COLOUR VISION

[Illustration: TYPES OF COLOUR VISION.

  W. DE W. A., DEL.      W. GRIGGS, LITH.
]




                             COLOUR VISION


                                 BEING

                          THE TYNDALL LECTURES
                           DELIVERED IN 1894
                                   AT
                        _THE ROYAL INSTITUTION_


                                   BY
              CAPT. W. DE W. ABNEY, C.B., D.C.L., F.R.S.,
                          LATE ROYAL ENGINEERS


               WITH COLOURED PLATE AND NUMEROUS DIAGRAMS


                                NEW YORK
                        WILLIAM WOOD AND COMPANY




CONTENTS.


                                                                      PAGE

  PREFACE                                                              vii


  CHAPTER I.

  The Eye                                                                1


  CHAPTER II.

  Simple Colours and their Mixture                                      15


  CHAPTER III.

  Three Colour Sensations Possible                                      32


  CHAPTER IV.

  The Young and Hering Theories of Colour Vision                        41


  CHAPTER V.

  General Aspect of Colour Blindness                                    58


  CHAPTER VI.

  Colour Blindness exhibited by Colour Discs and exhibited by
      Luminosity Curves of the Spectrum                                 74


  CHAPTER VII.

  Luminosity of Colours to Different Parts of the Retina                88


  CHAPTER VIII.

  Luminosity of a Feeble Spectrum and the Limit of the Perception
      of Colour                                                         98


  CHAPTER IX.

  The Extinction of Light from the Spectrum                            108


  CHAPTER X.

  The Extinction of the Perception of Light by the Colour Blind        122


  CHAPTER XI.

  Tobacco Blindness                                                    137


  CHAPTER XII.

  Examples of Colour Blindness due to Disease                          148


  CHAPTER XIII.

  The Holmgren Test for Colour Blindness                               167


  CHAPTER XIV.

  The Spectrum Test for Colour Blindness                               180


  CHAPTER XV.

  The Young and Hering Theories of Colour Vision Compared              187


  APPENDIX                                                             201


  INDEX                                                                229




PREFACE.


The writer had for some years past, in conjunction with General
Festing, and recently as Secretary and Member of the Colour Vision
Committee of the Royal Society, carried out a series of investigations
on colour vision, and selected that subject when he was invited, in
1894, to deliver the Tyndall Lectures at the Royal Institution.

The brief time allotted for these lectures--an hour on three successive
Saturday afternoons--restricted the discussion of some aspects of the
question, and confined its treatment in the main to those features most
readily explicable by the physicist, and to bringing into notice the
latest results which had been obtained from physical experiments. How
far the writer has succeeded in the task which he then outlined it is
for the reader to determine.

There was no intention in the first instance to publish these lectures.
After their delivery, many persons expressed a desire that the
information they contained should be rendered accessible to such as
were interested in the theory of colour vision, and in deference to
that desire the lecture-notes have been re-cast in book form. For the
reader’s convenience the matter is now divided into chapters instead
of into lectures, and a few additions have been made in the text to
explain some of the experimental work to those who have not facilities
for its repetition.

The writer has to acknowledge several debts of gratitude. First, to Mr.
E. Nettleship, for his kindness in looking over the proofs, and making
valuable suggestions whilst the work was passing through the press; and
also, as will be seen throughout its pages, for many of the interesting
cases of defective colour perception which have been examined by the
somewhat novel methods described. Next, the writer’s gratitude is
due to Professor M. Foster for the permission he has given to use his
admirable description of the Hering theory; and, lastly, to the Royal
Society for the permission it accorded to use various diagrams which
have served as illustrations to papers which have appeared in its
“Philosophical Transactions” and “Proceedings.”




COLOUR VISION.




CHAPTER I.


I must commence this course by saying that I feel the honour that has
been done me in asking me to undertake it, connected as it is with the
name of Tyndall, whose recent removal from our midst has been deplored
by all lovers of science, and by none more than by those who have had
the privilege of listening to him at this Institution. It is my duty
to speak on some subject of physics, and the subject I have chosen
is Colour Vision. I hope it will not be considered inappropriate,
since it was Thomas Young, the physicist, whose connection with this
Institution is well known, who first propounded a really philosophical
theory of the subject. Interesting as it may be to trace how old
theories have failed and new ones have started, I feel that for those
who, like myself, have but little time at command in which to address
you, the historical side of this question must of necessity be treated
incompletely.

Colour vision is a subject which enters into the domains both of
physics and physiology, and it is thus difficult for any one individual
to treat of it exhaustively unless he be a Helmholtz, who was as
distinguished in the one branch of science as he was in the other.
I am not a physiologist, and at the most, can only pretend to an
elementary knowledge of the physiology of the eye, but I trust it is
sufficient to prevent myself from falling into any grievous error. I
shall try and show you, however, that the subject is capable of being
made subordinate to physical methods of examination. I must necessarily
commence by a very brief description of those parts of the eye in which
it is supposed the seat of vision lies, but in terms which are not
too technical. As to the mere optical properties of the eye I shall
say but little, for they are not necessary for my purpose, although
more particularly adapted to mathematical treatment than the other
properties I have to describe.

The eye may be diagrammatically represented as in the figure which is
supposed to be a horizontal section of it, the figure being reproduced
from Professor Michael Foster’s Physiology.

[Illustration: FIG. 1.

  _Scl_ is the sclerotic coat. _Ch_ the choroid coat, with _CP_ the
    ciliary process. _I_ is the body of the Iris. _R_ is the retina
    or inner wall. _PE_ the pigment epithelium or outer wall. _L_ the
    lens held by the suspensory ligament _sp.l._ _VH_ is the vitreous
    humour. _ON_ the optic nerve, _ox_ is the optic axis, in this case
    made to pass through the fovea centralis, _f.c._
]

As far as the perception of colour is concerned, the principal part
of the eye which is not distinctly optical--_i.e._ for the production
of images--is the retina, and this it will be seen is in reality an
outcrop of the brain, the connection between the two being the optic
nerve. Owing to this connection, it is not easy to determine where the
seat of colour perception is located; but for the purpose of physical
investigation this is not of first-rate importance, nor does it affect
the discussion of rival theories except in a minor degree. There are
other subsidiary adjuncts in the eye to which, however, I must call
attention, as they have a distinct bearing on the apparent intensity
of some colours and of the hue that mixtures of others are perceived.
The first is what is called the “macula lutea,” or yellow spot, a spot
which it may be assumed exists in every eye. It is horizontally oval
in form, and is situated in the very centre of the retina, embracing
some 6° to 8° in angular measure. It has a brownish or yellowish tint,
and the retina at this part is slightly depressed, being bounded by a
slightly raised rim. In the centre of this area the retina becomes very
thin, having a depression about 1/100 of an inch or ·3 millimetres in
diameter, which is named the “fovea centralis,” where it is said that
vision is the most acute. This statement can be well credited when we
come to consider where the seat of the stimulation of sensation lies.
The colour which tints the yellow spot is strongest at the crater-like
rim, and fades away centrally and peripherally, and is said to be
wholly absent in the fovea centralis.

As the colour of this spot is yellow or brown in the living eye (and
that it is probably brown the absorption indicates), it follows
that white light passing through it must be deprived of some of its
components, though in differing degrees. If the seat of sensation is
at the outer layer of the retina, as we shall shortly see must be the
case, it will further be seen that when light of any colour which the
brown pigment will absorb more or less completely falls on different
parts of the oval area, the absorption must vary at each part, and the
intensity of the perceived light will be least at the rim and increase
centrally and peripherally. As the centre of the yellow spot or fovea
is coincident approximately with the point where the axis of the eye
cuts the retina, the image of an evenly illuminated object, when looked
at directly, must fall on the yellow spot. If, therefore, a patch of
such light, the image of which more than covers the spot, be observed,
it ought to exhibit a varying brightness of colour corresponding to
the strength of the colouring matter which exists at the different
parts. This it but rarely does, for habit and constant interpretation
of what should be seen prevents the mind from distinguishing these
variations; but if the colour brightness, as perceived by the different
parts, be submitted to measurement by proper means, the variations in
brightness of the image can be readily recognised. A very common method
of exhibiting the presence of the pigment is to look at a bright white
cloud through a layer of chrome alum. Chrome alum transmits red and
blue-green rays. Now as the spectrum-blue rays are those which the
pigment will absorb, it follows that the colour of the solution should
appear ruddy to the central part of the eye, but on the rest of the
retina it should appear of its ordinary purplish colour. At a first
glance, and before the eye has become fatigued, this is the case, but
the phenomenon soon disappears. Another way of forming an idea as to
what the yellow spot absorbs is to throw a feeble spectrum on a white
surface and cause the eye to travel along it. If the spectrum be viewed
so that it does not occupy more than about 40° of the retina, the
movement of the eye will show a dark band travelling along the green,
blue, and violet regions as the image of these parts of the spectrum
fall on the yellow spot, and their apparent brightness will increase as
they fall outside the absorbing area. This proves that an absorption
takes place in this area.

[Illustration: FIG. 2.]

The retina consists essentially of an inner and outer wall, enclosing
matter which is similar to the grey matter of the brain. On the inner
wall are the vessels which are connected with the optic nerve. The
outer wall is epithelium coloured with a pigment, and it is here that
the visual impulses begin, although the rays of light giving rise
to them have to pass through the thickness of the retina before so
doing. It has already been stated that the light has to pass through
the thickness of the yellow spot before a visual sensation is felt in
the centre of the field, and the experiments just given offer a fair
proof of the truth of the assertion, but there is still another which
is perhaps more conclusive. Suppose we have a hollow reflecting ball,
as shown in Fig. 2, and through an orifice A we project a beam of light
to B, which meets an obstruction, S, in its path, then A B would be
reflected from B to C on a screen C F, and the obstruction S would be
marked at C. If another beam from D was directed so as to meet the same
obstruction, its presence would be marked at F. Knowing the distance
of the centre O of the hollow sphere from F C and its diameter, and
measuring the distance between F and C and their respective distances
from the axis of the sphere, the distances S B and S E can be
calculated. This method is applied in the formation of what are known
as Purkinje’s figures. The simplest case is where a beam of light is
directed through the sclerotic and transmitted through the lens. Images
of the retinal vessels are distinguished as at S, and it is found that
they cast shadows, which are seen as dark lines in the glare of the
field of vision. The sensation of light must therefore come from behind
these vessels, and calculation shows that the seat of the sensation is
close to the pigmented inner wall of the retina.

Lying here is a layer of what are known as rods and cones, which have
a connection, either actual or functional, with the optic fibres which
largely compose the inner wall of the retina, and are connected with
the optic nerve. In the yellow spot the cones are much more numerous
than the rods, but in the peripheral part the reverse is the case. In
the fovea the rods appear to be altogether absent. The total number
of cones in the eye has been calculated to be about 3,000,000, of
which about 7,000 are in the small fovea. The number of cones will
give an idea of their dimensions. This detail has been entered into
as it has been supposed that these rods and cones are all-important
in translating light-waves into visual impulses. The inner wall of
the retina of most human eyes, as has been mentioned, is stained with
a black pigment, fuscin, though in albinos it is absent. What its
particular use may be is still unknown, for its change by light is so
slow that it can scarcely be the cause of vision. In the outer parts of
the rods is, however, diffused a substance highly sensitive to light,
called the “visual purple,” from its colour, and a theory founded on
chemical action, produced by a change in this substance, has been
promulgated. Fascinating, however, as such a theory must be, it lacks
confirmation. The fact that the cones do not contain it, and that in
the fovea are cones alone, renders it difficult to reconcile the theory
with the fact that this part of the retina possesses, we are told, the
greatest acuteness of sensation as regards light and colour.

The eyes of most vertebrate animals, it may be remarked, have this
visual purple, but in those of the bat, owl, hen, and some others the
colouring matter seems to be absent. Visual purple is an interesting
substance, however, and as it is found in the eye it probably exercises
some useful function, though what that function may be is at present
unknown. That images of objects can be formed on the retina, owing to
the bleaching of this substance, has been proved by experiment. The
purple is first changed to a yellow colour, and then passes into white.
These “optograms,” as they are called, can be fixed in an excised eye
if the retina be detached, and then be treated with a weak solution of
alum.

[Illustration: FIG. 3.]

Many persons are not aware of the extent of the field of view which the
eye embraces. Vertically it takes in about 100°, whilst horizontally
it will take in some 145°, more or less. The field is smaller on the
nasal than on the temporal side. When both eyes are used, the combined
field of view is larger horizontally, being about 180°. The field of
view which is common to both eyes is roughly a circle of about 90°.
There is, however, a marked difference in the distinctness with which
objects are perceived in the different parts of field of view. On the
fovea centralis two dots placed so as to subtend an angle of 60″ will
be perceived as double. That is to say, if a piece of paper, on which
are two dots 1/30 of an inch apart, be placed 10 feet away from the
observer, these dots will be seen as separated, whilst dots (in this
case they should be black and of good dimensions) placed half-an-inch
apart would still appear as one if viewed at the same distance near
the periphery of the retina. In the yellow spot the distance apart of
the cones is such that they subtend about the same angle as the dots
when they are seen separate, viz., about 60″; that is, they are about
16/100000 of an inch apart, and hence may have something to say to
the limit of separation. The field for the perception of colour is
different to that for light.

The diagrams (Fig. 3) will show the fields in a satisfactory manner.
The concentric circles are supposed to be circles lying on the retina
corresponding to parallels of latitude on a globe, and are not,
therefore, equi-distant when seen in projection. To make these circles
it must be imagined that we have a bowl, in the middle of which is a
thin rod standing upright and passing through the centre, and another
rod attached to it at the centre of the sphere of exactly the length
of the radius. If this last arm be opened to make an angle of 5° with
the fixed rod, and be twisted round like the leg of a compass against
the bowl, it will make a circle, the projection of which will give the
innermost circle of the diagram; if opened to 10° it will give the next
circle, and so on for every subsequent 10°. The lines passing through
the centre are 30° from one another, the line stretching from 360° to
180° being a line supposed to be vertical. By means of an instrument
called the perimeter, the field of vision for each eye can be measured.
With its aid any small object can be made to fall on any part of the
retina by directing the axis of the eye to a fixed point and moving
the object along one of the diameters. Suppose we wish to ascertain
the field for a white object, a small white disc is moved, say, along
the horizontal line, and the angles at which the retina just no longer
sees it are noted. This gives two points in the field, and they are
plotted on the chart--in Fig. 3 one touches the outside circle, and the
other is at an angle of about 65°. The field of vision is next tested
along another line, say 300° to 120°, and other points noted and marked
on the chart. When the whole circle has been examined, the various
points are joined together, and we have the boundary of vision for a
white object. The boundaries of the _colour_ perception for (say) small
red and green discs are found in the same way. The former is depicted
in the left-hand chart and gives the field for the right eye, and the
latter with that for white in the right-hand chart for the same eye.
It will be noticed that two boundaries are given, one taken at mid-day
and the other at 6 p.m. The brighter the colour, the larger is the
boundary in both cases, showing that the field of colour vision varies
according to the illumination. Now it is difficult from this method of
experimenting to determine whether the fields for different colours are
the same or differ in extent, as we have no information as to whether
the colours themselves which were used were physiologically equal. The
only way by which this can be satisfactorily determined is by using
spectrum colours each of known brightness and area. (Some preliminary
experiments made by myself regarding the colour fields will be found in
the appendix, and will be referred to later.) It must not be thought
that the various colour boundaries mark the limit at which _light_ is
perceived, but only the limit at which colour is seen; outside the
boundaries the objects appear of a nondescript colour, to which we
shall by-and-by call attention. The yellow spot lies within the circle
of 5°, and the blind spot on which no sensation of light is stimulated
is shown by the black dot about 15° away from the centre.

I have only attempted to sketch, in unphysiological language, the
primary apparatus with which our experiments in colour have perforce to
be made.




CHAPTER II.


It will be seen, then, that in measuring colour or light several
circumstances have to be taken into account. These are not simple, and
require differentiating one from another before the results of colour
measures can be finally laid down as correct, or as being held to be
applicable to all cases.

We must naturally ask, what is colour? The answer I should like to pass
over entirely. It can only be described as a sensation, just as we
should describe touch as a sensation. It has, however, one advantage
over most sensations, in that it is a sensation which can be submitted
to empyric measurement. The question whether certain phenomena, such
as the colours produced by simultaneous contrast, are subjective or
real, does not require answering for the purpose that we have in view,
but the results recorded may probably help to throw light on it.
Colour is an impression caused by the stimulation in the eye of some
apparatus, that lies near the outer wall of the retina, the effect of
the stimulation being conveyed by the optic nerve to the brain. If this
apparatus be complicated by being made up of distinct parts, each of
which transmits its own kind of impression to the brain, it is not only
quite possible, but more than probable, that when one part is absent or
injured the particular impression for which it is responsible will be
lacking, and that the sum of the impressions due to the remainder will
be unlike that perceived when they are all working together.

In every investigation, whether it be in physical or in any other
branch of science, it is better to work up from the simple to the
more complicated; and acting on this plan, it is better to commence
experimenting with simple rather than with complex colours, though
they may apparently produce precisely the same sensations. I shall,
with this in view, devote most of the remaining part of this chapter
to some necessary experiments with simple colours. The simple colours
are those of the spectrum, and are the result of motion in the ether,
which pervades all space. The motion is in the form of undulations or
waves, and each colour is due to a series of these waves, which have a
definite length. Thus, 6562 ten-millionths of a millimetre produces
to most of us a red colour in the spectrum (see Plate I.), occupying
the position indicated by a black line known as the C line in the solar
spectrum.

A table of wave-lengths of certain lines in the solar spectrum is given
below:--


TABLE OF WAVE-LENGTHS IN TEN-MILLIONTHS OF A MILLIMETRE.

  B, deep red           6866
  Lithium, cherry red   6705
  C, red                6562
  D, orange             5892
  E, green              5269
  b, green              5183
  F, bluish green       4861
  Lithium, blue         4603
  G, violet             4307
  H, extreme violet     3968

The rays in the different parts of the spectrum being due to these
simple vibratory motions, cannot be decomposed further. And it makes
no matter whether we _see_ them as different colours or not, they will
always issue at the same angle from the same prism (if the prism be
used to form the spectrum), when it is turned to the same angle to the
incident light. Milestones are useful along a road to tell us where
we are in reference to some central place, and these black lines in
the spectrum serve the same end. But they have the advantage over the
milestone, for whilst the last will tell us how far we are from, say,
York or London, the former tell us our distance from a zero point.
We thus have a scale of light of different wave-lengths laid down for
us, which we can apply to the study of the sensations stimulated in
the eye, and so have the means of instituting a comparison between
the colour vision of different eyes. A mixed or composite colour is
in a different category, however, to the simple colour, as you will
see directly. It is one which may be formed by any number of rays of
different wave-lengths falling on the eye. What these rays are we can
only tell by analysing the light and referring them to the spectrum.

The instrument before you is one which I have used before in this
theatre; but as the major part of my experiments have been carried
out with it, in case those who are present may not be acquainted with
it, it will be necessary to describe it very briefly. The general
arrangement of the apparatus is given in the accompanying diagram, Fig.
4.

[Illustration: FIG. 4.]

R R are rays coming from the source of light, be it sun light or the
electric light, and an image of the one or the other is formed by a
lens L₁ on the slit S₁ of the collimator C. The parallel rays produced
by the lens L₂ are partially refracted and partially reflected. The
former pass through the prisms P₁, P₂, and are focussed to form a
spectrum at D by a lens L₃. D is a movable screen in which is an
aperture S₂, the width of which can be varied as desired. The rays are
again collected by a lens L₄, and form a white image of the surface
of the last prism on the screen E. If the light passing through S₂ is
alone used, the image at E is formed of practically monochromatic
light. Part of the rays falling on P₁ are, as just said, reflected, but
as it and the refracted part are portions of the light passing through
the slit S₁, they both must vary proportionally. If then we use the
reflected portion as a comparison light to the spectrum colours, the
relative intensities of the two, though they may vary intrinsically,
will remain the same. The rays reflected from P₁ fall on G, a silver
or glass mirror, and, by means of another lens L₅, also can be caused
to form a white patch on the screen E, alongside the patch of colour.
At M, or anywhere in the path of the beams, an electro-motor driving a
sector with apertures which can be opened or closed whilst rotating, is
placed, and the illumination of either beam can be altered at will. To
obtain a large spectrum on the screen E, all that is necessary is to
interpose a lens of fairly short focus in front of L₄, when a spectrum
of great purity and brightness can be formed.

If it be required to measure the width of the slits S₂ (which we shall
see further on is often necessary), a small lens of short focal length
placed behind L₄ and near the slit will cast a magnified image on E,
and by means of a scale placed there, the widths of each slit, if there
are more than one, can be read off on the scale by bringing them
successively into the same colour.

[Illustration: FIG. 5.]

Originally the comparison light was a candle, and it answered its
purpose fairly well, and for obtaining absolute measures is convenient
at the present time. Fig. 5 will show its arrangement, but as both the
candle and the electric light may vary independently of each other, it
will be seen that for merely the comparison of the different spectrum
colours, the previous arrangement is the better. In both cases the two
beams--the direct and the comparison--may be made to cast shadows
by placing a rod in their path, the shadow cast by one light is then
illuminated by the other light. By moving the rod towards or from the
screen the shadows can be brought side by side.

With this instrument it is easy to demonstrate that a mixed colour
may be mistaken for a simple colour of the spectrum. In a glass cell
with parallel sides is a solution of potassium bichromate, which,
to myself and probably most of you, has a beautiful orange colour.
The spectrum of white light is now on the screen, and if this orange
liquid is placed in the path of the white light before it reaches
the prisms, all the violet, blue, and most of the green is cut off,
leaving some green-yellow, orange and red only on the screen. That
these form the orange colour of the bichromate is readily shown by
removing the auxiliary lens. The spectrum, which has its focus at D, is
now recombined into a patch of light, which is at once seen to be the
colour of the solution.

[Illustration: FIG. 6.]

The colour of the bichromate is therefore a complex or mixed colour
according to our definition, for it is made up of a large number of
simple colours. What I desire to show, however, is that this complex
colour can be mistaken by the eye for a simple colour. First, let us
interpose the cell with the bichromate in the path of the _reflected_
beam, and throw the patch of light formed by it on a white surface A
(Fig. 6), alongside the patch of light B formed by the spectrum. Next
let us pass a single aperture (Fig. 7), which can be opened and closed
by a screw arrangement, through the spectrum. By careful movement we at
length come to an orange ray, which is spread out by the apparatus to
form a patch on B, that to the majority (and the word majority is used
with intention) of people exactly matches the colour of the bichromate.
Thus we have a proof that, as far as the eye is concerned, the simple
and the complex colours are identical. This illustration of the want
of power of the eye to analyse colour might be repeated as often as
we like. We may pass coloured wools, for instance, through the length
of the spectrum and show that they have the property of appearing
bright in, and therefore of reflecting, some colours and of almost
disappearing in others--a sure indication that these colours are mixed
colours as they are made up of the rays which are reflected. Yet when
viewed in white light they can in many cases be matched with simple
colours in the way we matched the colour of the bichromate solution.
This tells us that there is something which requires investigating as
to the constitution of the perceiving apparatus, and points to the
probability that it is less complicated than it would be were it able
to differentiate, without the aid of the spectrum, between simple
and complex colours. If the eye had a separate apparatus--and when I
say apparatus I use the word for want of a better--for taking up the
impression of every simple colour, it might well be assumed that a
differentiation must take place.

There is one class of colours, it must be remembered, which can never
be mistaken for simple colours. I refer to the purples--mixtures of red
and blue--for there are no spectrum colours which unmixed can possibly
match them. All other colours, as no doubt will soon be apparent, can
be referred to some one spectrum colour, either in its pure state or
else mixed with some variable quantity of white light. We are all
familiar with the fact that there are three primary colours, and we
are naturally led to consider these in the light of the experiments
just made. As good a definition as any other of a primary colour is
that it is a colour which cannot be formed by the mixture of any two or
more colours. The original investigators in colour phenomena were the
artists, and they found that neither red, nor yellow, nor blue could be
formed by any mixture of pigments on their palette, but that all other
colours could be made by a mixture of two or more of these three. Hence
to these three were given the name of primary colours. When, however,
the physicist began to work with the simple colours of the spectrum, it
was speedily found that, at all events, the yellow was not a primary
colour, as it could be formed by a mixture of green and red, whilst a
green could not be formed by a mixture of any other two colours. This
we can prove with our apparatus.

[Illustration: FIG. 7.]

Three apertures, all of which can be opened or closed as required (see
Fig. 7), are placed in the spectrum, one in the red, one in the green,
and one in the violet. The last we shall not require at present, so it
is entirely closed; but we vary the width of the other two. We find
that with a little red added to a bright green, a yellow green is
produced; with more red added we have yellow; with still more red, an
orange. The relative brightness of the two colours mixed together can
be shown by removing the lens which recombines the spectrum to form the
patch of light. Each colour issues through its slit and forms its own
patch on a white screen which, for the purpose, we make rather larger
than usual. The two patches overlap in the middle (Fig. 8), and the
pure colours are seen one on each side of the mixed colours.

