting, in balsam, 65;
    as polarizing objects, 79.


COX AND WYMAN, PRINTERS, GREAT QUEEN STREET, LONDON.




Transcriber’s Notes


Punctuation and spelling were made consistent when a predominant
preference was found in the original book; otherwise they were not
changed.

Inconsistent hyphenation was not changed.

Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.

The Table of Contents was added by the Transcriber.

The index was not checked for proper alphabetization or correct page
references. Roman numeral references are to chapters, not to pages.

Page 106: =T= indicates a boldface, sans-serif “T”.