[Illustration: FIG. 8.]

Now, placing one slit in the yellow and another in the blue of the
spectrum, we find that whatever width of slit we take, no green is
produced, but that, in fact, a yellowish or a bluish white results,
and that when the two slits are properly adjusted, a pure white is
produced. Evidently since none of the intermediate spectrum colours
between the blue and the yellow can be made by their mixture, certainly
green cannot. Hence, with pure colours a green and not a yellow is one
of the primaries.

Further investigation on these lines has placed the violet of the
spectrum as a primary rather than the blue, but this is still a matter
of debate. Suffice it to say that a red and a green in the spectrum are
really two of the primary colours, and most probably the violet the
third. Experiment shows that there is no other primary colour in the
strict sense of the word. We thus arrive at the fact that, except the
primary colours themselves, every colour in nature may be made by a
mixture of two or three of these primaries.

Just a word of explanation as to why, with pigments, the primary
colours appear to be red, yellow, and blue, and not red, green, and
blue. The colour of a pigment, it must be recollected, is a complex
one. If we analyse a yellow--a yellow glass will be just as good an
example as anything else--we find it is made up of green, yellow,
orange, and red. A blue is made up of blue and green. If a yellow is
placed behind a blue glass, and we look at a white surface through
them, the only light that can get through the glass is the green. If
the light, coming through each glass _separately_, falls on the same
spot on a white surface, it will be either colourless or bluish white,
or yellowish white, whichever colour preponderates. As the light
reflected from mixed pigments is made up principally by the light
coming through the different particles, first coming through one and
then through another, and only partially by mixed lights, it will be
gathered why the primary colour, when deduced from experiments with
pigments, was yellow, and not green.

With the spectrum colours there is this fact to remember, that
though all intermediate colours between the pairs of primaries can be
formed by their mixture, yet in some cases the resulting colours are
_slightly_ diluted with white, and that they thus appear less saturated
than the spectrum colours themselves. The reason for this we shall be
able to account for when we consider the colour sensations themselves.

When making matches to simple or other colours by the method of
mixtures, we have to be careful of the conditions under which we
experiment. This can be shown by a very simple experiment. I will make
a match on B with the white light, which is thrown on the surface A
(Fig. 6), by mixing the red, green, and violet that pass through the
three adjustable apertures or slits already described. The apertures
are altered till the match appears to myself perfect. From an appeal
made to those of the audience who are at least 25 feet away from the
patches of light, as to the correctness of the match, I gather that
the match is to them imperfect. The mixed colours appear to them to
give a pinkish white. The reason of this defect in the match is due
to the fact that, as the lecturer is viewing the two square patches
of 2 in. side from a distance of 2 ft. 6 in., their images on his
retina extend beyond the boundary of the yellow spot, whilst the
audience receives the whole of the image on that portion of the retina
which is completely covered by it. To the lecturer only part of the
blue and green is absorbed by the yellow spot, and the part of the
retina outside it on which the image falls receives and records the
full intensity of these colours. To the audience the full amount of
absorption takes place, with the result that the patch of mixed colours
must appear too red when it is correct to the lecturer. In this case
habit makes the eye take an average of the different intensities which
must exist at the various parts of the image. We can, however, cause
a perfect agreement between all parties if the experimenter views the
surfaces in a mirror placed some 12 feet away and then makes the match,
for he is viewing the patches from what is practically a distance of
24 feet. If after making the match without the aid of the mirror the
lecturer’s eyes are directed a little to one side of the illuminated
surfaces, a match will no longer exist; the mixed colour, which is
to the audience pinkish, will now appear a bluish green to him. The
reason for this alteration in hue is that the whole of the images falls
outside the yellow spot.

It will now be quite apparent that we must discount any assertion in
regard to colour matches, unless we are told the distance of the eye
from the surface on which the match is made, together with the size
of that surface. This yellow spot is often provokingly tiresome in the
study of colour mixtures, and one might almost be justified in doubting
whether any _absolutely_ exact matches can ever be vouched for, owing
to the important region of the retina which it occupies.

The fatigue of the retina to colour after it has been presented to
the eye for any length of time is a difficulty, but in a less degree.
That the retina does experience fatigue can be shown by a very simple
experiment. The lecture theatre is now illuminated by the incandescent
light, and if we throw an image of the bright carbon points of the
electric arc light on the screen and steadily fix the eyes on the
image of the white-hot crater for some (say) twenty seconds, and
then we suddenly withdraw it, a _dark_ image of the points will be
seen on the partially lighted screen, and will appear to travel with
the eyes as they move away from the fixed point. This phenomenon is
due to the fact that the perceiving apparatus for white light gets
fatigued on the parts of the retina on which the bright image of the
white carbon points thrown on the screen fell, and that when the
source of brightness was removed, the less intense illumination of
the screen failed to stimulate the vision apparatus at those parts
to the same extent that they were stimulated over the rest of the
field. We can vary the experiment by placing a red glass in front
of the electric light, and, following the same course as before, we
shall see a greenish-blue image of the carbon points upon the screen.
In this case the retinal apparatus which has not been stimulated by
the red sensation will be capable of the maximum stimulation by the
feeble white light, whilst that part which has suffered fatigue will
not respond so freely to the red contained in the white light. If we
abstract a certain amount of red from the spectrum, its recombination
will give a white tinged with greenish blue, which is a counterpart of
the colour we feel when the eyes have been fatigued by the red light.




CHAPTER III.


Let me take you back again to matches of colour. We will now, however,
make the matches with the primary colours in the guise of pigments.
These colours themselves are complex colours, but as the eye cannot
trace any difference, or at all events very little difference, between
them and simple colours, a mixture of these complex colours should
answer nearly as well as do mixtures of the simpler colours. We have
here three discs, a red, a green, and a blue, and we can very closely
match these colours by a red, a green, and a blue in the spectrum.

By having a radial slit cut to the centre of these card discs, we can
slip one over the other so as to expose all three colours as sectors of
a single disc. Then we can place the compounded disc on the axis of a
rapidly rotating motor, and the colours will blend together, giving an
uniform colour. Any proportions of the three colours can thus be mixed,
and by a judicious alteration in them we now have them so arranged
that they give a grey. By inter-locking together (Fig. 9) a black disc
and a white disc, each with a diameter slightly larger than that of
the other discs, but equal to each other, and rotating them on the
same spindle behind the three colour discs, we can, by an alteration
in the proportion of black to white, form a grey which will match that
produced by the rotation of the three coloured sectors. In other words,
white, though degraded in tone, can be produced by the three complex
pigment colours, as we have seen can also be done by the mixture of the
three simple spectrum colours.

[Illustration: FIG. 9.]

The mixture of the three spectrum colours can match other colours than
white. For instance, it can be made to match the colour of brown paper.
By the colour discs also we can do exactly the same by introducing, if
necessary, a small quantity of white or black, or both, to dilute the
colour or to darken its tone.

Another application of the same principles enables us to produce an
artificial spectrum by means of a red, a green, and a blue glass. By
fixing these three glasses behind properly shaped apertures cut in a
card disc at proper radial distances from the centre, and rotating
the disc, we have upon the screen when light is passed through them a
ring of rainbow colours. If the beam of light be first passed through a
suitable rectangular aperture, the breadth of which is small compared
with its length, placed close to the rotating disc, and an image of the
aperture be focussed on the screen by a suitable lens, we shall have
a very fair representation of the spectrum--every colour intermediate
between the red and green, or the green and blue, being formed by
mixtures of these pairs respectively.

We have now given a very fair proof that vision is really
trichromic--that is, that it is unnecessary to have more than the
sensations of three colours to produce the sensation of any of the
others.

[Illustration: FIG. 10.]

There is one colour, if it may be called so, that has not been shown
you, and whether it is a simple colour or not cannot be stated. It
seems, however, to be the basis of all other colours, since they all
commence with it. It would, perhaps, be preferable to call it the
first perception of light instead of a colour. We can exhibit this in
a fairly easy manner by a little artifice. An incandescent lamp is
before you, and a current from a battery passing through the carbon
thread causes it to glow brightly. In the circuit, however, I have
introduced what is known as a resistance, which consists of a very
large number of square pieces of carbonized linen, pressed more or less
tightly together. By means of a screw the pressure can be varied. When
the pressure is somewhat relaxed, the resistance to the passage of the
current is increased, and the carbon thread glows less brightly; and by
a still greater release of pressure, the light can be made to disappear
altogether. A beaker (Fig. 10) which we have here is covered with thin
blotting paper, and when placed over the incandescent glow-lamp it
appears as a luminous yellow cylinder, the colour being due to that of
the light within it. We can next insert more resistance in the circuit,
and it becomes red, due to the ruddy light of the thread. By inserting
still more resistance into the circuit the red fades away, but in the
darkness of this lecture theatre the beaker is still a luminous object,
though faintly so. It has no colour, and the only sensation it provokes
is one of light. Taking off the beaker, we see that the carbon thread
is a dull _red_ and nothing more. The passage of this light through
the white blotting paper so reduces it that the red is non-existent,
and the initial sensation is all we perceive.

Placing a piece of red, green, or blue gelatine round the lamp, we
get the same effect, showing that the basis of all colour, be it red,
green, or any other colour, is what appears to us to be colourless.
This experiment is one which is full of interest, as it has a very
distinct bearing on diagnosing our colour sensations, and a variation
of it will have to be repeated under other conditions.

To go back, however, a little way, how does it arise that only three
sensations are necessary to give the impression of all colours? One can
understand that some definite period of the ether waves might be in
unison with the possible swing of one apparatus in the eye, and another
with another, but it is somewhat difficult at first sight to conceive
that more than one can be made to answer to wave motion of a period
with which it is out of tune, so to speak. A couple of illustrations
taken from physical experiments may help to suggest how this can happen.

[Illustration: FIG. 11.]

Fig. 11 is a double pendulum arranged as shown. The pendulum A is
heavily weighted, whilst the pendulum B is light, being only a
string with a small weight attached. This difference in weight was
made designedly, to prevent any great effect of the movement of B
being shown on A, though that of A must necessarily exercise a great
influence on B. The two pendulums are now of the same length. A is
set in motion, and as it swings, B also begins to swing, and soon
is oscillating with greater motion than A, and continues to do so.
The length of the pendulum B is next shortened, and A is again set
in motion. B takes up the motion, and increases its swing more and
more, but now the two pendulums are in opposite phases, and the motion
of A tends to diminish the swing of B, and continues to do so till,
after an interval of time, B is once more at rest, when it again will
start swinging. The fact is, that when A commences to swing, B also
commences; and as long as B and A are moving in the same direction the
impulses tend to make B increase its swing, but when they are moving
in the opposite direction, or rather, perhaps it should be said, when
A begins to start from the highest point of its swing downwards whilst
B is travelling upwards, the swing of B will gradually diminish. This,
of course, must happen when B is shorter or longer than A, since
their times of oscillation are then different. We can now picture
to ourselves that when in the perceiving apparatus in the retina
the moving parts--probably molecules or atoms--arrive at a certain
amplitude, there is then an impression of light, and that it is quite
possible that not only those waves whose motion is exactly of the same
period as that of the apparatus will set them in motion, but also those
waves which are actually of a very different period. If such be the
case, it can be seen that waves of light of some periods may set each
of the three kinds of perceiving apparatus in motion, and that possibly
the resulting impressions given by the sum of all three for a wave out
of tune with any of them may be even greater than when the wave period
is absolutely the same as one of them. For in the last case a maximum
effect may be produced on one apparatus, and the effects on the other
two may be insignificant; whilst in the first case the effects on two
of them may be so large that their combined effects may have a larger
value.

[Illustration: FIG. 12.]

The following diagram (Fig. 12), made on the principle of Lissajou’s
figures, shows graphically the motion of the pendulum. The pendulum,
with a pen attached, was started by an independent pendulum, which had
a different period, and the amplitude of the former registered itself
on paper which moved by clockwork round the axis of suspension. As
the two pendulums had different periods, the amplitude, as shown by
the traces made, first increased and then diminished till there was
no motion, and then started again. The trace is very instructive, and
deserves attention. It will be noticed that the amplitude, or length
of swing, increased rapidly at first, and then very gradually attained
a maximum. Having attained this maximum, the amplitude diminished very
slowly for some time, and finally came rather rapidly to zero, and the
pendulum for an instant was at rest.

[Illustration: FIG. 13.

The top figure is the red sensation on the Young theory; the middle is
the green sensation, and the lowest the violet or blue sensation.]

With the notion in our minds that the perceiving apparatus might
act in the way that the pendulum acts, we naturally apply it to the
theories which early investigators on colour vision propounded. Thomas
Young, whose name has already been mentioned, had propounded a theory
of vision, which depended on the existence of only three colour
sensations, and Von Helmholtz adopted it and explained the action of
the three sensations in reference to the spectrum as shown in the
diagram. These figures do not pretend to be absolute measures of the
sensations, but only of the form which they might take (Fig. 13).
The height of the curve at each part of the spectrum is supposed to
represent the stimulation given to each apparatus by the different
colours. Looking at the figures we see that each sensation has a
place of maximum stimulation, and that the stimulation falls off more
or less rapidly on each side of this maximum. It will, however, be
noticed that whilst the green sensation takes very much the form of the
pendulum amplitudes (Fig. 12) between its periods of rest, the other
two differ from it. In the case of the red sensation, the stimulation
falls very rapidly in the red as it reaches the limit of visibility of
the spectrum, and in that of the blue sensation the steep descent is
towards the extreme violet. When the three sensation theory is examined
in the light of the careful measurements that have been made, the
results tell us that these diagrams can only be taken as suggestive.




CHAPTER IV.


An independent investigator of this subject was Clerk Maxwell, who
experimented with a “colour-box” of his own design, by which he mixed
the simple colours of the spectrum, and the results he got are really
the first which are founded on measurement. He measured something,
but hardly arrived at the colour sensation. His colour-box took two
forms, both on the same principles, so only one will be here described,
the diagram and description being taken from his classic paper in the
Philosophical Transactions of the Royal Society for 1860.

“The experimental method which I have used consists in forming a
combination of three colours belonging to different portions of the
spectrum, the quantity of each being so adjusted that the mixture shall
be white, and equal in intensity to a given white. Fig. 14 represents
the instrument for making the observations. It consists of two tubes,
or long boxes of deal, of rectangular section, joined together at an
angle of about 100°.

[Illustration: FIG. 14.

Maxwell’s colour-box.]

“The part A K is about five feet long, seven inches broad, and four
deep; K N is about two feet long, five inches broad, and four deep; B D
is a partition parallel to the side of the long box. The whole of the
inside of the instrument is painted black, and the only openings are
at the end A C, and at E. At the angle there is a lid, which is opened
when the optical parts have to be adjusted or cleaned.

“At E is a fine vertical slit, L is a lens; at P there are two
equilateral prisms. The slit E, the lens L, and the prisms P are so
adjusted, that when light is admitted at E, a pure spectrum is formed
at A B, the extremity of the long box. A mirror at M is also adjusted
so as to reflect the light from E, along the narrow compartment of the
box to B C.

“At A B is a rectangular frame of brass, having a rectangular aperture
of six inches by one. On this frame are placed six brass sliders,
X Y Z. Each of these carries a knife-edge of brass in the plane of the
surface of the frame.

“These six movable knife-edges form three slits, X Y Z, which may be so
adjusted as to coincide with any three portions of the pure spectrum
formed by light from E. The intervals behind the sliders are closed by
hinged shutters, which allow the sliders to move without letting light
pass between them.

“The inner edge of the brass frame is graduated to twentieths of an
inch, so that the position of any slit can be read off. The breadth of
the slit is ascertained by means of a wedge-shaped piece of metal, six
inches long, and tapering to a point from a width of half an inch. This
is gently inserted into each slit, and the breadth is determined by the
distance to which it enters, the divisions on the wedge corresponding
to the 200th of an inch difference in breadth, so that the unit of
breadth is ·005 inch.

“Now suppose light to enter at E, to pass through the lens, and to be
refracted by the two prisms at P, a pure spectrum, showing Fraunhofer’s
lines, is formed at A B, but only that part is allowed to pass which
falls on the three slits, X Y Z. The rest is stopped by the shutters.
Suppose that the portion falling on X belongs to the red part of the
spectrum; then, of the white light entering at E, only the red will
come through the slit X. If we were to admit red light at X, it would
be refracted to E, by the principle in optics that the course of the
ray may be reversed.

“If, instead of red light, we were to admit white light at X, still
only red light would come to E; for all other light would be either
more or less refracted, and would not reach the slit at E. Applying the
eye at the slit E, we should see the prism P uniformly illuminated with
red light, of the kind corresponding to the part of the spectrum which
falls on the slit X, when white light is admitted at E.

“Let the slit Y correspond to another portion of the spectrum, say the
green; then if white light is admitted at Y, the prism, as seen by
an eye at E, will be uniformly illuminated with green light; and if
white light be admitted at X and Y simultaneously, the colour seen at
E will be a compound of red and green, the proportions depending on
the breadth of the slits and the intensity of the light which enters
them. The third slit Z, enables us to combine any three kinds of light
in any given proportions, so that an eye at E shall see the face of
the prism at P, uniformly illuminated with the colour resulting from
the combination of the three. The position of these three rays in
the spectrum is found by admitting the light at E, and comparing the
position of the slits with the position of the principal fixed lines;
and the breadth of the slits is determined by means of the wedges.

“At the same time, white light is admitted through B C to the mirror
of black glass at M, whence it is reflected to E, past the edge of
the prism at P, so that the eye at E sees through the lens a field
consisting of two portions, separated by the edge of the prism; that
on the left hand being compounded of three colours of the spectrum
refracted by the prism, while that on the right hand is white light
reflected from the mirror. By adjusting the slits properly, these two
portions of the field may be made equal, both in colour and brightness,
so that the edge of the prism becomes almost invisible.

“In making experiments, the instrument was placed on a table in a room
moderately lighted, with the end A B turned towards a large board
covered with white paper, and placed in the open air, so as to be
uniformly illuminated by the sun. In this way the three slits and the
mirror M were all illuminated with white light of the same intensity,
and all were affected in the same ratio by any change of illumination;
so that if the two halves of the field were rendered equal when the sun
was under a cloud, they were found nearly correct when the sun again
appeared. No experiments, however, were considered good unless the sun
remained uniformly bright during the whole series of experiments.

“After each set of experiments light was admitted at E, and the
position of the fixed lines D and F of the spectrum was read off on the
scale at A B. It was found that after the instrument had been in use
some time these positions were invariable, showing that the eye-hole,
the prisms, and the scale might be considered as rigidly connected.”

[Illustration: FIG. 15.]

With this instrument he made mixtures of three colours, to match with
white. By shifting the slits into various positions and taking as his
three standard colours a red near the C line, a green near E, and a
blue between F and G (see frontispiece), he obtained a variety of
matches, from which he formed equations. After eliminating, or rather
reducing the errors to the most probable value by the method of least
squares, he got from his matches with white a table of colour values
in terms of the three standard colours, from which the diagram of the
spectrum (Fig. 15) was made. (The heights of the dotted curves are
derived from the widths of the slits, and the continuous curve is
the sum of these heights.) Now what appears to be a properly chosen
colour does not necessarily stimulate only one sensation. Indeed the
probabilities are against it, except in the extreme red and extreme
violet. If colours intermediate to the standard colours be matched by a
mixture of the latter, we do not arrive at any solution of the amount
of stimulation of each sensation, since the chosen standard colours
themselves may be due to a stimulation of all three sensations. As a
matter of fact, Clerk Maxwell chose colours which do not best represent
the colour sensations. The red is too near the yellow, as is also the
green. The blue should also be nearer the violet end of the spectrum
than the position which he chose for it. We may take it, then, that
except as a first approximation, Clerk Maxwell’s diagrams need not be
seriously taken into account. The diagram itself shows that the colour
_sensations_ are not represented by the colours he chose. Supposing
any one in whom the sensation of green is absent were examining the
spectrum, there would, according to the diagram, be no light visible
at the green at E. Anticipating for a moment what we shall deal with
in detail shortly, it may be stated that in cases where it is proved
that a green sensation is absent, there is no position in any part of
the spectrum where there is an absence of light. Had he chosen any
other green, the same criticism would have been valid. The diagram as
it stands is really a diagram of _colour mixtures_ in terms of three
arbitrarily chosen colours, and not of colour _sensations_. It merely
indicates what proportions were needed of the three colours, which
he took as standards, to match the intermediate spectrum colours.
The negative sign in some of the equations--given in the appendix,
page 201--may be somewhat puzzling to those who have not made colour
matches, but not to those who have actually made experiments. It
means that where it is present no match of colour by a mixture of
the standard colours is possible; and that it would be only possible
if a certain quantity of the colour to which is attached a negative
sign were to be abstracted--an impossible condition to fulfil, but
one which may often occur in colour-matching experiments. Later you
will find that when colours are chosen as standards so that the
resulting equations give no negative sign for any colour, we have a
criterion as to the colours which give the nearest approach to the
true sensations. The next diagram (Fig. 16) of colour sensations is
due to Kœnig, who investigated the subject with Von Helmholtz. By a
modified method, which perhaps need not be explained in detail here,
he produced them, and they must be apparently not far from the actual
state of things, supposing this theory be proved to be true. For my
own part, I am under the impression that the positions of the colours
which most nearly approach the colour sensations might be slightly
altered in regard to the green and the blue, for reasons that will
subsequently be given when the later experiments of General Festing
and myself come to be described. For the immediate purpose of the
lecture, the curves are sufficiently accurate, and I will ask you to
notice what they tell us. It is presupposed in these diagrams that,
if the three colour-perceiving apparatus are equally stimulated, a
sensation of white will be produced; and the reverse, of course, is
true, in that white will give rise to equal stimulation of the three
apparatus. It follows, then, that in the parts of the spectrum where
all three curves of sensation are seen to take a part in the production
of a colour, such as at the E line, the colour is really due to the
extra stimulation of one or two of the apparatus above that required
to produce a certain amount of white. _The colour in every part of
the spectrum may be represented by not more than two sensations, with
a proportion of white._ In the orange and scarlet there are only two
sensations excited, without any sensible amount of white, as the
amount of violet sensation is extremely small. At the extreme ends of
the spectrum only one sensation--the red or the violet--is excited; but
in the region of the green the colour must be largely diluted with the
sensation of white. As an example, we may take the part of the spectrum
where the red and the violet sensation curves cut each other. At this
point the green sensation curve rises higher than the intersection
of the other curves. The red and the violet sensations have only to
be mixed with an equal amount of the green sensation to make white,
so that the height of the green sensation curve above the point of
intersection represents the amount of pure green sensation which is
stimulated. The colour is therefore caused by the green sensation,
largely diluted with white. A scrutiny of the curves will show that
at no point is the green sensation so free from any other as at this
point, if we regard white by itself as a neutral colour. Looking at
these figures, we can readily see what effect the removal of any one
or two of the three sensations would have upon the colour vision of
the individual. The probabilities, however, against two of the three
sensations being absent must evidently be very much smaller than that
there should be an absence of only one of the sensations, either red,
green, or violet.

[Illustration: FIG. 16.]

It will be well that we should also have before us the theory which is
the only serious rival to that of Young, viz., that of Hering. In the
report of the Colour Vision Committee there is an excellent description
of this theory. As it was furnished by Dr. Michael Foster, we may
be sure that the ideas of its originator are correctly given, and
therefore I will quote it in his words:--

“Another theory, that of Hering, starts from the observation that when
we examine our own sensations of light we find that certain of these
seem to be quite distinct in nature from each other, so that each is
something _sui generis_, whereas we easily recognise all other colour
sensations as various mixtures of these. Thus, the sensation of red and
the sensation of yellow are to us quite distinct; we do not recognise
anything common to the two, but orange is obviously a mixture of red
and yellow. Green and blue are equally distinct from each other and
from red and yellow, but in violet and purple we recognise a mixture
of red and blue. White again is quite distinct from all the colours in
the narrower sense of that word, and black, which we must accept as a
sensation, as an affection of consciousness, even if we regard it as
the absence of sensation from the field of vision, is again distinct
from everything else. Hence the sensations caused by different kinds
of light or by the absence of light, which thus appear to us quite
distinct, and which we may speak of as ‘native’ or ‘fundamental’
sensations, are white, black, red, yellow, green, blue. Each of these
seems to us to have nothing in common with any of the others, whereas
in all other colours we can recognise a mixture of two or more of
these. This result of common experience suggests the idea that these
fundamental sensations are the primary sensations, concerning which
we are enquiring. And Hering’s theory attempts to reconcile, in some
such way as follows, the various facts of colour vision with the
supposition that we possess these six fundamental sensations. The six
sensations readily fall into three pairs, the members of each pair
having analogous relations to each other. In each pair the one colour
is complementary to the other--white to black, red to green, and yellow
to blue. Now, in the chemical changes undergone by living subjects, we
may recognise two main phases, an upward constructive phase, in which
matter previously not living becomes living, and a downward destructive
phase, in which living matter breaks down into dead or less living
matter. Adopting this view, we may, on the one hand, suppose that rays
of light, differing in their wave-length, may affect the chemical
changes of the visual substance in different ways, some promoting
constructive changes (changes of assimilation), others promoting
destructive changes (changes of dissimilation); and on the other hand,
that the different changes in the visual substance may give rise to
different sensations.

“We may, for instance, suppose that there exists in the retina a
visual substance of such a kind that when rays of light of certain
wave-lengths--the longer ones, for instance, of the red side of
the spectrum--fall upon it, dissimilative changes are induced or
encouraged, while assimilative changes are similarly promoted by the
incidence of rays of other wave-lengths, the shorter ones of the blue
side. But it must be remembered that in dealing with sensations it
is difficult to determine what part of the apparatus causes them; we
may accordingly extend the above view to the whole visual apparatus,
central as well as peripheral, and suppose that when rays of a certain
wave-length fall upon the retina, they in some way or other, in some
part or other of the visual apparatus, induce or promote dissimilative
changes, and so give rise to sensations of a certain kind, while
rays of another wave-length similarly induce or promote assimilative
changes, and so give rise to a sensation of a different kind.

“The hypothesis of Hering applies this view to the six fundamental
sensations spoken of above, and supposes that each of the three pairs
is the outcome of a particular set of dissimilative and assimilative
changes. It supposes the existence of what we may call a red-green
visual substance of such a nature that so long as dissimilative and
assimilative changes are in equilibrium, we experience no sensation;
but when dissimilative changes are increased, we experience a sensation
of (fundamental) red, and when assimilative changes are increased, we
experience a sensation of (fundamental) green.

“A similar yellow-blue visual substance is supposed to furnish,
through dissimilative changes a yellow, through assimilative changes
a blue sensation; and a white-black visual substance similarly
provides for a dissimilative sensation of white and an assimilative
sensation of black. The two members of each pair are therefore not
only complementary but also antagonistic. Further, these substances
are supposed to be of such a kind that while the white-black substance
is influenced in the same way, though in different degrees, by rays
along the whole range of the spectrum, the two other substances
are differently influenced by rays of different wave-length. Thus,
in the part of the spectrum which we call red, rays promote great
dissimilative changes of the red-green substance with comparatively
slight effect on the yellow-blue substance; hence our sensation of red.

[Illustration: FIG. 17.]

“In that part of the spectrum which we call yellow, the rays effect
great dissimilative changes of the yellow-blue substance; but their
action on the red-green substance does not lead to an excess of
either dissimilation or assimilation, this substance being neutral to
them; hence our sensation of yellow. The green rays, again, promote
assimilation of the red-green substance, leaving the assimilation of
the yellow-blue substance equal to its dissimilation; and similarly
blue rays cause assimilation of the yellow-blue substance, and leave
the red-green substance neutral. Finally, at the extreme blue end
of the spectrum, the rays once more provoke dissimilation of the
red-green substance, and by adding red to blue give violet. When
orange rays fall on the retina, there is an excess of dissimilation of
both the red-green and the yellow-blue substance; when greenish-blue
rays are perceived, there is an excess of assimilation of both these
substances; and other intermediate hues correspond to various degrees
of dissimilation or assimilation of the several visual substances.
When all the rays together fall upon the retina, the red-green and
yellow-blue substances remain in equilibrium, but the white-black
substance undergoes great changes of dissimilation, and we say the
light is white.”

It has been said by the same writer that this theory is tri-chromic.
For my own part I do not see that it is so in the sense in which that
word is used in the theory of Young. It may be a tetra-chromic, for
as far as _colour_ is concerned the black-white sensation must be
excluded; but it appears to me that it cannot be strictly brought under
the head of tri-chromic.




CHAPTER V.


The readiest means of investigating the stimulation of the different
sensations necessary to produce colour is evidently by eyes in which
one or two sensations are absent, and this applies not only to the
Young theory, but also to that of Hering.

In former days, not much more than a century ago, the existence of
colour blindness, as it is now named, was a matter of great curiosity,
and in the Philosophical Transactions of the Royal Society of 1777,
the case of a shoemaker named Harris is described by a Mr. Huddart,
who travelled all the way from London to the Midlands on purpose to
see if all the alleged facts regarding the patient were true. Harris
mistook orange for green, brown he called black, and he was unable to
distinguish between red fruits and the surrounding green leaves. At
first, colour blindness was called Daltonism, from the fact that the
great chemist Dalton suffered from it, and investigated the variation
which he found existed in his vision from that of the majority of his
fellow-creatures. It was in 1794 that Dalton described his own case of
colour blindness. He was quite unaware of his defect till 1792, when he
was convinced of its existence from his observations of a pink geranium
by candle-light. “The flower,” he says, “was pink; but it appeared to
me almost an exact sky-blue by day. In candle-light, however, it was
astonishingly changed, not having any blue in it; but being what I
call a red colour which forms a striking contrast to blue.” He goes
on to remark that all his friends except his _brother_ (mark this
relationship), said: there was not any striking difference in the
two colours by the two lights. He then investigated his case by the
solar spectrum, and became convinced that instead of having the normal
sensations, he only had two or at most three. These were yellow, blue,
and perhaps purple. In yellow, he included the red, orange, yellow,
and green of others, and his blue and purple coincided with theirs.
He says, that “part of the image which others call red, appears to me
little more than a shade or defect of light; after that, the orange,
yellow and green seem _one_ colour, which descends pretty uniformly
from an intense and a rare yellow, making what I should call different
shades of yellow. The difference between the green part and the blue
part is very striking to my eye, they seem to be strongly contrasted.
That between the blue and purple much less so. The purple appears to be
blue much darkened and condensed.”

Dalton said a florid complexion looked blackish-blue on a white ground.
Blood looked like bottle green, grass appeared very little different
from red. A laurel leaf was a good match to a stick of sealing-wax.
Colours appeared to him much the same by moonlight as they did by
candle-light. By the electric light and lightning, they appeared as in
day light. Some browns he called red, and others black.

Mr. Babbage, in Scientific London (1874), gives an account of Dalton’s
presentation at Court.

Firstly, he was a Quaker, and would not wear a sword, which is an
indispensable appendage to ordinary Court-dress. Secondly, the robe
of a Doctor of Civil Laws was known to be objectionable on account of
its colour--scarlet, being one forbidden by the Quakers. Luckily, it
was recollected that Dalton was affected with that peculiar colour
blindness which bore his name, and that as cherries and the leaves
of a cherry-tree were to him of the same colour, the scarlet gown
would present no extraordinary appearance. So perfect evidence was the
colour blindness, that the most modest and simple of men, after having
received the Doctor’s gown at Oxford, actually wore it for several days
in happy unconsciousness of the effect he produced in the street. The
rest of the description we need not reproduce. Both the above cases
we shall see shortly come under the category of red-blindness in the
Young theory. Recent investigations tell us that such colour blindness
is by no means rare, nor can it have been then. Statistics, derived
from carefully carried out examinations made in various parts of the
world by an approved method of testing, show that about four out of
every hundred males suffer from some deficiency in colour perception,
but that so far as the more limited statistics regarding them are to be
depended upon, only about four out of every 1000 women suffer in the
same manner.

Colour blindness in a healthy subject is usually hereditary, and is
always congenital. It is curious to trace back in some instances the
colour blindness, where it is to be found, in a family. It often
happens that colour blindness--as the gout is said to do--skips a
generation. This is usually traced to the fact that the generation
skipped is through the mother. Thus, the maternal grandfather may
be colour blind, as may be the grandsons, but the mother will very
frequently have perfectly normal vision for colour. On the other hand,
the paternal grandfather may have defective colour perception, and this
may be inherited both by the grandsons and the father. The remark made
by Dalton regarding his brother’s eyesight points to the fact that his
own colour blindness was probably hereditary. Deaf mutes, Jews and
Quakers, seem to be more liable to colour blindness than other people,
statistics giving them 13·7, 4·9, and 5·9 as the percentages. It may
be well to point out that the deficiency in colour perception to which
we are alluding is totally distinct from that which may arise from
disease. This last form has such marked characteristics of its own that
it can at once be distinguished from the congenital form.

Of the four per cent. of males who suffer from congenital colour
deficiency of vision, a large number are not totally lacking in any
one or more colour sensations. Those in which one sensation, on the
Young theory, is entirely missing are called “completely red-, green-,
or violet-blind,” whilst those in which the sensation is but partially
deadened are called “partially red-, green-, or violet-blind.”
When two sensations are entirely absent, and such cases are very
rare indeed, they are generally said to have monochromatic vision;
that is, every colour to them is the same, as is also white, the
only distinction between any of them being the superior brightness
of some over others. The best illustration of this form of colour
vision is perhaps to say that the retinæ of such people have the same
characteristics in regard to sensitiveness as has a photographic
plate, the resulting prints in black and white representing what it
sees in nature. When we have to adopt the terms used by the followers
of Hering’s theory--the theory which obtains most followers amongst
the physiologists, since it endeavours to explain colour vision in a
physiological way, though it fails to meet all the requirements of the
physicist--we should restrict our terms to red-green and yellow-blue
blindness, still perhaps retaining the term monochromatic vision for
the rare cases specified above. As we must employ some terms to express
our meaning, we shall in these lectures adopt those of the Young theory.

Now taking a red-blind person and examining him with the spectrum, we
find that he sees no light at all at the extreme limit of our red,
and only when he comes to the part where the red lithium line marks
a certain red does a glimmer commence; he then sees what he may call
dark-green, or he may call dark-yellow. When questioned about what to
us are greens he also calls them green or yellow, some being bright,
others saturated hues, and others again paler. When he gets to the
bluish-green he calls it grey, and will say it is indistinguishable
from, and in fact will match with, a white degraded in tone. From this
point he will say he sees blue, near F pale-blue, and in the violet
dark-blue. Too much importance must not be attached to the nomenclature
adopted by the colour blind. They have to take the names of the colours
from the normal eyed. Yellow objects are generally brighter than red,
and having annexed the idea that what to them is bright red is called
yellow, they give it that distinguishing name. His limit of vision at
the violet end will be the same as the majority of mankind, but it will
be considerably shortened at the red end. The point in the spectrum
which he calls grey is an important point, and corresponds to the place
where the violet and green curves cut in Fig. 16. This point can be
very accurately determined by placing alongside the colour patch A
(Fig. 6) the white patch, which is reduced in brightness as required
by rotating sectors. As the slit is moved along the spectrum it will
eventually reach a point where he will say both patches of light are
exactly similar in hue. To the normal eye one will be white and the
other the kind of green indicated above (see frontispiece).

If a similar examination be made of the green-blind, the red end of the
spectrum will be called red or yellow, but the spectrum itself will
be visible between the same limits as it is to the person who has the
normal sense of vision. A grey stripe will be seen in the spectrum, but
in this case it will be a trifle nearer the red end of the spectrum
than the point which the red-blind calls grey; from this point to
the extreme violet, the green-blind will name the spectrum colours
similarly to the red-blind. The part of the spectrum where grey exists
to the green-blind is even more important than that part at which it
exists to red-blind, for it marks the place where the red and violet
curves cut each other in Fig. 16, and is in the majority of cases the
place in the spectrum where to the normal eye the green sensation is
unmixed with any sensation except that of white, as quite recently
explained. This green evidently is the colour which is most usefully
employed in making colour mixtures in order to obtain the three
sensation curves of the Young theory, since white can be added to the
colour matched. To avoid verbiage, we shall call the points where the
red- or green-blind see a grey in the spectrum their neutral points,
and the grey they see at those points their neutral colours. The three
curves we shall call the red, green, or violet curves, and the slits,
when placed in the red, green, or violet of the spectrum, as the red,
green, and violet slits.

We have already mentioned the case of those who possess monochromatic
vision, and shown in what respect they will differ in their description
of the spectrum from those more common cases of defective vision. If
the visual sensation they possess be the violet, they will see no light
at the extreme red of the spectrum, and very little in the orange. They
must match every colour with some shade of grey, for they will only
perceive that sensation, in what to ordinary normal eyes is white.
We need not detail how those who possess monochromatic vision due to
some other sensation would describe the different colours. The diagram
will tell us. Suffice it to say, that one colour will only differ from
another and from white in brightness.

It is a very remarkable fact how many people who are defective in
colour vision pass through a good part of their lives without being
definitely aware of it. It is very doubtful whether, in the majority
of cases, they themselves discover it. They may quite possibly
attribute the descriptions of colour which they hear, and which appear
to them absolutely false or meaningless, as due to mental or moral
defects in their friends. I have had two cases of this recently. One
was a gentleman of seventy-four, who had no conception that he had
anything but normal colour vision; his daughters, however, had a
suspicion that something was not quite right in it, and after a good
deal of persuasion brought him to me to examine. The first mistake
that he made was to state that he was sitting on a black velvet
chair, whereas the seat was a deep crimson plush. He laughed at his
daughter’s description of the mistake he made, and declared he was only
colour ignorant, and that she was the one who was colour blind. The
examination showed that colour ignorant he was, but that the ignorance
was due to complete red-blindness. For the seventy-four years he had
lived he was unaware of his deficiency, suspecting it in others, and
it was only an accidental circumstance which made him acquainted with
the true state of his colour perception. Another elderly gentleman,
in a high position in life, was also accidentally tested, and he
proved to be completely green-blind. He, too, was quite unaware of
his defect, and protested that, yachtsman as he was, he would never
mistake a ship’s lights; but a very brief test showed his friends
who were with him that his declaration had to be received with a
certain amount of reservation. Others there are who certainly do know
that some peculiarity exists in their sense of colour, and, foolish
as it may appear to be--though, after all, it is quite consistent
with a sensitive nature--they have tried to hide their defect from
their fellow-creatures. Such examples, no doubt, some of my audience
have met with, and experience tells me that they have just as much
reluctance to pass an hour in my darkened room as they would have to
occupy a police cell. In those few cases that have come voluntarily
to me for examination, the peculiarity in colour sense was first
brought to notice by the patient--if patient I may call him--failing
to distinguish between cherries and the cherry leaves, or strawberries
and the strawberry leaves. Such mistakes committed publicly are usually
the source of unbounded merriment and curiosity to schoolboys when made
by their schoolfellows, and I am bound to say that even persons of
graver years are not unapt to be amused at what they consider to be a
shortcoming in their fellow-creatures. To the student of colour vision
the discovery of curious cases of colour deficiency is looked upon in a
very different light--a good case of colour blindness, or still better
one of monochromatic vision, is eagerly sought after, with the hope of
submitting it to a rigid examination. When we look at the diagram (Fig.
16) we shall find why it is that the colour blind describe the spectrum
as they do. Literally for those whose vision is dichromic, it is made
up of two sensations alone, and the colours to which these sensations
give rise are mixed throughout a large part of the spectrum, the pure
unmixed sensations being at each end of the spectrum as they are in
normal colour vision. The annexed diagram (Fig. 18) gives the curves
for a red-blind person as made by observations under Clerk Maxwell’s
directions. The standard colours here have been badly selected, for
one of them stimulates the two sensations possessed.

[Illustration: FIG. 18.]

An easy and instructive experiment can be made to give an idea of
the kind of colour that these colour blind imagine as white, whether
they be red-, green-, or violet-blind. (For those who have only
monochromatic vision, as before stated, white is coloured with the
one colour they possess.) Three slits are now in the spectrum, one
near the extreme end of the red, another well in the violet, and the
third in that part of the spectrum in which the green-blind see their
neutral colour (see page 66). With the three colours issuing from
these apertures a match is made with the white patch, and in this case
the match is made as seen from a distant point, so that the resulting
deductions may be true to the audience. If a colour-blind person be in
this theatre, he will agree with me that the match is as correct to him
as it is to myself and the rest of you. So far we could not distinguish
his colour perception from the normal, but if he be red-blind, and
the red slit be covered, he will still say that the match holds good,
for, as a matter of fact, the red with which we helped to build up the
white is non-existent to him. The white that he now sees is to us the
greenish-blue patch which the mixed violet and green make. If he be
a green-blind person he will tell us the colour is a very pale blue,
but when the green slit is covered up and the red uncovered, the match
will once more be correct, though the purple, formed by the mixture of
red and blue, will appear to him to be a little darker than the white.
This is what one would expect, for you must recollect this green in
the spectrum he would call white or grey. If then, from what to him is
also white, though formed by the rays coming through the three slits,
we take away a certain amount of degraded white (green to us), he must
still see white, but darker. We have, however, met with what is an
apparent paradox. The green, coming through the now covered slit, he
calls white, as he also does the purple. To impress this point more
strongly upon you, I will place in front of the green slit a small
prism which has an angle of about one and a-half degrees. This is
just sufficient to throw the green colour on the neighbouring white
surface. Here we have both the colours which the green-blind calls
white side by side. If the brightness of each be the same, he would see
no difference in them. Is it possible that on any theory this can be
correct? To explain this apparent paradox, and without reference to
the mathematical proof that white subtracted from white leaves white,
we have only to look at our diagram (Fig. 16), and it is immediately
apparent how it arises. The red and the blue curves cut at this point;
and if we take away the green sensation entirely, the residue will be
a mixture of the red and blue, which is the identical purple colour
forming the patch.

If we are wishful to ascertain the colour that the violet-blind calls
white, we have only to cover up the violet slit and a yellow is left
behind as the result. I would have you remark that these colours which
are seen as white would only be of the hues shown you, supposing the
colour sensations were identical with those in normal vision. Whether
this is the case we cannot absolutely say, and the only way in which
this can be authoritatively settled is by examining some person who has
normal colour vision in one eye and defective colour sense, _not due
to disease_, in the other. One such person has been examined abroad,
but in what way I am unable to say. It is recorded that he sees the red
end of the spectrum as yellow with the eye that is defective. Another
person I have heard of in England, but so far have not had the good
fortune to get hold of him for examination. When I can lay my hands on
him, he will be able to help to confirm or disprove what should be a
general rather than a particular case.

So far I have only met with what appears to be one genuine case of
violet blindness. It is very remarkable, on account of the eccentricity
of the colour nomenclature. The only two colours which the subject
saw were red and _black_. He named all greens and blues as black, the
distinction between the two being that the former was “bright black”
and the latter “dark black.” Yellow he called white, and a glance at
Fig. 16 will show that at this place in the spectrum the neutral point
of a violet-blind should occur. By shifting the slit gradually into
the green, he called it grey, instead of “bright black,” though it did
not match the white patch when darkened. He called a green light a
“bright black” light. We shall have to refer to this case when we are
describing other investigations.




CHAPTER VI.


Another mode of exhibiting colour blindness, and one of the first
adopted, is by making mixtures of colours with rapidly rotating colour
discs. In my own experiments I have chosen a red, which is scarlet,
over which a wash of carmine has been brushed. It has a dominant
wave-length of 6300. The green is an emerald-green, and has a dominant
wave-length of 5150. The blue is French ultra-marine, with a dominant
wave-length of 4700. The card discs, of some 4 inches diameter, are
coated with these colours as pastes, and by making an incision in them
radially to the centre, as before described, and inter-locking them,
the compound disc can be caused to show sectors of any angle that
may be required. Outside these are the discs of black and white, the
proportions of which can be altered at will.

The light thrown on the rotating sectors being that from an electric
arc light, normal vision requires 118° of red, 146° of green, and 96°
of blue to match a grey made up of 75 parts of white and 285 parts of
black. For the last two numbers a correction has been made to allow for
the small amount of white light reflected from the black surface. This
correction has also been made in the subsequent matches which will be
described. Colour mixtures such as these are conveniently put in the
form of equations, and that given will then be shown as follows--

                  118 R + 146 G + 96 U = 75 W + 285 B.

(Here R, G, U, W, and B are used to indicate Red, Green, Blue, White,
and Black.)

This match was exact also for all the colour blind, for the deficiency
in one grey is also a deficiency in the other. With a red-blind,
however, very different matches can be made, as the red pigment is
a complex colour. There is in it, besides red, a certain amount of
yellow, whilst in the green there is, besides green, a small amount of
a red and a larger amount of yellow. The yellow will not only stimulate
the green sensation, but also the red where it is present. Although
in complete red-blindness the red sensation is totally absent, we may
expect that a mixture of red and blue, as well as of green and blue,
will enable a match to be made of the grey produced by the mixture of
white and black.

This was the case. We have the following proportions--

                      295 R + 65 U = 45 W + 315 B.

When the green disc is substituted for the red, the red-blind made the
following mixture--

                     229 G + 131 U = 120 W + 240 B.

It is worth noticing that the amount of blue in the first mixture is
about half that required for the second. This tells us that the amount
of green sensation stimulated in the first case is much less than in
the second. As red can be substituted for green, it should follow that
green, when rendered darker, should match the red. To try this a red
disc replaced the black disc, and a black disc replaced the blue. The
following match was then made--

                     131 G + 229 B = 340 R + 20 W.

It seems impossible to believe that these mixtures, so dissimilar in
colour, could ever form a satisfactory match. This last equation might
have been derived from the two first, in which case it would have
stood--

                     137 G + 223 B = 342 R + 18 W.

By a completely green-blind the following mixtures were made--

                     251 R + 109 U = 62 W + 298 B,

                                  and

                     277 G + 83 U = 107 W + 253 B.

In this case 363 Green are equivalent to 251 parts of Red mixed with 78
of White and 34 Black. The difference in the matches made by the two
types of colour blindness is very evident. In the one case the amount
of red required is much greater than the green, and in the other _vice
versâ_. Another instance may be given of colour matches made, by means
of discs, by a _partially_ green-blind person, whose case will be more
fully described when we treat of the luminosity of the spectrum to the
different classes of colour vision.

His matches were as follows--1st, That of the normal vision. 2nd,--

                  160 R + 80 G + 120 U = 72 W + 288 B.

The green was then altered to 200, when the following made a match--

                  65 R + 200 G + 95 U = 72 W + 288 B.

Using these two equations, we have the following curious result--that
120 G was matched by 95 R + 25 U. As the green disc is nearly twice as
luminous as the red to normal colour vision, this equation confirms
the result otherwise obtained, that his blindness to colour is a
deficiency in the green sensation. No mixtures of blue and red, or blue
and green, would match a grey formed by the rotation of the black and
white sectors.

I must now introduce to your notice a different method of experimenting
with colour vision. If we throw the whole spectrum on the screen, and
ask a person with normal vision to point out the brightest part, he
will indicate the yellow, whilst a red-blind will say the green, and
so on. This tells us that the various types of colour blind must see
their spectrum colours with luminosity differing from that of the
normal eye. The difference can be measured by causing both to express
their sense of the brightness of the different parts of the spectrum
in terms of white light, or of one another. Brightness and luminosity
are here used synonymously. On the two small screens are a red and a
green patch of monochromatic light--a look at the green shows that it
is much brighter than the red. Rotating sectors, the apertures of which
can be opened or closed at pleasure during rotation, are now placed in
the path of the green ray. The apertures are made fairly small, and the
green is now evidently dimmer than the red. When they are well open
the green is once more brighter. Evidently at some time during the
closing of the apertures there is one position in which the red and
green must be of the same brightness, since the green passes through
the stage of being too light to that of being too dark. By gradually
diminishing the range of the “too open” to “too closed” apertures we
arrive at the aperture where the two colours appear equally bright. The
two patches will cease to wink at the operator, if we may use such an
unscientific expression, when equality in brightness is established.
This operation of equalising luminosities must be carried out quickly
and without concentrated thought, for if an observer stops to _think_,
a fancied equality of brightness may exist, which other properly
carried out observations will show to be inexact. Now, instead of
using two colours, we can throw on a white surface a white patch from
the reflected beam, and a patch of the colour coming through the slit
alongside and touching it. The white is evidently the brighter, and so
the sectors are placed in this beam. The luminosity of (say) a red ray
is first measured, and the white is found to require a certain sector
aperture to secure a balance in brightness. We then place another
spectrum colour in the place of the first, and measure off in degrees
the brightness of this colour in terms of white light, and we proceed
similarly for the others. Now how are we to prove that the measures
for luminosity of the different colours are correct? Let us place three
slits in the spectrum, and by altering the aperture of the slits make
a mixture of the three rays so as to form white. The intensity of this
white we can match with the white of the reflected beam. We can then
measure the brightness (luminosity) of the three colours separately,
and if our measures are correct there is _primâ facie_ reason to
suppose that they will together make up the brightness of the white.
Without going through this experiment it may at once be stated that the
reasoning is correct, for within the limits of error of observation
they do so. Having established this proposition, we can next compare
_inter se_, the brightness of any or all of the rays of the spectrum by
a preliminary comparison with the reflected beam of white light. As in
the colour patch apparatus all colours and principal dark lines of the
solar spectrum are known by reference to a scale, in making a graphic
representation of the results, we first of all plot on paper a scale
of equal parts, and at the scale number where a reading is made, the
aperture of the sectors in degrees is set up. Thus, suppose with red
light the scale number which marked the position of the slit was 59,
and the aperture 10°, we should set up at that scale number on the
paper a height of 10 on any empyric scale. If in the green at scale
No. 38 the sectors had to be closed to 7°, we should set up 7 at that
number on the scale.

When observations have been made at numerous places in the spectrum,
the tops of these ordinates, as they are called, should be joined, and
we then get the observed curve of luminosity for the whole spectrum.
For convenience’ sake we make the highest point 100, and reduce the
other ordinates in proportion. For some purposes it may be advantageous
to give the luminosity curve in terms of a scale of wave-lengths. For
our purpose, however, it is in general sufficient to use the scale of
the instrument.

[Illustration: FIG. 19.]

Now, if we test the vision of the various types of colour blind by
this plan, we should expect to get luminosities at different parts of
the spectrum which would give very different forms to these curves.
We cannot hope, for instance, that a red-blind who sees no red in the
extreme end of the spectrum would show any luminosity in that region,
nor that the green-blind should show as much in the green part of the
spectrum as those who possess normal colour vision, since one of the
sensations is absent. With monochromatic vision there should be a still
further departure from the normal curve. That these differences do
exist is fully shown in Fig. 19. One of the most striking experiments
in colour vision is to place a bright red patch on the screen, and to
ask a red-blind to make a match in luminosity with the white. The
latter will have to be reduced to almost darkness--a darkness, indeed,
that makes the match almost seem incredible. You will notice that the
places in the spectrum where the red- and green-blind see grey are
by no means places of greatest luminosity. We shall find that these
luminosity curves are suggestive when making another investigation into
the form of the spectrum curves of the colour sensations.

Besides cases of complete blindness due to the absence of one or two
sensations on the Young theory, we have other cases, as was said when
remarking on the percentage of people who are colour deficient, in
which one or even two sensations are only more or less deadened. It
has often been said that with the theory provisionally adopted, such
cases are difficult to class as red or green deficient. As far as my
own observations go, I have never found this difficulty. The luminosity
curves of such observers, combined with other indications, give a ready
means of classing them. The main difficulty to my mind is to state
what is normal colour vision, but, as I have found that the very large
majority of eyes give the same luminosity to colours as my own, I have
taken my own colour perception as normal. In numerous experiments which
Lord Rayleigh has made in matching orange by means of a mixture of
red and green, he has come across several who have apparently normal
vision, as they see colours correctly in every part of the spectrum,
and yet some require much less red mixed with the green to make a match
with the orange than do others. What is yellow to them is decidedly
green to the majority. This has been classed as another kind of normal
vision; but the luminosity curves show that it may be equally well due
to a deficiency in the green sensation, and which would require more
green to make the necessary match. The limits of the visible spectrum
to these persons, as far as my examination of their cases goes, are the
same as my own.

Again, there are others in which the spectrum seems decidedly somewhat
shortened at the red end compared with my own, and the luminosity
curves point to them as being strictly colour deficient in the red
and nothing else. As they see all colours, they have been classed as
another form of normal vision. The deficiency in both these cases is
so small that white is their neutral colour, but there is evidence
that the hues are slightly changed. I do not wish any one to accept
my deductions as being more correct than those who hold differently,
but the results of examination by the luminosity methods appear to
me difficult to reconcile with any other view. There are, however, a
large number of cases in which, though complete red- or green-blindness
is wanting, there is no doubt that more than slight colour deficiency
exists. For instance, in Fig. 23 we have the curve of luminosity of
the spectrum as measured by a very acute scientific observer, and it
is compared with that of normal colour vision. He certainly is not
completely blind to any sensation. An inspection and comparison of
the two curves will show that he is defective in the green sensation,
although it is present to a large extent. The deficiency is obvious
enough. An endeavour to find his neutral point was most interesting.
At 39 in the scale he saw a little colour, but at 39·5 all colour
had vanished, and between the coloured patch and the white he saw no
difference. This similarity he saw till 47·3 in the scale, when he
began to see a faint trace of colour. There is a large piece of the
spectrum, then, which to him is grey. It must be recollected that
all three sensations were excited in this region, but some more than
others. Now, experiment has shown that, with normal vision, two per
cent. of any colour may be mixed with a pure colour without its being
perceived. It is not surprising, therefore, that although the red,
or the green, or the blue may be present in an intensity above that
required to form white, yet the resulting sensation should pass for
white. It may be remarked that red and white when mixed he never
mistook for yellow, and he always recognised yellows and red; yellowish
green, however, he called pale yellow.

[Illustration: FIG. 23.]

Another example of partial red-blindness is also instructive. Fig. 23
also shows it graphically. There is no doubt as to the nature of the
defect. The spectrum is slightly shortened, and the luminosity of this
part of the spectrum is less than that of normal vision. There was no
difficulty in distinguishing every colour, though the positions of
the colours from yellow to green seemed to be shifted; but no neutral
point could be traced. Apparently, both this case and the former are
about equally colour defective; but in this last the same reasons do
not apply for the existence of a neutral point. (For measures see page
214.)




CHAPTER VII.


We are now in a position to carry the investigations as to luminosity
a little further. When we look at small patches of light, we view the
colour through the yellow spot in the eye. If, when we have matched the
luminosity in the ordinary manner, we turn our eyes some 10° away from
the patches, we shall find that except at one place in the green the
equality in brightness no longer exists. By a little practice we can
make matches of luminosity when the eyes are thus diverted. This will
give us a different curve of luminosity, as the yellow spot absorption
is absent, and the difference in the heights of the ordinates between
the two curves will give us that absorption. Fig. 20 shows this very
well; and it will be noticed that the eye is appreciably not so
sensitive to the red and yellow at 10° from the axis as it is on its
central area. If we measure the areas of these two curves we get the
relative values of the light energy which is active on the two parts
of the eye, and these we found to be as 167 to 156. The heights at
which to put the maxima of the two curves were found from various
considerations, and the correctness of the deductions was verified by
directly comparing the intensities of two patches of white light some
10° apart, which, when looked at direct, were of equal intensities.
When one was compared with the other, the eye receiving one image
centrally and the other outside the yellow spot, the difference in
values was closely proportional to those of the above areas. The part
of this last curve showing a deficiency in red sensation is very
similar to that obtained from a person who is partially colour blind.
The absorption by the yellow spot derived from these measures is
graphically shown in the next figure (Fig. 21).

[Illustration: FIG. 20.]

[Illustration: FIG. 21.]

The question of the visual sensation at the “fovea centralis” (if it be
admitted that this is coincident with the visual axis of the eye, as is
usually accepted) may be very easily studied. When the luminosity of
the spectrum is examined at five or six feet distance, by throwing the
two patches on the whitened face of a small square of half-inch side,
we get a result differing from both of the above. The fovea appears
to be slightly more sensitive to red than the macula lutea, and is
generally less sensitive to the green rays (see Fig. 20). If a star,
or a distant light, be observed with the part of the retina, on which
the axis of the eye falls, as is the case in ordinary vision, and then
be observed with the eye slightly directed away, the difference in the
colours of the light is unmistakable. (The tables giving the measured
value of these curves will be found in the appendix, page 211.)

[Illustration: FIG. 22.]

Can we in any way find from these methods the colour sensation curves?
I think we can. Suppose we have a second instrument exactly like the
first placed side by side with it, we can then throw two patches of
colour on the two adjacent white surfaces, and we can mix with either,
or both of them, as much white light as we choose. From the second
instrument let us throw all the spectrum colours in succession on to
the one surface, and on to the other the three primary colours mixed in
such proportions as to match them accurately. This plan is, I venture
to think, a better way of obtaining the value of colours in terms of
standard colours than that adopted by Maxwell. This method gives the
values directly, and not by calculation from matches with white. Let
us place one slit near each of the extreme ends of the spectrum; that
in the red near the red lithium line, and another a little beyond G
in the violet of the spectrum, whilst the third slit should be in the
exact position in the green, where the _green-blind sees grey_. Now it
might be a matter of dispute as to whether one was entitled to make
this last one of the positions for the slits, for we use it entirely
on the assumption that two of the colour sensations which we suppose
we possess are identical with those of the green-blind. This might be,
or might not be, the case; but I think it can be shown very easily
that the assumption we are making is more than probably exact. Having
the slits in these positions, we may endeavour to match the spectrum
orange. We mix the red and the green lights together, and find that the
best mixture is always paler than the orange, but by adding a small
quantity of white to the orange we at once form a match. In the same
way if we have a greenish-blue to match, we shall find that we can only
make the match when we add a little white to the simple colour. Now
let us shift the position of the slit in the green just a little--a
very little--towards the blue, and again try to match orange. Do what
we will we cannot find apertures to the slits which will give us the
colour, though it be diluted with white. It will be too blue or too
red, but never exactly orange. This tells us that there is too much
blue in the green we are using. Next, shift the slit a little towards
the red below our fixed position, and endeavour to match the blue.
We shall find that this, too, becomes impracticable. The blue is
either too green or too violet, telling us that our mixture contains
too much green. As the neutral point of the colour blind is the only
position for the green slit which enables us to make a good match to
both the orange and the blue, it follows that this must be the point
where these two colour sensations are so arranged as to be in the
proportions required to form white when green is added; that is, that
there is neither an excess of red nor an excess of violet. To come
back to our measures of colour. We can make up every spectrum colour
with these three colours, and finally divide the luminosity curve into
the _colour luminosity_. In all these matches the violet luminosity is
very small indeed compared with the red or green. A match with white
is now made by a mixture of all these colours, and you will see, from
the images of the slits on the screen, that the _luminosity_ of the
violet is almost a negligible quantity compared with the others. We
may, therefore, as a first approximation, divide up the luminosity
curve into two parts, one being the luminosity of the green in the
different colours and the other of the red. The green, however, is
made up of red, of violet, and of an excess of green sensation, which
in this case comes practically to a mixture of white with the green
sensation. How can we tell how much is green and how much is white?
Suppose I, as a normal-eyed person, compare the luminosity of the
colour coming through the red slit with that coming through the green
slit, and then get the green-blind to do the same, it is evident that
any excess in the luminosity as measured by myself over that measured
by the green-blind must be due to the green sensation, and we can also
see how much red and violet make up his white. We shall not be far
wrong, then, in apportioning the constituents of the white thus found
between the green and the red; the violet being, for the time being,
negligible. We must subtract the red sensation from the green _colour_
curve and add it to the red colour curve: the two curves will then be
very closely the curves of the _red and green sensations_. By causing
the green-blind to make mixtures of red and violet for all the colours
of their spectrum, we can arrive at what must be finally taken away or
given to these curves, though such addition or subtraction of violet
will be small when the luminosities are considered. The accompanying
figure (Fig. 22) gives an idea of the shape and general features of
these curves. It may be remarked that we can check the general accuracy
of the measures of the colour mixtures by calculating or measuring the
areas of the two colour curves, the red and the green. If accurate,
they should bear the same ratio that the _luminosities_ of the two
colours bear to each other (when mixed with a little violet, which
is practically negligible) to form white light. So far, then, we can
utilize the luminosity methods to calculate and to trace the sensation
curves for the normal eye. It will not escape your notice that the
maximum heights of these two component curves are nowhere near the
parts of the spectrum where the colour is the purest. Another check to
these curves may also be made by taking the difference in the ordinates
of the luminosity curves of the colour blind and the normal eyed. Too
much stress must not, however, for the moment, be laid on this, as
this method depends on the absolute correctness of the scale of the
ordinates in the curves. It must be recollected that to the former
white light is deprived of at least one constituent sensation which is
perceived by normal eyes. This, in all probability, renders the white
less luminous to them than those possessing normal vision, so that the
comparisons of luminosities are referred to different standards.

It may seem a very simple matter to ascertain the correct scale, but it
is not, except by the extinction method, which will be described later.
At one time General Festing and myself tried to obtain a comparison
by finding the limiting illumination at which a book could be read.
We got results, but for the purpose in question the values are not
conclusive. What we really were measuring was the _acuteness of vision_
in different coloured lights. As a good deal depends upon the optical
perfection of the eyes under examination, besides the illumination,
we must be on our guard, even if there were nothing else against the
method, against taking any such measures as being conclusive.




CHAPTER VIII.


Before quitting the measurement of luminosity, it may be as well to
see whether the curves described are the same whatever the brilliancy
of the spectrum may be. We can easily experiment with a very reduced
brightness. Upon the screen we have an ordinarily bright spectrum. As
the slit, through which the white light forming the spectrum comes, is
narrowed, there is an evident change in the relative brightness of the
different parts, though the energy of every ray must be proportionally
reduced. The red is much more enfeebled than the green, and in
brightness the green part of the spectrum looks much more intense
than the yellow, which is ordinarily the brightest part. This we have
assured ourselves of not only by casual observation, but also by direct
measurement. Perhaps I can make this even more decisive to you. Two
slits are now in the ordinarily bright spectrum, one in the red, and
the other in a green which is near the E line of the solar spectrum.
Instead of using one lens to form a single colour patch of mixed light,
two parts of a lens, appropriately cut and of the same focal length as
the large combining lens, are placed in front of the slits, one bit of
lens before each. This artifice enables us to throw the patch of red
on one white surface, and the patch of green light on another adjacent
to it. By opening or closing one or other of the slits the brightness
of the two patches of light are so arranged that there is no manner
of doubt but that the red is the brighter of the two. The absolute
energies of the rays forming each of the patches are proportionally
reduced by closing the slit of the collimator, as before. At one stage
both patches appear of about the same intensity. This might be taken
for an error in judgment, but to make the change that takes place
perfectly plain to you, the rotating sectors are introduced in front of
the two slits, and the rays now pass through them. The apertures of the
sectors are gradually closed, and we now come to such a reduction that
the red is absolutely invisible; but the green still shines out. It is
losing its colour somewhat, and appears of a bluish tint. The reason of
this change of hue in the latter we shall shortly see. The sectors are
withdrawn and the red re-appears, and is as bright as the green. The
slit of the collimator is next opened, and there is no doubt that the
red is much brighter than the green, as it was purposely made at the
beginning of the experiment. The same class of experiment might have
been repeated with the green and violet or the red and violet, and the
same kind of results would have been obtained. The violet would have
been the last to disappear when the green was so reduced in luminosity
that it appeared in the ordinary brilliant spectrum to be equal to
the violet ray selected. When the green was of the luminosity given
by a slit equal in width to that of the violet, the violet would have
disappeared first, owing to its feeble brightness to begin with. Now,
if we measure a feebly illuminated spectrum we must adopt some special
means to exclude all light, except that of the comparison light and the
ray to be measured. This we can do by the box which is shown in the
next diagram (Fig. 24).

[Illustration: FIG. 24.]

At one end of a box, shown in plan, is an eye-piece, E. The other
end has at its centre a white square of paper of 1½-inch scale. The
monochromatic beam _a_, coming from the spectrum through the slit S
and the reference beam _b_ of white light, are reflected from glass
mirrors M₁, M₂ to apertures in opposite sides of the box, and from
close to these apertures by the right-angled prisms P₁ P₂, so as to
fall on and cover S. Rods R₁, R₂ are inserted in the box in the
paths of the beams, so that the opposite halos of S are illuminated.
Diaphragms inside the box cut off any stray light, and rotating sectors
placed at A and B regulate the intensity of the beams as required.
The sector A is rotated with a previously determined-on aperture;
the white light coming through B is altered till the luminosity of
the two on the screen, as seen through E, are the same. Every part
of the spectrum can be measured in this way; the result is shown in
the diagram. Fig. 25 (the measures will be found at page 215 in the
appendix). In this case the orange light at D where it fell on the
screen was equal to 1/132 of an amyl-acetate light, which, in its turn,
is closely ·8 of a standard candle. In the same figure the luminosity
curve of the ordinary bright spectrum is given for reference, and it
can be seen how the point of maximum luminosity is shifted into the
green, lying almost over the E line of the solar spectrum. The maximum,
of course, has been made 100 as before, for had it been drawn to the
same scale as the other, the form of the curve would not have been
demonstrated. There is a remarkable resemblance between it and the
curve of luminosity of the monochromatic vision, and such a resemblance
can scarcely be fortuitous. As a matter of fact, in this we seem to
have come to the final curve for low luminosities, and is almost the
same as that observed when the spectrum is reduced to such an extent
that it is colourless throughout, a condition that it can assume, as we
shall see very shortly. When the spectrum is rather more luminous, it
gives a curve of luminosity which is similar to that of the ordinary
spectrum when measured by a red-blind person. Here, then, we have an
indication that a person with normal vision passes through a stage of
red-blindness, as the intensity is diminished before he arrives at
absolutely monochromatic vision.

[Illustration: FIG. 25.]

This investigation is of practical as well as theoretical interest, as
General Festing and myself quickly discovered when we first made it.
The curious colour of a moonlight landscape is entirely accounted for
by it. White light becomes greenish-blue as it diminishes in intensity,
and the reds and yellows, being reduced or absent, are not reflected by
surrounding objects. Hence, moonlight is cold, whilst the sunlight is
warm owing to their presence.

When measuring these low luminosities, the various colours will in a
great measure disappear. Part of the spectrum will be of that peculiar
grey which was shown you in the experiment with the incandescent
light (p. 34). By further experiment it is possible to arrive at an
approximate determination of the point where all colour vanishes from
the different parts of the spectrum. We use the same apparatus (Fig.
24) as before, the only difference being that each of the sectors
is movable during rotation. The apertures of those through which
the colour passes are reduced till all colour on the screen just
disappears, the point being arrived at by a comparison with the white,
which is itself also reduced. The apertures of the first sector alone
need be noted, and from these readings the diagram (Fig. 26) is made
(for measures, see page 216).

[Illustration: FIG. 26.]

This extinction of colour is one which often occurs, but is seldom
noticed. The figure tells us that the orange is about the last colour
of the spectrum left, some of the others still appearing as greys. The
next to retain its colour is the green, and the most rapid to lose them
are the red and violet. It must not be supposed that the colours remain
of the same hue up to the time that they vanish. Pure spectrum red (red
sensation) remains the same up to the last, but the scarlet becomes
orange, and the orange yellower, and the green bluer. This is what
would be predicted from the Young theory if the order of extinction of
sensation be red, green, violet. This we shall see is the case. At
nightfall in the summer the order of disappearance of colour may often
be seen; orange flowers may be plainly visible, yet a red geranium
may appear black as night; the green grass will be grey when the
colour of the yellow flowers may yet be just visible. An early morning
start in the autumn before daybreak will give an ample opportunity of
satisfying oneself as to the order in which colours gradually re-appear
as daybreak approaches. Red flowers will be at the outset black,
whilst other colours will be visible as grey. As more light comes from
the sky the pale yellow and blue flowers will next be distinguished,
though the grass may still be a nondescript grey. Then, as the light
still increases, every colour will burst out, if not in their full
brilliance, yet into their own undoubted hue.




CHAPTER IX.


Not only, however, may we lose a sense of colour, but we may also
lose all sense of light by reducing the energy of the different rays.
We have seen that colour goes unequally from the different parts of
the spectrum. We may therefore prognosticate that the light itself
may disappear more rapidly from some parts than from others. You will
scarcely, however, I think, be prepared for the enormous difference
which exists in the stages of disappearance of the grey of the reduced
red and of that of the reduced green.

[Illustration: FIG. 27.]

But how are we to measure this extinction of light at the different
parts of the spectrum? This is a problem which I have attacked during
the last few years by a variety of methods; but as is the case with
almost every scientific problem, when the mode of attack is reduced to
its simplest form, it yields the more readily to solution. If we have
a box, like that figured in Fig. 27, and combine it with our colour
patch apparatus, the problem is solved. B B is a closed box 3 feet
long and about 1 foot high and wide, having two similar apertures 1½
inch in diameter in the positions shown. The aperture at the side is
covered on the inside by a piece of glass _a_, ground on both sides,
and a tube T is inserted, in which diaphragms, D, of various apertures
can be inserted at pleasure. The most convenient form of diaphragm is
that supplied with photographic lenses--an iris diaphragm. E is a tube
fitted at the end of the box through which the screen S is viewed. S
is black except in the centre, where a white disc is fastened to it. A
mirror, M, placed as shown, reflects the light scattered by the ground
glass on to the screen S. The rotating sectors are placed where shown,
and are in such a position that they can be readily adjusted by the
observer. The patch of any desired colour of the spectrum is thrown
on _a_, and an appropriate size of diaphragm used, so that when the
sectors are not less than 5° to 10° open, the light totally disappears.
We can now make observations throughout the whole spectrum, and knowing
the value of the different apertures of the diaphragm and the angular
opening of the rotating sectors, we can at once find the amount of
reduction of the particular part of the spectrum that is being required
in order to just extinguish all traces of light from the white disc
at the end of the box. From these measures we can readily construct
a curve or curves which will graphically show the reduction given
to the different parts of the spectrum. Fig. 28 gives the curve of
extinction for ordinary normal colour vision. The spectrum was of such
a brilliance that the intensity of the square patch of light formed on
_a_ of the orange light (D) was exactly that of an amyl-acetate lamp,
placed at one foot distance from the receiving screen. Knowing this,
the actual luminosity of all the other rays of the spectrum can be
derived from the curve of luminosity (see Fig. 20). Extinguishing the
various parts of the spectrum by this plan, it is found that the red
rays cease to stimulate the retina sufficiently to give any appearance
of light long before the green rays are extinguished. It is only the
rays in the extreme violet of the spectrum, and which consequently
possess very feeble luminosity, that make any approach towards
requiring the same amount of reduction as the red rays.

[Illustration: FIG. 28.]

There is the fact to remember in making these measures in the extreme
red and the extreme violet, that the luminosities of the colours are
so small that the illumination of the prism itself, by the white
light falling on it, has to be taken into account, since it forms
an appreciable portion of the patch of feeble colour. By placing a
proper shade of blue or red glass in the front of the collimator
slit this white light disappears or becomes negligible, and when the
absorption of the coloured glass is known from measurement, we can
get a very accurate measure of the extinction of these parts. Some
people may propound the idea that the rotating sectors may in such
kind of measurements give a false result. Now such a criticism is
quite fair, and it is absolutely necessary that it should be answered.
Well, to test the accuracy or the reverse of the assumption that such
measures are correct, the following small piece of simple apparatus
was devised. A and B (Fig. 29) are two mirrors placed at angles of 45°
to the angle of incidence of the beam. The path the beam takes can be
readily ascertained from the figure. This piece of apparatus was placed
in position in front of the spectrum, and the reflected beams used
to form the patches of colour. For convenience only a small pencil of
light was allowed to issue from the prism, a diaphragm of some ½-inch
in diameter being placed in front of it. This allows a spot of any
desired colour to fall on the screen, the ground glass being removed.
The slit through which the spectrum colours pass is moved along the
spectrum, and a position is arrived at where the last glimmer of light
disappears.

[Illustration: FIG. 29.]

The mirrors A and B may both be of plain glass blackened with smoke on
one side, or one may be plain glass and one silvered, or they both may
be silvered. This, with the power possessed of altering the aperture
of the slit of collimator, puts us in possession of ample means of
making our measures. We may also use the ground-glass arrangement and
use different diaphragms, which puts a further power of variation in
our hands. I may at once state that the resulting measurements fell on
the curves, obtained by measurements made with the rotating sectors, a
sufficient proof that the sectors may be used with confidence. There is
still another method which avoids a resort to the sectors. A tapering
wedge of black glass can be moved in front of the colour slit, and a
different thickness of glass will be required to cause the extinction
of each colour. Recently I have modified the extinction box, more
particularly for the purpose of using it where the spectrum is to be
formed of a feeble light, such as that of an incandescent lamp or a
candle. If a really black wedge could be obtained, this would seem to
be the best method, but no glass is really black. We have, therefore,
to make a preliminary study of the wedge to ascertain accurately the
absorption co-efficients for the different rays, a piece of work which
requires a good deal of patience, but which, when done, is always at
command.

In Fig. 28 two branches of the curves are given at the blue end of the
spectrum; one is shown as the extinction for the centre of the eye,
and the other of the whole eye. Of course the former observations were
made by looking direct at the spot. This may appear a very easy matter,
but it is not really so simple as it sounds. It is curious how little
control there is over the absolute direction of the eyes when the
light has almost disappeared. The axes of the eyes are often directed
to quite a different point. When the extinction for the whole eye is
made, the readings are really much easier, as then the eye roams where
it likes, and a final disappearance is noted. When the eye has once
been invested with a roving commission, it is hard to control it. In
making these observations it was therefore advisable to have data for
the first branch of the curve, before commencing to observe for the
later. The main cause of difference between the two branches of the
curve is due to the absorption by the yellow spot.

It might be thought that with the curves (Fig. 28) before us, we have
learnt all we can regarding the extinction of light, but is it so?
Surely we ought to know something as to the reduction necessary for
extinction of the different parts of the spectrum when they are all of
equal luminosities and of ordinary brightness.

We arrive at this by simple calculation. Supposing we have two
luminosities, _one double the other_, it does not require much thought
to find out that you have to reduce the greater luminosity twice as
much as the other in order for it to be just extinguished. In other
words, if we multiply the extinction by the luminosity, we get what we
want. Now, in the curves before us, we have taken the luminosity of the
yellow light near D as one amyl-acetate lamp, and that has a height
in the curve showing the spectrum luminosity very closely approaching
100. We may, therefore, multiply the extinctions of a ray by the value
of its ordinate in the luminosity curve and divide the result by 100,
and this will give us the extinction of each colour, supposing it had
the luminosity of an amyl-acetate lamp. A portion of the curve so
calculated is shown in the same diagram (Fig. 28) as a dotted line. It
appears at the violet end as an approximately horizontal line, and then
starts rapidly upwards, and would, if carried on to the same scale,
reach far out of the diagram; but at the extreme red it would be found
to bend and again become horizontal. I would have you notice that the
same is true not only for the extinction observed with the centre of
the eye through the yellow spot, but also for the whole eye. Such
straight, horizontal parts of the curve must mean something.

[Illustration: FIG. 30.]

In the diagram (Fig. 16) of colour sensations we see that in each
of these two regions there is but one sensation excited, viz. the
violet and the red. Now, if these sensation curves mean anything, the
reduction necessary to produce the extinction of the same sensation
when equally stimulated should prove to be the same, for there is
no reason to the contrary, but exactly the reverse. _Primâ facie_,
then, taking the Young theory as correct, we may suppose that these
horizontal parts are due to the extinction of one sensation. Let us
treat it as such, and go back to the original extinction curve shown
in the continuous lines. The parts of the curve which lie over the
fairly horizontal dotted line, at all events, should be the extinction
curve of the same sensation, but more or less stimulated or excited.
As before explained, if we have double the stimulation at one part of
the spectrum to that we have at another, the reduction of the greater
luminosity to give extinction will be double that of the lesser. If,
then, we take the _reciprocals of the extinction_, it ought to give
us a curve which is of the form of some colour sensation; and when we
arrive at the maximum, we may for convenience make that ordinate 100,
and reduce the other ordinates proportionally. This has been done in
Fig. 30 in the curves C and D. For the sake of a name my colleague and
myself have named such curves “persistency curves.” Perhaps some other
name might be more fitting; but still a poor name is better than none
at all.

When the persistency curve was scrutinized to see what might be
taken as its full signification, I must confess that the result
astonished us somewhat, though we ought not to have been surprised.
The persistency curve C, when applied (in a Euclidean sense) to the
curve of luminosity recorded for the men who had monochromatic vision,
almost exactly coincided with it. In other words, by far the largest
part of the extinction was due to the extinction of the sensation
which in the monochromatic vision was alone excited. If this be not
the case, there is something in colour vision which no theory which
I am acquainted with can account for. Then, again, the persistency
curve agrees with the curve of luminosity when the intensity of the
spectrum is very feeble, which is another coincidence of a remarkable
character which some theory should explain. [Fig. 30 gives, besides
the persistency curves, the luminosity curves of the normal eye, of
monochromatic vision, and of the violet-blind; and an exaggerated curve
of the difference between the normal luminosity curve and that of the
violet-blind, and others which I think will be found useful for general
reference.]

What sensation is it that is last extinguished, and which is possessed
by a certain class of colour vision? In the Young theory it can only
be the violet sensation. It is certainly not the green, and much less
the red. It does not correspond, however, very well with the violet
sensation shown in Fig. 16, but more with one which should be in the
blue.

In making the extinctions of light, it is quite necessary that certain
precautions should be taken to avoid error. All my audience know that
when going from bright daylight into a cellar, in which only a glimmer
of light is admitted, but little can be seen at first, but that, as the
eye “gets accustomed” to the darkness, the surroundings will begin to
be seen, and after several minutes what before was blackness comes to
be invested with form and detail. So it is with the extinction of light
in the apparatus described. Observations carried on before the full
sensibility of the eye is attained are of no value. A recorded set of
observations will show this. A light of a certain character was thrown
on the extinction box, to be extinguished, and the observer entered
the darkened room from the full glare of daylight. The eye was placed
at the eye end and kept there, and the extinctions were made one after
the other till they became very fairly constant. The following is the
result:--

    Times of Observation.   Readings.
  At the commencement          1·0
  After         38 sec.        3·2
  After         53 sec.        4·9
  After  1 min. 11 sec.        6·9
  After  1 min. 44 sec.       10·5
  After  2 min. 43 sec.       17·0
  After  3 min. 44 sec.       27·5
  After  4 min. 52 sec.       43·0
  After  5 min. 59 sec.       63·0
  After  6 min. 41 sec.       78·0
  After  7 min. 28 sec.       89·0
  After  8 min. 32 sec.       96·0
  After 10 min. 46 sec.      103·0
  After 12 min.              103·0

  (For convenience the first reading is unity; the other numbers are
                the _inverse_ of the extinction value.)

The eye apparently, under the conditions in which these observations
were made, was at least 100 times more sensitive to very faint light
after twelve minutes than it was at the beginning, and that then
concordant readings could be made. It will now be quite understood that
before any serious measures can be made this interval must elapse,
and also that the light, finding its way to the end of the box to
illuminate the spot, should never be strong, otherwise the eye might
lose its sensitiveness.




CHAPTER X.


Before considering the subject of the extinction of light by other
types of colour vision, attention must be called to what has already
been brought before you. The various colours of the spectrum have to
be reduced to the following amounts before they suffer extinction, the
orange light at D being of the value of one candle. (See appendix, page
217, for complete tables.)

                    Reduction in
                     Millionths.                 Remarks.

  B              10,000 or 1/100         approximately pure red
                                           sensation

  C               1,100 or 1/909         rather more scarlet

  D                  50 or 1/20000       orange light

  E                 6·5 or 1/154000      a green chosen by Maxwell
                                           as a standard colour

  F                15·0 or 1/67000       beginning of the blue

  Blue Lithium     85·0 or 1/11700       a good sample of blue

  G               300·0 or 1/3300        approximately pure sensation
                                           of violet.

If we make these same colours all of the luminosity of one amyl-acetate
lamp (·8 of a candle), we find that the numbers are as follows:--

              Reduction in
               Millionths.

  B              300
  C              225
  D               48
  E                3·3
  F                 ·9
  Blue Lithium     1·1
  G                1·1

These numbers are remarkable, and we may enforce what they mean in
this way. The energy of radiation, and of light also when of ordinary
luminosity, varies inversely as the square of the distance from an
incandescent body when of small dimensions. But from the above it seems
that a white screen receiving the rays from an amyl-acetate lamp in an
otherwise perfectly dark place, and having a colour which stimulates
the red sensation alone, would be invisible at 58 feet distance, for
there would not be enough energy transmitted to stimulate the red
perceiving apparatus sufficiently to give the sensation of light. If
it were an orange light, such as sodium, of the same luminosity, we
should have to move it from the screen 142 feet before the same result
was attained. With the green light at E, the distance would be 550
feet, and with the violet the distance would be increased to 1000 feet.
The reduction in intensity of white light, which, when of ordinary
brightness, is warm, would make it colder, for the red would disappear,
and finally the residue of light, just before extinction, would become
a cold grey, due to the absence of all colour. The changes in hue that
would occur are variable, the variation being due to the loss of colour
of the different rays for different amounts of reduction, and then
their final extinction. We can place two patches of white light on the
screen, and gradually reduce one in intensity, keeping the other of its
original value. No one would expect that the two would be dissimilar
in hue, as they appear to be when the former is moderately near the
extinction value. If we wish to see this perfectly, we should use an
extinction box and view it away from the surroundings, which must be
more or less slightly illuminated.

It has already been stated that the persistency curve for persons who
have normal colour vision is closely the same as that recorded for
those who are of the monochromatic type. As this is so, we must expect
to find that the persistency curve of these last is the same as their
luminosity curve. We put this to the test of experiment and found that
our reasoning was correct, for the persistency curve could be almost
exactly fitted to it. (See table, pages 217 and 222.) The slight
difference between them can be credited to the fact that the whole eye
may have been brought into use during the extinction observations, the
centre of the eye not being exclusively used. The Figure 31 shows
both the extinction and the persistency curves, and also the curve of
luminosity for the normal eye.

[Illustration: FIG. 31.]

The former were derived from a case P. sent for examination. P. and
Q. are brothers, each of whom possesses but one colour sensation, and
examination showed that their vision was identical. Mr. Nettleship
has kindly given me the following particulars regarding them:--“Their
acutes of vision (form vision) in ordinary daylight is only one-tenth
of the normal. A younger sister and brother are idiotic and almost
totally blind, and in one of these the optic nerves show clear
evidence of disease. Hence, the colour blindness of P. and Q. must
almost without doubt be considered as the result of disease, perhaps
ante-natal, involving some portion of the visual apparatus.” A lack
of acuteness of vision would be expected from the small amount of
light they perceive compared with normal vision. The fact that two of
a family, not twins, possess exactly the same colour sense, and that
their extinction curves are entirely different to those suffering from
post-natal disease, but similar to those of normal vision, point to
their colour blindness as falling in the same general category as that
of the congenital type. To this I shall refer again.

[Illustration: FIG. 32.]

We may reason still further. With the red- and green-blind the violet
sensation is still present, and we may therefore expect that their
extinction curves, and consequently their persistency curves, should
be alike, and should also agree with that made from your lecturer’s
observations. A study of Figures 32 and 33 will tell you that such is
practically the case. The former shows the luminosity, the persistency,
and the extinction curves of a completely red-blind subject, and the
latter the same curves for a green-blind subject (see pages 223 and
224). Both were excellent observers, and their examination was easy,
owing to the acquaintance with scientific methods. The accuracy of
their results may be taken as unquestionable. Each of them may be taken
as a representative of their own particular type of colour blindness.
There is an agreement between them at the violet end, but a deviation
at the red end of the spectrum. The general form of the curves
indicates that the same sensation is extinguished last in all. Now,
have we any other criterion to offer? We have. In the first instance,
we have the violet-blind person to compare with the others, and also
another observer who had monochromatic vision, but whose sensation
was different to that of the two monochromatic cases we have so far
brought to your notice. We have already stated the peculiarities in
colour nomenclature of the violet-blind case. His curve of luminosity
for the spectrum was taken (page 227), and when compared with the
curve of normal luminosity, it became evident that in the red and up
to the orange his measures were those which a normal eye would make;
but that the luminosity fell off in the green, and finally disappeared
to an immeasurable quantity in the violet (see Fig. 30, curves M and
F). If his measures of spectrum luminosity are deducted from those of
the normal eye, and the ordinates be increased proportionately to make
the maximum difference 100, the figure so produced, when compared with
the _luminosity_ curve obtained from the monochromatic observers, was
found to be the same, and consequently with the persistency curves
above referred to. Endeavours were made to gain a good extinction
curve, but the results were not as successful as could be desired; but
it was ascertained that, without doubt, his most persistent sensation
was not more than 1/180 as lasting as that of the normal eye, or to put
it in another way, his green at E was only extinguished when the energy
falling on his eye was 180 times greater than that at which it vanished
with the normal eye. This plainly teaches us that the missing sensation
was that which, when present, is ordinarily the most persistent.

The next is a case of monochromatic vision, which differs from those
previously brought before you, and I cannot do better than describe it
in the words which General Festing and myself employed in our paper in
the “Philosophical Transactions.”

[Illustration: FIG. 33.]

The patient (B. C.) had been examined by Mr. Nettleship, who kindly
secured his attendance at South Kensington for the purpose of being
examined by the spectrum and other tests. [Mr. Nettleship states
that this case is without doubt a genuine case of congenital colour
blindness, without any trace whatever of disease.] B. C. is a youth
of 19, who has served as an apprentice at sea. His form vision is
perfect, and he is not night blind. He can see well at all times,
though he states that on a cloudy day his vision seemed to be slightly
more acute than in sunshine. He was first requested to make matches
with the Holmgren wools in the usual manner, with the result that he
was found to possess monochromatic vision. He matched reds, greens,
blues, dark yellows, browns, greys, and purples together; and it was a
matter of chance if he selected any proper match for any of the test
colours. Finally, when pressed, he admitted that the whole of the heap
of wools were “blue” to him, any one only differing from another in
brightness. The brighter colours he called “dirty” or “pale” blue,
terms which eventually proved to be synonymous. We then examined
him with patches of monochromatic spectrum colours by means of the
colour patch apparatus. He designated every colour as “blue,” except a
bright yellow, which he called white, but when the luminosity of this
colour was reduced he pronounced it a good blue. So with white, as the
illumination was decreased, he pronounced it to pass first into dirty
blue, and then into a full blue.

Colour discs were then brought into requisition, and it was hard at
first to know how to make the necessary alterations, owing to the terms
he employed to express the difference which existed between the inner
disc and the outer grey ring. By noting that a pale “blue” passed into
a pure blue when the amount of white in the outer ring was diminished,
and that the inner disc was described as “pale” or “dirty” when the
outer ring was described as “a very full blue,” we were enabled to make
him match accurately a red, a green, and a blue disc separately with
mixtures of black and white.

The following are the equations:--

  360 red   = 315 black + 45 white.
  360 green = 258 black + 102 white.
  360 blue  = 305 black + 55 white.

With these proportions he emphatically stated that all were good blues,
and that the inner disc and outer ring were identical in brightness and
in colour.

It may be remarked that this is a case of congenital colour blindness,
and that there is reason to believe that some of his ancestors were
colour blind.

Before using the discs an attempt was made to ascertain the luminosity
of the spectrum as it appeared to him. His readings, however, were so
erratic that nothing could be made out from these first observations,
except to fix the place of maximum luminosity, the terms “pale” and
“dirty” puzzling us as to their real meanings. After the experience
with the discs we had a clue as to what he wished to express by pale or
dirty blue, which only meant that the colour or white was too bright,
and on making a second attempt he matched the luminosities of the two
shadows as easily as did P. and Q., the other cases of monochromatic
vision. The method adopted was to diminish the white light illuminating
one shadow to the point at which he pronounced it a good blue, when
a slight alteration in the intensity was always sufficient to secure
to his eye equality of luminosity between it and the coloured shadow
without his perceiving any alteration in the saturation.

[Illustration: FIG. 34.

B. C.’s Luminosity and Extinction Curves.]

The curve of luminosity, Fig. 34, is a very remarkable one, being
different in character to that of P. and Q., the maximum being well on
the D side of E. A great falling off in the luminosity when compared
with that measured by the normal eye will be noticed both in the
blue and in the red. (For measures see page 225.) The evidence was
therefore presumptive that B. C.’s colour sensation was neither red nor
blue, but probably a green.

The next test was made to throw light on this point. He made
observations of the extinction of the different parts of the spectrum.
His observations were very fair, except on the violet side of F, where
they became slightly erratic, but by requesting him to use all parts
of his retina to obtain the last glimpse of light, a very concordant
curve resulted, as shown in Fig. 34. Some of his observations at this
part were evidently made with the centre of the retina, for they gave
readings which, when the “persistency” curve was calculated, and
these observations treated as part of the extinction, agreed with the
luminosity curve. We may, therefore, conclude that B. C. has a region
in the retina in which there is an absorbing medium corresponding to
the yellow spot of the normal eyed. This is diagrammatically shown in
Fig. 34 by the difference in height of ordinates in the persistency
(dotted) and the luminosity curves. On the red side of the maximum the
two curves are practically identical, except from Scale number 54. At
this point it is probable that the white light which illuminated the
prism vitiated the readings to some degree. At the violet end something
similar, doubtless, occurs, but it is masked by the difference that
exists in the extinction by the central part of the retina and that of
the whole eye.

It must, however, be remarked that the amount of reduction of the
intensity of a ray to produce extinction is very different for B. C.
and for the normal eyed, or for the red- and green-blind or for P. and
Q. B. C. can bear nearly 200 times less reduction for the rays near
E. We have already pointed out that the same is practically the case
with M., whom we presume to be violet-blind. We may therefore deduce
the fact that the monochromatic vision in this case is of a totally
different type to that of P. and Q., and that the last sensation to be
lost is the same as that of M. If any violet sensation were present in
either, the fact would be made evident by the order of the extinction.
The sensation of B. C. is thus apparently the green sensation, though
that this particular sensation is exactly the same as that absent in
the green-blind is not certain.

The observations made by the different types of the colour blind
seem to me to throw great light on the theory of colour vision. They
show that when the violet sensation is present, according to the
Young theory, the extinction shows its presence; and that where this
sensation is absent, the reduction of light necessary to produce
extinction is greatly less, and may with great certainty be attributed
to a different sensation being the final one to disappear.




CHAPTER XI.


I have so far spoken only of normal, or physiological, colour
blindness; a peculiarity, or defect, present at birth, and, as far
as is at present known, irremediable, but not associated with any
defect of the visual functions, or with any disease or any optical
peculiarities. What the nature and seat of this defect may be--whether
in the eye or in the sensorium--is at present unknown, although some of
the characteristics of the deficiency in colour sensation, I believe,
seem to indicate the existence of a special part of the brain endowed
with the functions for perceiving colour.

But cases are well known to medical men in which colour vision,
normal to start with, fails in greater or less degree in connection
with disease. This part of the subject is large and very complex, and
requires for its full elucidation an acquaintance with the diseases
and disorders of the eye. Many of the phenomena accompanying acquired
colour blindness, however, are of great interest to the physicist
in his study of colour vision, more particularly in regard to the
test of the truth of any particular theory. Through the kindness of
several medical men, and Mr. Nettleship in particular, I have had the
opportunity of examining by the colour apparatus several types of
colour blindness due to disease. One feature, common, I understand,
to all, or nearly all, cases, is the presence of some disease of the
optic nerve. Defective sight--from loss of transparency of the cornea,
the crystalline lens, or other transparent parts of the eye--does not
interfere with the perception of colour; nor is true colour blindness,
as I am informed, well marked, if present at all, in disease limited
to the choroid and retina (see Fig. 1). Even in cases of the disease
of the optic nerve, medical authorities tell us that great differences
exist in the amount of colour defect, and that _although the colour
defect always goes along with some other serious visual loss, either
of form, light, or field_, the relation between these several factors
of the visual defect is by no means always the same, so far as can be
judged by the tests commonly used by ophthalmic surgeons. They tell us
that in some cases of disease of the optic nerve, colour vision when
tested by the wool test, which will be described shortly, may be almost
perfect, whilst the capacity for reading test letters of the alphabet
may be extremely bad, and _vice versâ_. It seems that in some cases
these discrepancies cannot be accounted for; but in others the facts
can be explained by the limitation of the disease to certain fibres of
the optic nerve. Thus, if those fibres which supply the yellow spot
region of the retina are alone involved, direct, or central, vision
will be much damaged both for form and colour, whilst a little further
from the centre of the field, the visual functions in such a case are
often quite normal. From what has been said in the opening chapters,
this will be understood to be that the colour vision is perfect, but
the definition of form more or less imperfect. We are told that cases
of this type have long been known and are comparatively common, and
often favourable as regards recovery; that the mischief may affect one
optic nerve, or both; that when both are diseased the malady is usually
due to the action of some toxic substance, and that of all substances
known to have this particular effect on the optic nerves tobacco is
the most important. I dwell a little on this variety--damage to form
and colour sense at the centre of the visual field of each eye from
limited, and usually curable, disease of the optic nerve--on account
of its interest to myself in the investigations I have made, and also
on account of the degree of practical importance which it assumes in
connection with the proper reading of signals and coloured lights.
These cases of “tobacco amblyopia,” as it is pathologically called,
are, of course, always found in men; and it may occasionally happen
that such a man, if an engine driver, signalman, or a look-out man on
board ship, may still see form sufficiently well to see his signals,
but may mistake their true colours. From evidence given before the
Committee of the Royal Society on Colour Vision, it appears that the
disease causing this type of colour blindness is usually produced by
the over-use of tobacco, aided by mental depression and a low state
of health. As we have no sumptuary laws, cases of tobacco blindness
must frequently occur, and it should be the care of all who have the
management of railways or shipping to take measures for preventing
persons suffering from this disease from occupying posts which require
perfect colour vision in order to prevent the possibility of loss of
life.

Congenital colour blindness can at once be discovered, and its
possessor be excluded from any post in which normal colour perception
is necessary, but with this type a single examination is no safeguard,
as it may be developed at any period of a man’s career. The disease
is, I believe, a progressive one, and at first is most generally
unrecognised, the deficiencies of vision being usually slight at its
commencement. It is very often brought to the notice of the sufferer
by finding he is unable to read. The words at first seem only slightly
indistinct, but later become undecipherable, and as time goes on he is
unable to even see the letters. He or his friends then usually think
it time to consult the specialist. In tobacco amblyopia the area of
insensibility is central, and it may subtend a very small angle or one
which covers a considerable portion of the field. I am not aware that
it ever extends over it all, but it very generally covers the yellow
spot. Now as the eye naturally receives the image on the centre of the
retina, it follows that, as the ability to distinguish some colours is
absent in that particular region, the patient is practically colour
blind, though he can distinguish them on most parts of the retina which
are not affected. As regards form vision, it was mentioned in the first
chapter that in a healthy eye it is much more acute at the centre
than towards the periphery, and instances were given of the angular
distances apart that black dots on a white ground were required to be
placed to allow their being seen as separate objects when the images
were received on the centre of the retina, and at the periphery. Sharp
definition may be said to be almost confined to 3° of angular distance
at the centre, and most probably this is a happy state of affairs, for
if we could see equally distinctly with the whole field of vision, the
mind would be distracted from the object which it wished primarily to
contemplate.

Bearing in mind the want of definition beyond 3°, and the
indistinctness caused by a diseased central area, it will not be
surprising to find that form vision in these cases is imperfect
throughout, though the colour perception outside such area may be
unimpaired. But, practically, men suffering from this disease are
colour blind to coloured objects, such as a signal light on a railway
or a ship’s light at sea. They may see that there is light at the
distant signal or on the bow of a vessel, but will be unable to
interpret correctly the colour. The colours which fail to make visual
impressions are the reds and greens. Some will distinguish yellow, and
very nearly all will distinguish blue with the centre of the eye. If
a bright spectrum be thrown on the screen, and a tobacco-blind person
be requested to name the colours of the different parts pointed out
to him, it is often the case that as his eyes follow the pointer he
will tell you that in the extreme red he sees no light, but in the
bright red he sees dull white. The bright yellow he will tell you is
a pale yellow or white, according as his case is a moderate or bad
one; the green he will call white, and the blue and violet he will
designate correctly. At the same time that his eye is turned away to
another colour, he will see the true colour of the part of the spectrum
which he has just incorrectly named, but it will disappear again as he
turns his eyes back again. This tells us that his sense of colour is
apparently unaffected outside the diseased area.

[Illustration: FIG. 35.

_Left Eye._    _Right Eye._]

At page 10, a description has been given of the manner in which the
field for colour and light has been determined, and if this same method
be pursued with persons suffering from this form of colour blindness
we get some remarkable results. Fig. 35 is the chart of the eye for
red and for white, which was made by a case of tobacco blindness. The
yellow spot is entirely affected, and, as is very common, it extends
to the blind spot in the eye. At no place within that area can red be
seen, though blue is immediately recognised. The extent of the field
for white is that found under normal conditions, and except for the
diseased area the same is true for the red. The fields for both eyes
are given: that for the left eye in the left-hand chart, and that for
the right eye in the right-hand chart. The small dark spots within the
5° area are places where the colour sensation is most defective. The
part in the central dark area shaded with lines in this direction ////
shows the portion of the field which is insensitive to red, though not
to _light_, whilst the remainder of the shaded central area indicates
the extent of the field which is sensitive to red. The field for light
generally is also shown by the (approximately) rectangular unshaded
area. Although the area occupied by the insensitive part of the retina
is small compared with the whole, yet it is in that part which is used
for distinct vision.

For testing for colour the apparatus, Fig. 3, arranged so that the
patch of colour has the white patch alongside, is the most useful, but
it is as well then to use a surface of patch about ½ inch square only,
and thus to confine the image as nearly as may be to the spot on the
retina which is defective. These cases of central scotoma are by no
means very easy to test; for it frequently happens that before they are
able to distinguish that there are two patches side by side, they have
to approach very close to the screen. If this be the case, however,
it will usually be found that the patches of ½ inch side are still
efficient, as the near approach of the eyes to the screen indicates
a wide area as being affected, so that the image still lies within
the diseased retinal area. In some instances the colours named will
vary very considerably; sometimes, for instance, a red will be named
as grey, and then immediately after as pale red. This is generally
due to the diseased area being small, and a very slight change in the
direction of the axis of the eye causes it to be seen in nearly its
true colour, part being viewed with the diseased and part with the
healthy portion of the retina. With the wool test, which we shall
describe later, it is the commonest thing possible for colour-blind
persons who have a central scotoma to match accurately the different
test-skeins, for the reason that the images of the skeins of wool are
so large that they are received on the parts of the retina which are
not diseased. These same colours, however, if presented to them in
small patches, will inevitably show the defect in vision.

With this end in view, I have had a set of brick-clay pellets some
3/16-inch in diameter, painted with water-colours mixed with soluble
glass solution of the same colours as the wools. These are placed in
a shallow tray, and presented to patients affected with this central
colour blindness to pick out all the pellets which match reds and
greens. They will tell you that they see neither one nor the other,
though they will pick out the blue pellets unerringly. A red pellet
they will match with a red, green, grey, or a brown one, and a green
one with the same. If, however, you instruct them to direct their eyes
a few degrees away from the tray, they will tell you they see all the
colours, and as they endeavour to pick them out, they, with a natural
instinct, direct their eyes again to the collection, when once more
the colours vanish. It is almost piteous sometimes to see the distress
which this simple test occasions. The sight of the colours for an
instant and their immediate disappearance in the cases that I have
tried, seem indicative of something terrible, for they usually have no
idea of the cause of this (to them almost miraculous) phenomenon. I
have seen these colour blind tested with a pair of ordinary bull’s-eye
lanterns, placed side by side, with diaphragms of moderate size with
coloured glasses, which can be changed at will, in front. At twelve
feet distance they will often see both lights as one, but as they
approach they will make out two lights and call them both white, or
sometimes they will make a guess and call a green red, or _vice versâ_.
It goes without saying that such eyesight is useless for reading
signals, and indeed for any purpose whatever. Sometimes, but I believe
this is rare, no colour whatever can be distinguished.




CHAPTER XII.


I will now give in full the result of the examination of a patient who
was suffering from tobacco blindness. X., aged thirty-six, a commercial
traveller, was suffering from rather severe tobacco amblyopia. The
scotoma was a very marked one, and the loss of colour sensation most
complete. Mr. Nettleship, who furnished the case, has kindly added the
following remarks on the case:--

His acuteness of vision was 6/36 with the right eye and 6/60 with the
left. He smoked half-an-ounce of “shag” daily and drank about four
pints of beer. His sight had been failing for about two months. As is
common in early stages of this disease the ophthalmoscope revealed no
decided changes at the optic discs.

He passed the test of the Holmgren wools satisfactorily, proving that
the usual vision was normal for colour, but failed at once with the
pellet test.

[Illustration: FIG. 36.]

The objects in view were to test his perception of the spectrum
colours, and then the extent of his retinal field for colour. This
last is not recorded here. The spectrum colours were reduced to
uniform luminosity between λ 4600 and λ 6600. Diaphragms containing
holes of different sizes were placed in front of the last prism, and
thus a round spot of monochromatic light of the same luminosity was
produced upon the screen when a slit was passed through the spectrum.
From the red end to λ 5270 he called the whole of the colours white,
and from that point he began to see blue, called the colours bluish
and blue. When the full illumination for all the colours was used,
the same results were obtained. From this examination it would appear
that he was totally deprived of the sensation of any colour except of
blue. A subsequent examination of his perception of the luminosity of
different rays, however, has to be taken into account, for in the first
examination he had no light of pure white with which to compare the
colours. In the next experiments, a strip of white light was placed in
juxtaposition to the colour, and the results were slightly different.
The table below gives his luminosity measures (Fig. 36). Col. I. is
the empyric scale number, II. is the wave-length, III. the luminosity
of the colour to the normal eye, IV. the luminosity to X., and V. the
ratios of III. to IV.

In the diagram, his luminosity curve X. is shown, its area being 1400
against 1650 for the normal eye. His central perception of light, as
arrived at by the extinction method, was only two-thirds of that of
the normal eye; hence his area of luminosity should be 1100. As it is
1400, the ordinates of the above curve should be multiplied by 0·8, to
compare with that of the normal eye.

  -----+-------+----------+----------+-----+--------------------+----------------
  I.   | II.   | III.     |  IV.     |  V. |                    | Spectrum
  -----+-------+----------+----------+-----+                    | colour to
  Scale|Wave-  |Luminosity|Luminosity| IV. |  Colours to X.     | normal
  No.  |length.|to the    |to X.     | --  |                    | eye.
       |       |normal eye|          | III.|                    |
  -----+-------+----------+----------+-----+--------------------+----------------
  60   |  6730 |    7·3   |     0    | 0   |Sees only the white |Red.
       |       |          |          |     |stripe              |
  57   |  6423 |   32     |    10    | 0·31|Calls red yellowish,|Scarlet.
       |       |          |          |     |and white bluish    |
  55   |  6242 |   65     |    38    | 0·65|  „      „          |
  53   |  6074 |   96     |    86    | 0·89|Both one colour     |Red-orange.
  51   |  5920 |   99     |    90    | 0·91|  „      „          |Orange-yellow.
  47   |  5660 |   92     |    83    | 0·90|Calls green a little|Greenish-yellow.
       |       |          |          |     |blue; white he      |
       |       |          |          |     |sees as white       |
  43   |  5430 |   69     |   625    | 0·90|  „      „          |Yellowish-green.
  40   |  5270 |   50     |    46    | 0·92|  „      „          |Green.
  32   |  4910 |    8·5   |     9    | 1·06| Sees blue as blue, |Greenish-blue.
       |       |          |          |     |and white yellowish |
  31   |  4960 |    7     |     8    | 1·14|  „      „          |Blue.
  26   |  4680 |    3     |     3    | 1·00|  „      „          |Blue.
  -----+-------+----------+----------+-----+--------------------+----------------

His readings of luminosity were made without any hesitation, and were
concordant for each observation, which is not to be wondered at, as the
matches, except at the blue end, were practically matches of different
mixtures of black and white.

It appears that the white which X. sees as white is the same as the
orange sodium light, and that the red he sees is yellowish. The mixture
of this yellowish-white with the blue makes white. He sees a little
blue in the spectrum colour at λ 5720, so it must be taken that at
that point of the spectrum he begins to see colour--a point which is
considerably lower than that given by his preliminary examination
of the spectrum colour, and due, no doubt, to the fact that in this
experiment he had the white light of the positive pole of the electric
light to compare with it. It seems probable that what X. called
yellowish was really a sensation of white mixed with a very small
quantity of red sensation, for he saw no yellow in the orange, in
which that colour would be most easily distinguished on account of its
luminosity. Red light, when strongly diluted with white light, to the
normal eye is often called orange.

As, practically speaking, the colour vision of X. is confined to blue
and white, it is of interest to note the difference in luminosity at
the different parts of the spectrum that is registered by him and by
P., who had blue (violet) monochromatic vision. To facilitate the
comparison, the luminosity curve of the latter is shown in the diagram.

[Illustration: FIG. 37.

The thin line curve is the normal curve.]

Perhaps another case of a patient suffering from tobacco blindness may
be quoted, as it will show the differences that exist in recognising
the colours of the spectrum, and that the shorter the visible limit of
the spectrum at the red end, the more pronounced is the extent of the
colour blindness. G. suffered from a very well-marked tobacco scotoma,
occupying a considerable area. His curve of luminosity of the spectrum
is shown in Fig. 37. The horizontal band beneath will show the colours
which the spectrum colours appeared to match.


TABLE OF LUMINOSITY FOR G. _See_ page 153.

  -------------------------------------------------+
  Scale No.                                        |
     +---------------------------------------------+
     | Wave-length.                                |
     |      +--------------------------------------+
     |      | Reading.                             |
     |      |    +---------------------------------+----------------
     |      |    |                                 |    Colour of
     |      |    |       Colours named by G.       | spectrum to the
     |      |    |                                 |   normal eye.
  ---+ -----+----+---------------------------------+----------------
  57 | 6423 |  0 |                                 | Scarlet.
  55 | 6242 |  3 | No colour                       |
  53 | 6074 | 11 | Colour “yellow,” white “blue”   | Red-orange.
  51 | 5919 | 34 |   „        „       „     „      | Orange-yellow.
  50 | 5850 | 60 |   „        „       „     „      |
  49 | 5783 | 64 | Colour “gold,” white “sky-blue” | Yellow.
  45 | 5538 | 59 |                                 |
  40 | 5270 | 40 | Both white                      | Green.
  35 | 5042 | 18 |     „                           |
  30 | 4848 | 10 |     „                           |
  29 | 4807 |  6 | Colour “very pale blue,”        | Blue.
     |      |    |     white as white              |
  26 | 4707 |  4 | Colour “blue,” white “white”    |
  20 | 4518 |  3 |   „       „      „      „       |
  10 | 4248 |  2 |   „       „      „      „       | Violet.
  ---+------+----+---------------------------------+----------------

G. was tested for light sense by the extinction method, and it appears
that the final sensitiveness to light at the central part of the eye
was nearly 12 times less than a person possessing normal sense. I may
mention that I have examined one, if not two cases in which the patient
was not only tobacco blind, but also congenitally colour blind. Though
interesting for record, they need not be given in full here.

With these specimens of examination I must leave the cases of tobacco
blindness. Although very important, they by no means constitute the
sole cases of colour deficiency due to disease. I will give as an
instance a case of loss of colour sensation due to progressive atrophy
of both eyes which was examined, with Mr. Nettleship’s aid. When
tested with spectrum colours--a patch of white light being placed in
juxtaposition with the colour--it was found that W. S. was absolutely
blind to colour from 26·75 (λ 4733) on the scale of the spectrum to the
termination of the red of his spectrum, which was close to 63 on the
scale (λ 7082). Above scale No. 26·75 W. S. saw blue, and his spectrum
was continued normally in the violet. His luminosity curve (Fig. 37)
was made without any difficulty, and, compared with my own, shows a
slight deficiency in brightness from the red to the yellow, but his
perception of luminosity increases as the blue is approached.


TABLE OF LUMINOSITY FOR W. S. _See_ page 155.

  ---------+------------+---------+---------+----------
           |            |         | Spectrum| Spectrum
           |            |         | colours | colours
           |            |         |  named  | to normal
  Scale No.|Wave-length.| Reading.| by W. S.|   eye.
  ---------+------------+---------+---------+----------
     60    |    6728    |   3·4   | Grey    | Scarlet.
     58    |    6520    |  15·0   |  „      |
     56    |    6330    |  41·0   |  „      |
     55    |    6242    |  43     |  „      |
     54    |    6152    |  69     |         |
     52    |    5996    |  94     |         |
     50    |    5850    | 100     |  „      | Orange.
     48    |    5720    |  96     |         |
     45    |    5538    |  88     |         |
     42    |    5373    |  74     |         |
     40    |    5270    | 61·5    |  „      | Green.
     38    |    5172    |  45     |         |
     35    |    5042    |  30     |         |
     30    |    4848    |  12     |  „      |Blue.
     25    |    4675    |   6     | Bluish  |
     20    |    4518    |   4     |         |
     15    |    4376    |   3     | Blue    | Violet.
     10    |    4248    |   2·5   |  „      |
  ---------+------------+---------+---------+----------

He was subsequently tested with colour discs--Ultra-marine (U),
Red-royal (R), Emerald-green (G), Chrome-yellow (Y), White (W), and
Black (B).

It was found that--

            165 (U) + 48 (R) + 147 (G) = 75 (W) + 285 (B).

The black reflected 3·4 of white; hence the true equation is--

  (i). 165 (U) + 48 (R) + 147 (G) = 84·7 (W) + 275 (B).

 (ii). 120 (U) + 240 (Y) = 196 (W) + 164 (B) (corrected)--
        With 260 (U) + 100 (Y) he sees blue.
             250 (U) + 110 (Y) he sees light-blue.
             242 (U) + 118 (Y) he sees no blue.

This last in connection with (ii) shows that his blue perception is
neutralised by the yellow, although the yellow to him was matched with
white.

I have already shown you a chart of the insensitive area of the retina
found in a tobacco-blind case, and it may be advisable that you should
see an example of the curtailment that exists, both for light and
colour, in the field of vision of eyes in which there is progressive
atrophy of the optic nerves. The large black area shows the part of the
field that was encroached upon. The dark spots show small areas which
are also insensitive. The field for colour shown by the inner shaded
area is also encroached upon, and practically the patient was blind in
a great part of his field. His form vision was also very bad, and his
colour perception feeble. The three charts given in these lectures
were brought by Mr. Nettleship, for the information of the Colour
Vision Committee of the Royal Society, and by his permission they are
reproduced here.

[Illustration: FIG. 38.

_Left Eye._    _Right Eye._]

Two other cases I may give in some detail, one in which the sensation
of colour is totally absent in the left eye, the right eye being
normal; and the other in which there is colour blindness of a very rare
character. The first case is that of a lady, whom we will call Miss W.
It appears from the history of this lady that she had a slight stroke
of paralysis which affected her left side, and that she subsequently
found her left eye was deprived of all sensation of colour. It is said
by the specialists who examined her retina that this is a case of
atrophy of the optic nerve. She had very little difficulty in matching
the most brilliant spectrum colours with the white patch of light.
Her curve of luminosity is given in Fig. 39 (see table, page 228). At
19 of the scale, which is well in the blue, she had very little sense
of light, though her extinction curve shows that it extended to some
distance beyond. The eye in which normal vision existed was, during the
examination of the defective eye, bound up with a handkerchief, and
when occasionally she was allowed to use both eyes, her astonishment
was great to see the colours which she had matched with the white. The
curve of luminosity taken with her right eye coincided with my own,
which throughout we have taken as normal. From her extinction curve we
gather that there was a marked diminution of sensitiveness to light
in her left eye compared with that of normal vision. Apparently, in
that eye she only has 1/25 of the normal sensitiveness to light near E
in the green, but her extinction curve takes the same general form as
that of the normal eye. The difference between the sets of ordinates
of the two indicates the difference in sensitiveness for each part of
the spectrum.

[Illustration: FIG. 39.]

Her persistency curve as calculated occupies the same position and is
of about the same dimensions, when the maximum is made 100, as that of
the normal eye, as it is therefore of red- and green-blind, and also
of the two cases of monochromatic vision. We have in Miss W. a type of
colour blindness which no present theory of colour vision accounts for
without straining; and it would probably have to refer it to the seat
of sensation rather than to the retina alone.

[Illustration: FIG. 40.

The thin line curve is the curve of luminosity for the normal eye.]

The second is a case of congenital colour blindness and with no trace
of disease, brought by Mr. Nettleship to the same Committee. He found
that this lady, N. W., mistook blue for red, and it was with some
curiosity that this case was examined. Her first examination was as to
colour sense with the spectrum colours, a patch of monochromatic light
being placed in juxtaposition with an equal patch of white light. At
62·5 (λ 6890) of the scale the light of the spectrum disappeared. As
the slit moved along the spectrum, and the white was approximately
reduced to equal luminosity, she described all the red as grey, and
of the same colour as the white until 53·5 (λ 6110). At this point
she said the colour was brownish compared with the white, and this
hue continued to her till 48 on the scale (λ 5720), when she said the
colour was “neither brown nor green, but both.” From 48 on the scale
she described the colour as green, when it changed quite suddenly at
31·5 (λ 4905). From this point and in the blue she again began to see
grey; the grey at this end of the spectrum, and also of the white
patch, she called brownish-grey. This name must evidently have been a
mental distinction, as she described the red end and the white as grey
only, and not brownish-grey; and, indeed, she was tested again over
that part of the spectrum, and adhered to the previous naming. It would
appear to be due to low luminosity, which made the grey appear to her
what she called brownish, rather than to any actual difference in hue.

  ---------+------------+---------+--------------------------+----------
  Scale No.|Wave-length.| Reading.|  Colours named by N. W.  | Spectrum
           |            |         |                          | colours
           |            |         |                          | to normal
           |            |         |                          | vision.
  ---------+------------+---------+--------------------------+----------
    60     |    6728    |   3     | Both grey                |  Red.
    58     |    6520    |  10     |     „                    |
    56     |    6330    |  30     |     „                    |
    54     |    6152    |  52     | Colour “brownish,”       |
           |            |         |   white “grey”           |
    52     |    5996    |  70     |     „     „     „        |
    50     |    5850    |  81     |     „     „     „        |  Orange.
    48     |    5720    |  87     | Colour “brownish-green.” |
           |            |         |   white “grey”           |
    46     |    5596    |  90     | Colour “green,” white    |
           |            |         |   “grey”                 |
    44     |    5481    |  88     |     „           „        |
    42     |    5373    |  82     |     „           „        |
    40     |    5270    |  62·5   |     „           „        |  Green.
    38     |    5172    |  46     |     „           „        |
    35     |    5042    |  23     |     „           „        |
    32     |    4924    |  12·5   |     „           „        |
    31     |    4886    |  10     | Colour “brownish-grey,”  |
           |            |         |  white “brownish-green.” |
    30·5   |    4862    |   8·5   |    „      „      „       |  Blue.
    25     |    4675    |   5     |    „      „      „       |
    20     |    4518    |   3     |    „      „      „       |
    15     |    4376    |   2·5   |    „      „      „       |
    10     |    4248    |   1·5   |    „      „      „       |  Violet.
     0     |    4010    |   0·2   |    „      „      „       |
  ---------+------------+---------+--------------------------+----------

Her curve of luminosity in the spectrum was next taken, and her
readings are given in the table above. The curve is shown in Fig. 40.
The shaded band beneath it applies to her curve. Miss W.’s luminosity
curve is also repeated in the same figure for the sake of comparison.

An endeavour was made to form a series of colour equations with her
eyesight by placing three slits in different parts of the spectrum, but
without success, although a match with white was made in two positions.
One slit was in the orange-red (52 of the scale), another at E, and the
third at G; mixtures were made which she said matched the white, but
they were so erratic that it was useless to measure the apertures. When
the slit in the violet was covered up, a white patch being alongside as
a comparison, she called the mixture of red and green “brownish-green”;
when the slit in the red was covered she called the mixed light of
green and violet “green”; and when the green slit was covered up she
called the purple colour a “different kind of brown.”

When the first slit was moved into the red near the lithium line she
called the colours “green,” whenever the green slit was uncovered. A
piece of red glass was placed in the white reflected beam, forming
a red patch, and a patch of the blue at scale No. 30·5 (λ 4862) was
placed alongside, and she matched them in luminosity and in colour.
(The dominant colour of the signal glass in question was λ 6220.) She
finally was tested with colour discs.

To make white she required

                 130 G + 113 R + 117 U = 72 W + 218 B.

She was then tried with the blue and green discs alone and made a
match--

                     258 U + 102 G = 65 W + 295 B.

An attempt was made to match with the green and red discs alone, but
this failed.

She matched the red disc alone with black and white, and also the blue
disc alone--

                   360 R = 56 W + 304 B (corrected),
                   360 U = 60 W + 300 B (corrected).

With any proportion of R and U mixed together she matched a grey of
approximately the same intensity as above, as it might be supposed she
would from the last two equations.

Taking the intensity curve of the light reflected from the red disc, it
was found to contain a great deal of the part of the spectrum which she
called brownish, viz., from 33·5 to 48 on the scale, whereas the blue
reflected a trifle of this portion of the spectrum, as did also the
green; and this may account for her making a match to grey of U and G,
and not of R and G, but it is hard to see why she matched U alone and
also R with the grey.

Reviewing the case, it seems that any perception of colour is very
small, and that the sensations are green and much less red. From the
equations it also seems that she would have matched green with white
and black alone, and that 360 G = 75 W + 285 B. Perhaps the explanation
of the matches and names of colours may be that a proportion of colour
may be mixed with another without being perceived, but this colour so
hidden has still the capability of neutralising a certain quantity of
the complementary colour.




CHAPTER XIII.


You have been taken through much experimental work, and possibly it
may be thought that there has been too much of it; but now that we
are coming to the more practical part of the subject, it will become
apparent that a good working hypothesis is absolutely necessary before
effectual tests for colour vision can be carried out, and that the
reasons for its adoption should be given in full. The question of
colour blindness is one of very practical importance, as in certain
occupations it is essential that colours should be accurately and
quickly known, and that no guess-work should be allowed. Lives have
without doubt been lost by a want of proper knowledge of colours,
both at sea and on railways. The evidence that such is the case is,
as a rule, it is true, merely negative, though there are cases extant
where great losses which have occurred can be traced to a deficiency
in colour perception. If there be no proper system of tests for
ascertaining the defects of signal or look-out men in their colour
sense, it is palpable that positive evidence cannot be forthcoming,
and this is very much the state of things which exists up to the
present time. We hear of collisions at sea and vessels foundering
in consequence of the rule of the road not being followed, but at
the investigations which follow we have no record that the question
of colour perception of the look-out man has been gone into, though
there may be conflicting evidence as to whether a red light or a green
light was shown. That danger from colour blindness is incurred has for
some time been recognised by the Board of Trade, as it insists that
all officers of the Mercantile Marine must be tested for their sense
of colour, and that their certificates must be endorsed as having
failed to pass the colour test should they do so. For my own part,
I think endorsement of their certificate is quite inadequate, for
it is still open for shipowners to employ them (of course at their
own risk). A rejection for colour vision should entail a withholding
of the certificate altogether; for it surely is as dangerous that a
signal should be misread as it is that the logarithm of the sine of
an angle should be misunderstood. If a candidate fails in theoretical
navigation, he is not allowed a certificate, but if he only fails in
a very practical part of his examination, his certificate is merely
endorsed.

The system employed by this department _was_ a defective one, and we
know of many instances in which candidates have passed the colour
test, though they ought to have been rejected, and are at present in
the service. The subject of testing for colour vision was brought
prominently forward some two or three years ago, and a Committee of the
Royal Society, to which I acted as secretary, was requested to consider
the methods at that time in force on the railways and in the mercantile
marine, and to find one which was not open to objection. It recommended
the system that had been elaborated by Holmgren, a Swedish physicist,
and known as Holmgren’s test, which has long been in force in Sweden
and elsewhere. This system has, I am glad to say, been adopted by the
Board of Trade, and by most of the railway companies in the United
Kingdom. There have been numerous indications that this change of
method was necessary. Only within the last month (April, 1894), for
instance, I was informed by the Medical Officer who had to examine the
employés on a certain railway in Scotland by the Holmgren test that he
had found some, amongst others an engine-driver, who were colour blind,
and presumably unfit for the posts they occupied owing to this defect.

There is one popular objection which is always made against this
test, or indeed against any proper test, viz., that the examination
is not made under the same conditions which absolutely exist, nor
with the very lights which the candidates have to distinguish from
one another--that is, the red and green lights. Let me beg of you to
remark, that as a mere matter of guessing, the chances are equal that a
man would name the light shown correctly. If you turn a man’s back to
the light, and if he has a coin in his pocket and deliberately calls
heads red and tails green, he will have a good chance of passing the
test; for, if he guessed rightly three or four times, no one would
fail to pass him on his answers. The great point in a test is to cause
the candidate to _do something_ to show that he appreciates colour. It
is this _doing something_ and saying nothing which is the important
feature in the Holmgren test. A man may be ignorant of the names of
colours--colour ignorant it is called--but he cannot be ignorant of
the colours themselves if he has normal colour vision. As a matter
of fact, the colour blind may possibly distinguish between red and
green lights by having carefully noted, under ordinary conditions
of atmosphere, their different brightness, and by their difference
in saturation with their neutral colour. If external conditions are
altered, as they are in actual daily life, these slight indications
vanish, and the quick naming of the colour to be read becomes a mere
matter of chance. A proper test should include all variations that can
occur in these respects. It cannot be too strongly impressed upon every
one that a man who is colour blind to colours in ordinary daylight is
equally so in lamplight, although some shades of colour which are well
distinguishable by daylight may disappear when the artificial light is
used as the source of illumination.

Now, on what scientific principles should a colour test be founded? We
must hark back to a theory for a moment, and as it has been shown that
for all essential purposes that of Young answers, we will use it as
a good working hypothesis, and it was from this theory that Holmgren
himself reasoned. The red- or green-blind see a grey in a part of their
spectrum, which to us who possess normal colour vision is green. If
then we present such a green to them, they would match it with a grey.
If, however, we have a yellowish-green, which is pure green mixed with
red, the complete green-blind will not see the green in it, but only
the red. The colour to him would be very pale red, and as he sees all
such greens and yellows and reds as red more or less saturated, that
is, more or less mixed with his neutral colour, any one of these he
would match with a green. The red-blind, on the other hand, would see
all these colours as green, and he too might make similar matches with
them. Suppose now we have a pink skein: the green-blind would see it
as a white or bluish-white, for a purple is white to him, and he would
match with it either greys or colours having a slight excess of blue
in them; for a green is to him a neutral colour. The red-blind, on
the other hand, would see but little green in the pink; blue would
predominate, so he would choose mauves or blues amongst other matches.

Acting on these principles, Holmgren selected his test colours. He
chose wools as the most convenient for handling, and also because they
present the same colour without sheen when looked at in any direction.
His first test colour is a very pale green which contained no blue. Its
paleness is a point in its favour. The colour is quite distinguishable
by us normal-visioned persons, but it might appear as grey to the
red- and green-blind; for as we who possess normal vision may mix a
small percentage of colour with our neutral colour (white) without
it being perceived, so may they with theirs (white and green). As the
green, when it is to us saturated, would be nearly neutral coloured
to them, the very diluted colour which we see in the skein would to
them be masked by the addition of white. In any case, if any colour
be visible to them, it must be on the red side of the neutral points.
A candidate is given this skein of wool, and from a heap of over a
hundred skeins, of varying degrees of saturation, amongst which are
drabs, yellows, yellow-greens, blue-greens, purples, pinks, greys, and
so on, he is asked to select others which appear to him to be of the
same colour as the test-skein, though they may be darker or lighter. He
will, if colour blind, select some of the colours already indicated.
The second test-skein is a pink, which is a purple diluted with white,
but much less so than the green, to which it is nearly a complementary
in daylight. The candidate is required to select colours which match
this, and according to his selections is he pronounced as having
normal colour vision or as being colour defective (either completely
or partially) to the red or to the green. The case of violet blindness
is not important in reading the signals ordinarily used, and therefore
in this test no special test-skein is employed. Let us consider what
colour we should use. The neutral colour to this form of colour
blindness is yellow. If, therefore, we pick out a pale yellow skein,
the candidate would pick out greys to match it; or if we gave him the
pink skein to match, since he has no blue (violet) sensation, he would
match it with a pure red or with a purple.

Where monochromatic vision is under examination, all skeins would be
matched with one another indiscriminately--blues, reds, greens, greys
will all be a match, some lighter and some darker than the test-skein.
I have been told by some who have carried out examinations for colour
blindness that this matching is by no means so uncommon as is often
imagined. In future it is hoped that most of those who make these
matches may be examined by the spectrum test, as it may turn out that a
proportion of them will be most valuable theoretical cases.

In making an examination with the Holmgren test, it is almost
unnecessary that the candidate should take up a skein out of the heap
of wools to form a preliminary diagnosis. The colour blind will not
at once pick out an evident match, but will hesitate and evince a
desire to appear very accurate in his choice. This indicates at once
that there is something amiss. He probably will pick up a skein of the
right colour, place it against the test-skein, lay it down and again
take it up. Or he will pick up a skein which is evidently incorrect and
do the same thing, but perhaps he will return it to the heap and take
up another which is equally bad.

He will fumble over making his matches, and eventually have a heap by
him which will at once tell the examiner that he is colour defective. I
may as well give you an idea of the colours which the colour blind will
pick out by a simple experiment. The heap of wools is on the table, and
in the pure white of the electric arc light, which is thrown on it from
the lantern, every colour is distinct in hue and in intensity. On one
side are placed the two important test-skeins, the pale green and the
pink. There can be no doubt but that in that heap of wools there are
a large number which can be matched with each of them. The red-blind,
be it recollected, sees no red, and if I can place in front of the
lens of the lantern some medium which cuts off the red as completely
as possible, the audience as well as myself will see the colours
approximately as the red-blind would do. Such a medium is found in the
same blue-green glass that is used for signals on most railways and on
board ship. The green-blind, on the other hand, see no green, and if a
medium can be found which when placed in the path of the light allows
no green to pass, the colours in the heap being deprived of the green
would be such as would very nearly be the same as this type of colour
blind would see. This glass is covered with a film of collodion in
which fuschin and blue have been dissolved. It transmits a fine purple
and should answer our purpose. That these two media are what we require
can be readily demonstrated by placing them in front of the slit of
the collimator of our colour apparatus and throwing the spectrum on
the screen. The spectrum of white light is now on the screen, and
when we place the blue-green glass in front of the slit, we see that
the red is very nearly entirely extinguished, whilst if we substitute
for it the dyed collodionized glass the green is absent. Now, placing
the first glass in front of the lantern lens and switching on the
current, the wools are illuminated with the bluish-green light. The
green test-skein appears green, and we can proceed to make our matches,
picking out the colours which appear the same, but taking no heed as to
their lightness or darkness. A dozen skeins are now picked out, and I
think the audience will agree with me that the matches as viewed in the
green light are accurate. The glass is now withdrawn, and the ordinary
white light falls upon the skeins in my hand. They are a strangely
variegated lot as now seen; we have green shades, yellows, and browns,
and greys. Such a variety would tell me that I was colour deficient,
but would not be, perhaps, decisive as to what was the exact character
of the deficiency. For if the pink glass is placed in front of the
lantern you will find the same matches, with one or two exceptions,
might have been made. The blue-green glass is once more placed in the
beam, and this time I match the pink skein with the wools. A certain
number are picked out, and the audience will agree with me that the
matches are fair ones. When, however, the glass is withdrawn from the
light and we see what colours have been selected, we find that they
consist of pale blues, mauves, pinks of various shades, and cerise, and
violet. The red in the pink did not affect my eyes any more than would
it the red-blind. I am evidently then in this light red-blind, for if
the pink glass replaces the blue-green, the matches are impossible.
While this coloured light is illuminating the heap I will make matches
again. When made, the white light is again thrown on the selected
skeins, and this time we have bluish-green and neutral tint together
with pinks. The reason of this is evident, there is no green visible;
the bluish-green contains besides blue a certain amount of yellow,
which, in its turn, contains red, and the grey must be pink. To the
green-blind, for reasons already given, the blue-green looks white, as
does the pink, and therefore the two are matched together. The grey
is also degraded white to him, and therefore he also matches that
with them. The matches which the violet-blind would make can be well
exemplified by placing in the beam of light a yellow glass, or a glass
coated with collodion in which “brilliant yellow” has been dissolved.
By this plan, then, we can in some measure produce the effect of colour
blindness on ourselves, and very interesting it is to compare theory
with the results obtained in this manner. There is no necessity to
have recourse to the electric light for this purpose. If matches are
made with such media held before the eyes in ordinary daylight, the
same results will be obtained. I have often examined through these
same media the matches made by the colour blind, and been able at once
to settle the nature of the defective vision from which they were
suffering. It must be remembered that the colours transmitted through
these two glasses are not _absolutely like_ the whites which the two
classes of colour blind see respectively, though they approach it.

We can imitate even more exactly the matches that they would make by
matching white light with a mixture of red, green, and violet of the
proper hues, and covering up the red or green slit, and then placing
the test-skein and the matches in the colour so formed. From the other
skeins viewed in the same light can be picked out the matches which
would be possible. There is very little chance, if any, of a mistake
about them being made when this plan is adopted.




CHAPTER XIV.


Holmgren’s test, although a qualitative one, is most accurate in
allowing a diagnosis to be formed, but it sometimes happens that a
candidate is not satisfied that he has failed in passing the test, and
wishes for another examination. Such a re-examination is best carried
out by the spectrum method, which I will now describe.

The test with the spectrum is a very decisive one, and can be carried
out with the patch apparatus (Fig. 3), page 19. Personally, I like to
have some idea of the kind of colour blindness, if any, which exists by
first using the Holmgren test. Should these tests show that a candidate
is colour blind in any degree, a very excellent beginning is to try and
find his neutral point in the spectrum--if he has one. To arrive at
it we place two patches of light on the screen, one of colour and the
other of white, the rotating sectors being in the last-named beam, and
ask him to say when the two colours appear alike. It must be remembered
that white is coloured from the effect of contrast as long as the
colour alongside differs from it. A good _point de depart_ is with the
slit in the yellow, then to move it into the red, and then gradually
to push it into the green. When here, if colour blind, he will say,
“The two patches are nearly alike, but that the white is rather pink or
green,” as the slit gets further towards the blue. The operator, whilst
changing the colour, alters the sectors so that the luminosities are
about the same. A point will be reached when the colour blind will say,
“Now they are both alike, but one is rather darker than the other.”
The sectors are altered until he says they are both alike, and the
observation is satisfactory when he declares the two patches of light
are both alike in colour and in darkness. It is curious how misleading
the word brightness is to some people who are uneducated. I find it
much safer to ask which is the darker colour, rather than which is
the brighter. A little patience will always enable you to get a good
observation. The place in the spectrum which is the neutral point is
now noted. The neutral point is again found, but this time commencing
in the blue. The same procedure is adopted as before, and we thus get
a second reading for it, and the two will be found to be very close
to one another. In difficult cases, four or five observations may be
made, and the mean taken as a close approximation. So far the spectrum
test has not shown whether the observer is red- or green-blind, except
by comparing the position of the neutral point with that usually
found by the two types. We have, however, an unerring criterion by
the luminosity method. The red is placed beside the white, and he is
asked to say which he considers the darker; he will give an answer of
some kind, and probably protest that the two colours are not alike. A
soothing answer will disarm his objection, and he will quickly see what
you mean. If he be red-blind he will match in brightness a brilliant
red and a feeble white; if he be green-blind he will make a match very
similar to normal vision. In the case of the red-blind the slit is then
moved into the extreme red, when he will say he sees but one patch of
light, whilst the green-blind will see it as a person of normal vision
would do. If time permits, the whole luminosity curve may be taken and
registered. This is not essential, but interesting for reference. Where
complete colour blindness exists, it should be possible to cause him to
match a green with a red. To do this a second instrument, as described
in page 18, may be used, but it is quite sufficient if a piece of
red glass, such as is used for railway signals, or of bottle-green
glass, be placed in the white beam. There is then a red or green patch
alongside the patch of spectrum colour. The red will stimulate the red
sensation of the green-blind, but not being spectrum red it contains
a certain amount of yellow, which stimulates the green sensation if
the observer be red-blind. The green is of such a colour that it will
stimulate both the red and the green sensations. In the path of the
reflected beam between G and the prisms (Fig. 3) a sheet of plain
glass is inserted, which reflects a proportion of white on to the red
patch. The sectors are placed in this beam. If the red glass is being
used, the slit is moved into the green near E, and the colour blind
will say that both are the same colour, but one darker than the other.
By opening or closing the slit in the spectrum, he will possibly say
that both colours are alike and of the same darkness, but he may say
one is paler than the other, in which case the white light must be
increased or diminished by means of the sectors till equality of tone
is established. This applies to the red-blind and the green-blind. The
former will require a very bright red to match a feeble green, whilst
with the latter the red will require a fairly light green. When the
green glass is used the spectrum colour patch should be red, and the
match be made as before. With the violet-blind the neutral point will
be in the yellow, and with monochromatic vision matches can be made
throughout the spectrum. So far it will be seen that no mention of any
colour is required. It may next be advisable to ask him the names of
colours. This is best done by placing the white patch of light over the
spectrum colour patch, and opening and closing, as may be required,
the sectors. If the sectors are closed it is very probable that
correct guesses may be made, for then the colours will be saturated,
and the colour blind, if they are intelligent, will know that a green
to them is white or pale in colour compared with red, though of the
same hue. If white be mixed with the red the wrong name is bound to
be given, for they will be unable to distinguish it from the green,
because it is then a less saturated colour. Passing from green to red
and mixing the colour more or less with white, the most--I was going
to say grotesque--telling mistakes are made. A further excellent test
is to place a cell containing a solution of bichromate in the path of
the reflected beam, and cause the observer to match its colour with
the light coming through two slits, one in the red near C, and the
other in the green near E. Defective colour perception will be well
demonstrated. There are various other artifices which can be employed
in the spectrum test, which would be too long to recount here, and if
there be two sets of apparatus the tests are practically unlimited in
number.

There are cases in which an observer who may have normal vision may
wish to be reported as colour blind. A seaman’s life is not always a
happy one, and a boy on a training-ship, knowing that a failure in
colour vision will free him from a sea life, may be anxious to be told
he has failed in colour vision. By “coaching” in the Holmgren test he
might manage to obtain a “failure,” but a malingerer is sure to be
detected by the spectrum method of testing. He may call diluted red
green, and he may declare he sees a neutral point in the spectrum, but
if he be tested with the diluted colours near his supposed neutral
point, he is sure to fall into a trap. He will make a mistake in
calling a patch green when it ought to be white, or white when it ought
to be green, if he were truly colour deficient--indeed, a malingerer
has no chance of escaping detection with the spectrum tests. It is not
an uninteresting experiment to get an acute observer who has normal
colour vision, and is accustomed to the spectrum test, to feign colour
blindness, and examine him in this manner. He never fails to make such
mistakes as would lead to his detection.

With the partially colour blind the same procedure may be adopted. In
examination by the Holmgren wool test, slight mistakes will be made in
matching the first two test-skeins. With the spectrum test the red will
require a greater dilution with white before it will be matched with a
green, even if it can be matched at all. Measures of the luminosity at
four or five positions in the spectrum, extending from near the extreme
red to the blue, will give an unerring criterion of the kind and extent
of colour blindness from which they are suffering. The existence of
a neutral point in the spectrum is sufficient to indicate that their
blindness is of a nature to be dangerous in certain occupations. To
some it may be a difficulty how a neutral point can be found in such
cases, since all sensations are more or less present. The reason,
however, was explained on page 96.




CHAPTER XV.


Examples of colour blindness have been brought to your notice, and
various measurements made by persons possessing normal and defective
colour vision have been recorded, but no attempt has been made to
discuss the two leading rival theories that have been laid before you.
Regarding these theories you may expect me to say something, and to
avow myself a partisan of one or the other. This last I must decline
to do, though it will have been seen by the line that I have taken in
these lectures that the Young theory attracts me. There are, however,
difficulties in adapting it to explain several facts of colour vision
which seem to render it, to say the least, incomplete. For instance,
to explain the colours produced by simultaneous contrast, the Young
theory has to betake itself into psychological ground. I will show
you some excellent examples of contrast colours. We have upon the
screen a patch of white reflected light, superposed over a patch of
red light. Placing a thin rod in the paths of the two beams, we have
two shadows--one illuminated by white and the other by red, and lying
between them a mixed light of red and white. The shadow illuminated by
the white does not appear white, but a bluish-grey. When the spectrum
colour is changed to orange the blue is intensified, whilst when it is
green, what should be white appears of an orange-salmon colour. Other
colours give the white different hues which I need not describe.

These contrast colours are usually said to be _complementary_ to the
spectrum colours employed, though it must be recollected that what a
complementary colour should be is determined by the quality of the
white light which the two, when mixed, are made to match. But recent
measures of my own show that they are not truly complementary in most
instances, whatever the white light may be. But whether they are or not
does not much matter when the explanation offered by the followers of
the Young theory is considered, for it is asserted that such contrast
colours have no real existence, but are psychological, or--what this
comes to be--simply delusions. If they are not real colours felt by
the retina, they have a very good resemblance to them, and the same
series of delusions are so persistent and so constant for all normal
vision that they can always be measured as having a constant value. I
bear in mind the experiment in which the contrast colour, after being
produced, is isolated in the eye from the colour producing it and the
background, and the continuance of the hue produced by the contrast.
This _retention_ may be psychological, but there are no grounds to my
mind for saying that its _production_ is due to the same cause, more
especially as experiments have been arranged to show that one eye may
see a contrast colour, whilst the other may see it of its uncontrasted
hue. In this last experiment it can scarcely be conceived that one eye
should be subject to delusion, whilst the other was free from it. If,
then, we may presume that they are real colours, the Young theory fails
to explain them, and the explanation offered by the Hering theory is
much more acceptable, as it propounds the idea that the retina has to
be considered as a whole, and that if (say) red light is at work at one
part its complementary colour (blue-green) must be felt at another.
It would be still more acceptable had it happened that the contrast
colours were truly complementary, and if the same action was noticeable
when the adjacent part of the retina was not also stimulated.

For what I may call the straightforward part of colour vision, dealing
with ordinarily bright colours, the Young theory is amply sufficient;
but when we come to the feeble luminosities and the colour fields,
it is again difficult to adapt to explain the phenomena observed.
When we reduce the luminosity of a coloured ray sufficiently we feel
the sensation of grey light: no colour is felt. Why is this? On the
Hering theory it is capable of the explanation that we have the white
sensation left unextinguished, but I fail to see any explanation on
the Young theory. When we take colour fields with pure colours (see
appendix, page 208), we are met with the unexplained difficulty that
the colour from a bright spot of light vanishes almost suddenly towards
the periphery of the retina, and is replaced by a bright _white_ light,
and that the extent of the field depends on the brightness of the
colour. This, perhaps, is the most telling observation which can be
recorded against the Young theory as it stands at present. It has this
support, however, in the _sequence_ of the phenomena observed, viz.,
when the boundary for the colour which we will suppose to be pure red
is being taken (as described at page 11), that close to the point where
it bursts into pure white, it assumes a pink colour (_i.e._, a mixture
of red and white), whilst, if the red be scarlet, containing according
to this theory a little green sensation, it becomes orange before
white, showing that the red sensation is dimmed slightly before the
green, and so with the other colours. What are called “after images”
I have not touched upon so far, nor shall I here, for it is at this
point that we step into very debateable ground. The colours perceived
in them are, as yet, not capable of being put to the test of physical
measurement, and I must leave the psychologist or the physiologist to
account for them in their own way.

Viewing the Hering theory from a physical standpoint, and in the light
of colour measurement, it appears to be deficient in several respects.
To take one point. We have seen that when blue and yellow are mixed
together to make white the sum of the luminosities of the two colours
separately is equal to the luminosity of the white produced. According
to the Hering theory, the yellow colour contains a certain amount
of the white-black sensation besides the yellow sensation, as does
also the blue colour besides the blue sensation. The theory tells us
that when white is produced by the mixture, the blue sensation undoes
the work that the yellow sensation has done, and the white sensation
is alone left behind. If this be the case, the sum of the separate
luminosities cannot be the same as that of the white produced, but
should be greater. The theory also has to be strained sometimes to make
it fit in with other observed facts. Take, for instance, the case of
persons who are called red-blind and green-blind on the Young theory.
We are told by the Hering theory that both are red-green-blind--that
is, blind to both green and red, and only see blue and yellow--and that
the only difference between them is that the former has his spectrum
slightly shortened at the red end, the maxima of the yellow-blue
sensations being shifted a little further towards the violet end of the
spectrum. The natural question to ask is: Why this shift occurs? Surely
it is more rational to adopt a theory which does not require such a
supposition? If the sensitive matter acted upon by the yellow-blue
rays be always of the same chemical composition, the shift cannot
occur. It might, perhaps, be allowed that one shift was practicable,
but, unfortunately, the shifts must become numerous when the cases
of partial colour blindness are to be accounted for, and this would
necessitate a constantly varying chemical composition of this matter,
and of that acted upon by the red-green rays.

Again, in the extinction of the spectrum, the red and the green
sensations in quantities to neutralize one another should be
extinguished nearly together, even allowing for what physiologists
tell us is the case, that the breaking down, or dissimulation, of cell
tissue continues longer than its building up, but we find a large
difference between the two. As already indicated, the luminosity curve
of the feeble spectrum favours the theory of Hering being that here
we only have the white-black sensation, and naturally the persistency
curves must be scored in its favour. But the cases of B. C. and M.,
it seems to me, cannot be explained by the theory without any undue
straining or assumptions. If we try and fit the cases of colour
blindness due to tobacco scotoma to the theory, we find that in many
cases yellow is not recognised, though blue is invariably. If the blue
be active, the yellow should also be so.

And here I may remark that it has been assumed that the two classes
of colour blindness are due to different causes. A question to ask
ourselves is whether all colour blindness may not have been caused
originally by disease. In the congenital form, it is true, no disease
of the retina is traceable in the eye, and it is usually hereditary,
but it does not follow that the want of response of the perceiving
apparatus to certain sensations may not have been due to what, for want
of a better expression, I may call an hereditary partial paralysis
of the perceiving apparatus. If this be so, we have a connecting link
between the two classes, and then a perfect theory should explain both
classes on the same grounds. The suspicion that the monochromatic
vision of P. and Q. might possibly be due to disease before birth,
owing to the behaviour of their eyes under certain conditions, would
then be explicable. I have no desire to press this view, though it
seems to me to be one which is not out of all reason, taking analogies
from other defects which are hereditary.

It has been usually accepted that the fields for blue and yellow in
the eye are approximately the same, as are those of the green and red,
and this has been taken as showing the interdependence between the
two pairs according to the Hering theory. It has already been pointed
out that the question of extent of fields requires still further
investigation beyond that which it has received, and measures made by
the method given on page 208 seem to cast a doubt as to whether this
interdependence can be upheld. It will be noticed that the fields
do not extend proportionately on the nasal and temporal sides (see
also Fig. 3). It should also be remarked that the order of extent of
field for the different colours does not follow the same order as
their disappearance. A point that is sometimes raised in favour of
Hering’s theory is the negative image formed after the eye is fatigued
by looking at bright red or bright green. The negative images (see
page 30) are said to be the complementary of these colours. The Young
theory tells us that the red or the green sensation suffers fatigue
by one or other colour, and that when the eye subsequently rests on a
grey surface the other two sensations are chiefly stimulated and cause
the complementary colour. It is said that it is easier to produce a
_negative green_ image than a _negative red_ image, and the adherents
of Hering tell us that this is due to the fact that destructive action
is more readily carried out than constructive. In the Young theory, it
is held that the green sensation is always mixed with white, whilst
the red is fairly pure, and thus, for equal luminosities, the surplus
green sensation is much less stimulated than the red, which offers a
consistent explanation of this fact. There are several other minor
difficulties in the way of accepting Hering’s theory as it stands from
a physical point of view, but we need not discuss them now.

The final sensation curves for the spectrum colours on the Young theory
are still under consideration, and are not definitely fixed, though
the observations made have been very numerous. Recently Helmholtz, in
the last edition of his “Physiological Optics,” has calculated, from
Kœnig’s observations, that no one of the three sensations is singly
stimulated by any colour, even at the extreme ends of the spectrum, and
he makes the three fundamental sensations vary considerably from those
given in these pages. Every colour he states is considerably mixed with
white light. The calculations by which he arrived at this conclusion
are of a complicated nature, and I think if he had had besides the
colour equations of Kœnig, the luminosities and the extinction measures
before him, there might have been a modification of his views, for
these last give evidence to the contrary.

There is a possible modification of the Young theory which would
account for a good many of the phenomena that are unaccounted for by
it in its present form, though it may raise new difficulties in the
minds of some. Let us suppose that each of the three sensations were
compounded of fundamental _light and of colour_ in fixed and definite
proportions, and not in the same proportion in each; and further that
the apparatus in the eye which was responsible for each sensation
had two functions, one of which was to respond to the fundamental
light sensation and the other to the colour. One essential difference
between this modification of the Young theory and that of Hering
is that, whilst in the latter the white sensation is a sensation
_distinct from the colour sensations_, in the former it is a _definite
part_ of them. The fact that the sensation of colour is lost before
the sensation of light is one of the greatest significance, and any
theory to be accepted must offer a reasonable explanation of it. If
the modification suggested be made, it accounts for the existence
of this residuum of light equally as well as Hering’s theory, and
without its drawback. It is not hard to imagine the apparatus which
gives rise to two sensations, on the assumption of different kinds of
atomic motion, induced by the ether motion, or at least three kinds are
possible. When extinction of _colour_ is made, the ether vibrations
would have sufficient energy to induce but one kind of motion; and
when all _light_ was extinguished from the same ray, they would not
be capable of inducing any sensible motion whatever. In the case of
Miss W., who saw all colours as white, it might be that disease had
entirely prevented the first kind of motion in all three sensations,
and that in P. and Q. the red and green sensations were absent or
paralysed in their entirety, whilst the blue sensation was left in full
operation. In B. C. the blue and red sensations would be similarly
absent, leaving the green sensation unchanged. The coincidence of
their persistency and luminosity curves would then indicate that
the proportions of fundamental light and colour remained the same
throughout. Other examples and considerations seem to indicate that
the proportion of colour to fundamental light is greatest in the red
sensation, next in the green, and least in the blue. This would explain
why with increasing intensities blue appears white sooner than green,
and much sooner than red. The proposed modification would also offer
the necessary explanation as to the disappearance of colour from the
field.

Looking at colour vision from what I may call an evolutionary point
of view, the “light-colour” theory commends itself as probable. There
are many reasons for thinking that the visual sensation first evolved
was that of light, subsequently followed by that of colour. The first
evolved colour sensation would appear to have been the blue, and
the last the red. The discussion of this hypothesis would carry me
beyond my limits, and I must leave it thus baldly expressed for your
consideration.

For my own part, whatever theory of colour sensations may prove to
be the right one, I lean strongly to the idea that the cause of
vision will be found in chemical action, induced by the impact of
the different wave-lengths of light falling on sensitive matter. A
white substance may absorb all the wave-lengths found in the spectrum,
and if it have three sets of molecules, one of which has an atom or
atoms vibrating with the same period as the waves of light which show
a maximum for one sensation and another for another, and so on, the
requirements for the colour sensations are met. It may be that the
sensitive part of the retina is like a photographic plate, but with
this essential difference--that the sensitive material is constantly
changing. A photographic plate receives an impression which is not
recognisable by the eye, though it can be shown that a change in the
material does take place during the impact of light, by electrical and
other means. When the eye receives an impression of light, Dewar has
shown that in this case also a current of electricity is generated.
Recent published experiments of my own have demonstrated that with
a low intensity of light, the chemical change that occurs in a
photographic salt is by no means proportionate to that which takes
place with a greater intensity. In the eye, too, there is a limit of
sensibility to very feeble light. Again, the curves of the stimulation
of the colour sensations to the spectrum are closely of the same form
as the curves of sensitiveness of the various sensitive salts used by
photographers. These are analogies and, of course, must not be pressed
too far. There must be such a complexity in the sensitive material
in the eye, both chemical and physiological, that it may be that the
changes induced by light on the sensitive surface of the retina have
to be considered from both aspects. The purely chemical change is
naturally that to which a physicist is most prone to incline, and his
bias must be discounted, as must also that of the physiologist.




APPENDIX.


The following is extracted from Maxwell’s paper.

The following table contains the means of four sets of observations by
the same observer (K.):--


TABLE IV. (K.)

  44·3 (20) + 31·0 (44) + 27·7 (68) = W.
  16·1 (28) + 25·6 (44) + 30·6 (68) = W.
  22·0 (32) + 12·1 (44) + 30·6 (68) = W.
   6·4 (24) + 25·2 (36) + 31·3 (68) = W.
  15·3 (24) + 26·0 (40) + 30·7 (68) = W.
  19·8 (24) + 35·0 (46) + 30·2 (68) = W.
  21·2 (24) + 41·4 (48) + 27·0 (68) = W.
  22·0 (24) + 62·0 (52) + 13·0 (68) = W.
  21·7 (24) + 10·4 (44) + 61·7 (56) = W.
  20·5 (24) + 23·7 (44) + 40·5 (60) = W.
  19·7 (24) + 30·3 (44) + 33·7 (64) = W.
  18·0 (24) + 31·2 (44) + 32·3 (72) = W.
  17·5 (24) + 30·7 (44) + 44·0 (76) = W.
  18·3 (24) + 33·2 (44) + 63·7 (80) = W.


X.--REDUCTION OF THE OBSERVATIONS.

By eliminating W from the equations above by means of the standard
equation, we obtain equations involving each of the fourteen selected
colours of the spectrum, along with the three standard colours; and by
transposing the selected colour to one side of the equation, we obtain
its value in terms of the three standards. If any of the terms of these
equations are negative, the equation has no physical interpretation as
it stands; but by transposing the negative term to the other side it
becomes positive, and then the equation may be verified.

The following table contains the values of the fourteen selected tints
in terms of the standards. To avoid repetition, the symbols of the
standard colours are placed at the head of each column:--


TABLE VI.

  Observer (K.)  (24)  (44)    (68)
  44·3 (20) =   18·6   + 0·4   +  2·8
  16·1 (28) =   18·6   + 5·8   -  0·1
  22·0 (32) =   18·6  + 19·3   -  0·1
  25·2 (36) =   12·2  + 31·4   -  0·8
  26·0 (40) =    3·3  + 31·4   -  0·2
  35·0 (46) =  - 1·2  + 31·4   +  0·3
  41·4 (48) =  - 2·6  + 31·4   +  3·5
  62·0 (52) =  - 3·4  + 31·4   + 17·5
  61·7 (56) =  - 3·1  + 21·0   + 30·5
  40·5 (60) =  - 1·9   + 7·7   + 30·5
  33·7 (64) =  - 1·1   + 1·1   + 30·5
  32·3 (72) =  + 0·6   + 0·2   + 30·5
  44·0 (76) =  + 1·1   + 0·7   + 30·5
  63·7 (80) =  + 0·3   - 1·8   + 30·5

Mr. James Simpson, formerly student of Natural Philosophy in my class,
has furnished me with thirty-three observations taken in good sunlight.
Ten of these were between the two standard colours, and give the
following result:--

                       33·7 (88) + 33·1 (68) = W.

The mean errors of these observations were as follows:--

  Error of (88) = 2·5; of (68) = 2·3; of (88) + (68)
                = 4·8; of (88) - (68) = 1·3.

The fact that the mean error of the sum was so much greater than the
mean error of the difference, indicates that in this case, as in all
others that I have examined, observations of equality of tint can be
depended on much more than observations of equality of illumination or
brightness.

From six observations of my own, made at the same time, I have deduced
the “trichromic” equation--

                   22·6 (104) + 26 (88) + 37·4 (68) = W      (2)

If we suppose that the light which reached the organ of vision was the
same in both cases, we may combine these equations by subtraction, and
so find

                   22·6 (104) - 7·7 (88) + 4·3 (68) = D      (3)

where D is that colour, the absence of the sensation of which
constitutes the defect of the dichromic eye.

The sensation which I have in addition to those of the dichromic eye
is therefore similar to the full red (104), but different from it in
that the red (104) has 7·7 of green (88) in it which must be removed,
and 4·3 of blue (68) substituted. This agrees pretty well with the
colour which Mr. Pole[A] describes as neutral to him, though crimson
to others. It must be remembered, however, that different persons of
ordinary vision require different proportions of the standard colours,
probably owing to differences in the absorptive powers of the media of
the eye, and that the above equation (2), if observed by K., would have
been

                       23 (104) + 32 (88) + 31 (68) = W      (4)

and the value of D, as deduced from these observers, would have been

                     23 (104) - 1·7 (88) - 1·1 (68) = D      (5)

in which the defective sensation is much nearer to the red of the
spectrum. It is probably a colour to which the extreme red of the
spectrum tends, and which differs from the extreme red only in not
containing that small proportion of “yellow” light which renders it
visible to the colour blind.

    [A] Philosophical Transactions, 1859, Part I., p. 329.

From other observations by Mr. Simpson the following results have been
deduced:--


TABLE A.

              (88) (68)  |              (88) (68)
  (99·2 +)  = 33·7  1·9  |  100 (96) =  108    7
  31·3 (96) = 33·7  2·1  |  100 (92) =  120    5
  28   (92) = 33·7  1·4  |  100 (88) =  100    0
  33·7 (88) = 33·7  0    |  100 (84) =   61   11
  54·7 (84) = 33·7  6·1  |  100 (82) =   47   21
  71   (82) = 33·7 15·1  |  100 (80) =   34   33
  99   (80) = 33·7 33·1  |  100 (78) =   22   47
  70   (78) = 15·7 33·1  |  100 (76) =   10   59
  56   (76) =  5·7 33·1  |  100 (72) =    1   92
  36   (72) =  0·3 33·1  |  100 (68) =    0  100
  33·1 (68) =  0   33·1  |  100 (64) =    0   83
  40   (64) =  0·2 33·1  |  100 (60) =    3   60
  55·5 (60) =  1·7 33·1  |
  (57 -)    =  0·3 33·1  |

In the table on the left side (99·2 +) means the whole of the spectrum
beyond (99·2) on the scale, and (57 -) means the whole beyond (57) on
the scale. The position of the fixed lines with reference to the scale
was as follows:--

A, 116; a, 112; B, 110; C, 106; D, 98·3; E, 88; F, 79; G, 61; H, 44.

The values of the standard colours in different parts of the spectrum
are given on the right side of the above table, and are represented by
the curves of Fig. 9, Plate II., where the left-hand curve represents
the intensity of the “yellow” element, and the right-hand curve that of
the “blue” element of colour as it appears to the colour blind.

The appearance of the spectrum to the colour blind is as follows:--

From A to E the colour is pure “yellow,” very faint up to D, and
reaching a maximum between D and E. From E to one-third beyond F
towards G the colour is mixed, varying from “yellow” to “blue,” and
becoming neutral or “white” at a point near F. In this part of the
spectrum the total intensity, as given by the dotted line, is decidedly
less than on either side of it, and near the line F, the retina close
to the “yellow spot” is less sensible to light than the parts further
from the axis of the eye. This peculiarity of the light near F is even
more marked in the colour blind than in the ordinary eye. Beyond F the
“blue” element comes to a maximum between F and G, and then diminishes
towards H, the spectrum from this maximum to the end being pure “blue.”

The results given above were all obtained with the light of white
paper, placed in clear sunshine. I have obtained similar results when
the sun was hidden, by using the light of uniformly illuminated clouds,
but I do not consider these observations sufficiently free from
disturbing circumstances to be employed in calculation. It is easy,
however, by means of such observations, to verify the most remarkable
phenomena of colour blindness, as, for instance, that the colours
from red to green appear to differ only in brightness, and that the
brightness may be made identical by changing the width of the slit;
that the colour near F is a neutral tint, and that the eye in viewing
it sees a dark spot in the direction of the axis of vision; that the
colours beyond are all blue of different intensities, and that any
“blue” may be combined with any “yellow” in such proportions as to
form “white.” These results I have verified by the observations of
another colour-blind gentleman, who did not obtain sunlight for his
observations; and as I have now the means of carrying the requisite
apparatus easily, I hope to meet with other colour-blind observers, and
to obtain their observations under more favourable circumstances.


MEASUREMENTS OF COLOUR FIELDS.

Some experiments in the measurement of the colour fields in the
horizontal direction with the pure spectrum colours will help to show
what importance is to be attached to the luminosity of the colour and
the size of the spot of light with which the observations are made. A
yellow and a blue of the spectrum were taken of such hues that when
mixed they formed a patch of white light similar to the electric light.
Their luminosities were measured, and the yellow found to be 1·6 of the
light of an amyl-acetate lamp or 1·28 standard candles; the blue was
1/24 of this luminosity. The fields for these two colours were measured
by automatically throwing spots of each colour separately on a white
card which moved round a centre over which the eye was placed. The
light was subsequently diminished to ½, ¼, and ⅛ of the above values,
and readings again made. The following results were obtained with a
spot of ·7 inch diameter:--

                    Yellow.                       Blue.
  Light.  --------------------------   ---------------------------
          Nasal side. Temporal side.   Nasal side.  Temporal side.

  Full        33°          45°             35°          45°
   ½          24°          36°             26°          38°
   ¼          18°          24°             22°          32°
   ⅛          11°          15°             19°          30°

With a spot of ·3 inch diameter the following were obtained:--

  Full        24°          32°             21°          27°
   ½          17°          28°             16°          22°
   ¼          13°          16°             14°          20°
   ⅛           8°          10°             13°          16°

It will be evident how the field contracts as the light is diminished
in brightness, and also that the blue field does not diminish equally
with the yellow field, but is more persistent. Again, it will be
noticed that the luminosity of the blue, for the same extent of field
to be covered, has to be much lower than for the yellow.

The diminished area of the spot of light also diminishes the field, and
the same order of diminution of field is obtained as with the larger
spot.

Another set of experiments, made with the same aperture of slit passed
through the spectrum, and the field taken at different points, give the
following results:--

  Spectrum scale.   Nasal side.  Temporal side.
  (See Fig. 41,
   page 210.)

      58·6              18°          35°
      54·6              27°          46°
      50·6              33°          47°
      46·6              25°          30°
      42·6              21°          21°
      38·6              17°          17°
      34·6              22°          30°
      30·6              25°          33°
      26·6              33°          40°
      22·6              37°          44°
      18·6              28°          40°
      14·6              22°          34°
       8·6              20°          30°

Here we see that although the luminosity of the colour spots varies
at the spectrum luminosity, the fields do not vary proportionally;
when the luminosities of the green, yellow and red are made equal, the
fields become nearly equal on the nasal side. The field for the blue,
however, then becomes vastly larger than that for the others, showing a
peculiarity which is very remarkable.

[Illustration: FIG. 41.

_Spectrum Scale._]

Recently published experiments on colour fields have been so largely
based on the exigencies of the Hering theory, that it is somewhat
difficult to decide their significance from any other aspect.


TABLE I.--LUMINOSITY CURVES FOR THE NORMAL EYE (see Fig. 20).

  -------+--------------+----------------+--------------+-----------
    I.   |     II.      |     III.       |    IV.       |    V.
  -------+--------------+----------------+--------------+-----------
  Scale  | Wave-length. | Outside yellow | Yellow spot. |  Fovea
  number.|              |     spot.      |              | centralis.
  -------+--------------+----------------+--------------+-----------
    64   |    7217      |                |              |
    63   |    7082      |      ..        |     1        |
    62   |    6957      |       1        |     2        |    2
    61   |    6839      |       2        |     4        |    4
    60   |    6728      |       3·5      |     7        |    8
    59   |    6621      |       7·5      |     12·5     |   15·5
    58   |    6520      |      12·5      |     21       |   24
    57   |    6423      |      19        |     33       |   37·5
    56   |    6330      |      27·5      |     50       |   60
    55   |    6242      |      35        |     65       |   77
    54   |    6152      |      43        |     80       |   90
    53   |    6074      |      52·5      |     90       |   97
    52   |    5996      |      61·0      |     96       |  100
    51   |    5919      |      71·0      |     99       |  100
    50   |    5850      |      79·0      |    100       |   98
    49   |    5783      |      84        |     99       |   95
    48   |    5720      |      85        |     97       |   90
    47   |    5658      |      83·5      |     92·5     |   85
    46   |    5596      |      81·0      |     87       |   79
    45   |    5538      |      77·0      |     81       |   72·5
    44   |    5481      |      72.5      |     75       |   66
    43   |    5427      |      68·0      |     69       |   59
    42   |    5373      |      62·5      |     62·5     |   51
    41   |    5321      |      57        |     57       |   45
    40   |    5270      |      52        |     50       |   40
    39   |    5221      |      46        |     42·5     |   32
    38   |    5172      |      41·5      |     36       |   27·5
    37   |    5128      |      37·5      |     29·5     |   22·0
    36   |    5085      |      33·5      |     24       |   18
    35   |    5043      |      30·0      |     18·2     |   14
    34   |    5002      |      26·5      |     14·2     |   10
    33   |    4963      |      24        |     10·5     |    8·4
    32   |    4924      |      21        |      8·5     |    6·5
    31   |    4885      |      18·5      |      7·0     |    5·5
    30   |    4848      |      16·5      |      5·5     |    4·0
    29   |    4812      |      14·5      |      4·7     |    3·5
    28   |    4776      |      13·0      |      4·0     |    3·0
    27   |    4742      |      11·5      |      3·5     |    2·0
    26   |    4707      |      10·5      |      2·8     |    2·4
    25   |    4675      |      9·4       |      2·3     |    2·1
    24   |    4639      |      8·2       |      1·82    |    1·9
    23   |    4608      |      7·3       |      1·6     |    1·5
    22   |    4578      |      6·3       |      1·4     |
    21   |    4548      |      5·7       |      1·2     |
    20   |    4517      |      5·0       |      1·08    |    1·0
    19   |    4488      |      4·5       |       ·94    |
    18   |    4459      |      4·0       |       ·86    |
    17   |    4437      |      3·6       |       ·78    |
    16   |    4404      |      3·1       |       ·70    |
    15   |    4377      |       2·7      |       ·62    |    ·62
    14   |    4349      |       2·3      |       ·56    |
    13   |    4323      |       2·1      |       ·50    |
    12   |    4296      |       1·9      |       ·45    |
    11   |    4271      |       1·65     |       ·40    |
    10   |    4245      |       1·4      |       ·34    |
     9   |    4221      |       1·2      |       ·30    |
     8   |    4197      |       1·0      |       ·26    |
     7   |    4174      |        ·88     |       ·22    |
     6   |    4151      |        ·75     |       ·18    |
     5   |    4131      |        ·63     |       ·16    |
     4   |    4106      |        ·50     |       ·14    |
  -------+--------------+----------------+--------------+-----------


TABLES II. AND III.--CURVES OF LUMINOSITY OF A PARTIALLY RED-BLIND AND
OF A PARTIALLY GREEN-BLIND PERSON (see Fig. 23).

  -------------+------------+-----------------------
               |            |      Luminosity.
  Scale number.|Wave-length.+----------+------------
               |            |Red-blind.|Green-blind.
  -------------+------------+----------+------------
       64      |    7217    |   0      |      1
       62      |    6957    |   1      |      2
       60      |    6728    |   2      |      7
       58      |    6520    |   6      |     21
       56      |    6330    |   12     |     50
       54      |    6152    |   26     |     80
       52      |    5996    |   49     |     96
       50      |    5850    |   70     |     98
       48      |    5720    |   77     |     93
       46      |    5596    |   77     |     83
       44      |    5481    |   70     |     70
       42      |    5373    |   61     |     55
       40      |    5270    |   47     |     40
       38      |    5172    |   34     |     27
       36      |    5085    |   23     |     18
       34      |    5002    |   14     |     10
       32      |    4924    |   8·5    |      5·5
       30      |    4848    |   5·5    |      3·0
       28      |    4776    |   4·0    |      2·5
       26      |    4707    |   2·7    |      2·0
       24      |    4639    |   1·8    |      1·8
       22      |    4578    |   1·35   |      1·4
       20      |    4517    |   1·1    |      1·1
  -------------+------------+----------+------------


TABLE IV.--LUMINOSITY OF SPECTRUM REDUCED IN INTENSITY, SO THAT D =
1/132·5 AMYL LAMP 1 FOOT DISTANT (see Fig. 25).

  -------+---------+-----------+------------+---------------
         |         |    Mean   |            |   Persistency
         |         |  reading, | P. and Q.’s|  curve for the
  Scale  |  Mean   | reduced to| readings,  |   centre of
  number.| reading.|  100 max. | 100 max.   |   the eye.
  -------+---------+-----------+------------+---------------
   55·6  |    ·5   |     ·6    |     2      |      2
   53·6  |   5·5   |    7·0    |     3·6    |      3·6
   51·6  |  13     |   16·7    |     8      |      8
   49·6  |  23     |   29·7    |    22      |     22
   47·6  |  40     |   50·0    |    44      |     44
   45·6  |  57     |   71·2    |    69      |     69
   43·6  |  70     |   87·5    |    93      |     93
   41·6  |  79     |   98·7    |   100      |     99·5
   39·6  |  78     |   97·5    |    99·5    |     98·5
   37·6  |  74     |   92·5    |    96      |     93
   35·6  |  66     |   82·5    |    89      |     84
   33·6  |  55     |   68·7    |    77·5    |     71
   31·6  |  44·5   |   55·2    |    61      |     53·5
   29·6  |  35     |   43·7    |    45·5    |     36·5
   27·6  |  24     |   30·0    |    33·5    |     24
   25·6  |  17     |   21·7    |    25      |     16
   23·6  |  13     |   16·7    |    18      |     10
   21·6  |  10     |   12·5    |    13      |      8
   19·6  |   8     |   10·0    |     9·5    |      6
   13·6  |   3     |    3·7    |     4·2    |      3
    9·6  |   2     |    2·5    |     2·5    |      2
  -------+---------+-----------+------------+---------------


TABLE V.--LIMIT OF COLOUR VISION (see Fig. 26.).

  --------+---------+---------------+------------+----------------
  Scale   | Wave-   | Mean reading  | Luminosity | Luminosity of
  Number. | Length. | of the colour | of the     | the rays when
          |         | limit of the  | ordinary   | each colour
          |         | spectrum D,   | spectrum.  | disappears,
          |         | being 1 amyl  |            | each ray having
          |         | lamp in       |            | the original
          |         | 1/10000ths.   |            | luminosity of
          |         |               |            | 1 amyl lamp
          |         |               |            | in 1/100000ths.
  --------+---------+---------------+------------+----------------
   61     |  6839   |    120        |    4       |     48·0
   60     |  6728   |     67        |    7       |     46·9
   58     |  6520   |     26        |    21      |     54·6
   56     |  6330   |     13        |    50      |     65·0
   54     |  6152   |      9·5      |    80      |     76·0
   52     |  5996   |      9·0      |    96      |     86·4
   50     |  5850   |      9·0      |   100      |     90·0
   48     |  5720   |      9·0      |    97      |     87·3
   44     |  5481   |      9·5      |    75      |     71·3
   40     |  5270   |     10·5      |    50      |     52·5
   36     |  5085   |     12·5      |    24      |     30·0
   32     |  4924   |     18        |     8·5    |     15·3
   28     |  4776   |     32        |     4·0    |     12·8
   24     |  4639   |     55        |     1·8    |     12·0
   20     |  4517   |     90        |     1·08   |      9·7
   16     |  4404   |    160        |      ·70   |     11·2
   12     |  4296   |    250        |      ·45   |     11·0
   8      |  4197   |    400        |      ·26   |     10·4
   4      |  4106   |    700        |      ·14   |      9·8
  --------+---------+---------------+------------+----------------


TABLE VI.--EXTINCTION BY CENTRAL PORTION OF NORMAL EYE (see Fig. 28).

  -------+------------+--------------+-----------+----------+-----------
    I.   |    II.     |     III.     |    IV.    |    V.    |    VI.
  -------+------------+--------------+-----------+----------+-----------
         |            |      E.      |    L.     |          |Persistency
         |            | Reduction of |           |          |   curve
         |            | original     | Luminosity|(E × L) / |  650 / E
  Scale  |Wave-length.| luminosity   |    of     |  100     |(Maximum =
  number.|            | in millionths|  original |          |   100).
         |            | to cause     |   beam.   |          |
         |            | extinction.  |           |          |
  -------+------------+--------------+-----------+----------+-----------
    64   |    7217    |  55,000      |           |          |
    63   |    7082    |  30,000      |     1     | 300·0    |
    62   |    7957    |  15,000      |     2     | 300·0    |
    61   |    6839    |    7500      |     4     | 300·0    |
    60   |    6728    |    3750      |     7     | 262·5    |
    59   |    6621    |    1900      |    12·5   | 237·5    |   ·34
    58   |    6520    |    1050      |    21     | 220·5    |   ·62
    57   |    6423    |     650      |    33     | 214·5    |   1·0
    56   |    6333    |     380      |    50     | 190·0    |   1·71
    55   |    6242    |     272      |    65     | 176·8    |   2·38
    54   |    6152    |     196      |    80     | 156·0    |   3·32
    53   |    6074    |     140      |    90     | 126·0    |   4·64
    52   |    5996    |      97      |    96     |  93·12   |   6·70
    51   |    5919    |      57      |    99     |  56·43   |  11·40
    50   |    5850    |      35      |   100     |  35·0    |  18·6
    49   |    5783    |      24      |    99     |  23·76   |  27·1
    48   |    5720    |      17      |    97     |  16·49   |  38·2
    47   |    5658    |      12·6    |    92·5   |  11·65   |  51·6
    46   |    5596    |      10·2    |    87     |   8·87   |  63·7
    45   |    5538    |       8·6    |    81     |   6·97   |  75·6
    44   |    5481    |       7·4    |    75     |   5·55   |  87·8
    43   |    5427    |       6·7    |    69     |   4·62   |  97·0
    42   |    5373    |       6·55   |    62·5   |   4·09   |  99·5
    41   |    5321    |       6·5    |    57     |   3·705  | 100
    40   |    5270    |       6·55   |    50     |   3·27   |  98·5
    39   |    5221    |       6·65   |    42·5   |   2·83   |  97·5
    38   |    5172    |       6·85   |    36     |   2·46   |  95·0
    37   |    5128    |       7·2    |    29·5   |   2·12   |  90·0
    36   |    5085    |       7·6    |    24     |   1·82   |  81·3
    35   |    5043    |       8·15   |    18·2   |   1·48   |  80·0
    34   |    5002    |       8·8    |    14·2   |   1·25   |  74·0
    33   |    4963    |      10·2    |    10·5   |   1·07   |  63·0
    32   |    4924    |      11·6    |     8·5   |    ·988  |  56·0
    31   |    4885    |      13·6    |     7·0   |    ·952  |  47·7
    30   |    4848    |      16·3    |     5·5   |    ·896  |  40·0
    29   |    4812    |      20·5    |     4·7   |    ·963  |  31·7
    28   |    4776    |      26·0    |     4·0   |   1·040  |  25·0
    27   |    4742    |      31·0    |     3·5   |   1·085  |  20·9
    26   |    4707    |      38·5    |     2·8   |   1·078  |  16·9
    25   |    4674    |      46·0    |     2·3   |   1·058  |  14·1
    24   |    4639    |      56·0    |     1·82  |   1·019  |  11·6
    23   |    4608    |      67·0    |     1·6   |   1·072  |   9·7
    22   |    4578    |      80      |     1·4   |   1·120  |   8·41
    21   |    4548    |      95      |     1·2   |   1·140  |   7·22
    20   |    4517    |     107      |     1·08  |   1·156  |   6·1
    19   |    4488    |     124      |      ·94  |   1·165  |   5·23
    18   |    4459    |     140      |      ·86  |   1·204  |   4·64
    17   |    4437    |     160      |      ·78  |   1·228  |   4·1
    16   |    4404    |     180      |      ·70  |   1·260  |   3·60
    15   |    4377    |     200      |      ·62  |   1·240  |   3·25
    14   |    4349    |     220      |      ·56  |   1·232  |   2·95
    13   |    4323    |     240      |      ·50  |   1·200  |   2·7
    12   |    4296    |     270      |      ·45  |   1·215  |   2·4
    11   |    4271    |     300      |      ·40  |   1·200  |   2·18
    10   |    4245    |     335      |      ·34  |   1·139  |   1·94
     9   |    4221    |     375      |      ·30  |   1·125  |   1·73
     8   |    4197    |     430      |      ·26  |   1·118  |   1·51
     7   |    4174    |     490      |      ·22  |   1·078  |   1·32
     6   |    4151    |     510      |      ·18  |    ·918  |   1·27
     5   |    4131    |     640      |      ·16  |   1·024  |   1·01
     4   |    4106    |     750      |      ·14  |   1·050  |   0·86
  -------+------------+--------------+-----------+----------+-----------


TABLE VII.--EXTINCTION BY WHOLE EYE

(see Fig. 28).

  --------+---------+--------------+------------+---------+------------
     I.   |   II.   |     III.     |    IV.     |    V.   |    VI.
  --------+---------+--------------+------------+---------+------------
  Scale   | Wave-   |     E.       |     L.     |(E × L) /| Persistency
  number. | length. | Reduction of |            |   160   | curve
          |         | original     | Luminosity |         | 650 / E
          |         | luminosity   | of         |         | (Maximum=
          |         | in millionths| original   |         | 100).
          |         | to cause     | beam.      |         |
          |         | extinction.  |            |         |
  --------+---------+--------------+------------+---------+------------
     38   |   5172  |      6·9     |    41·5    |   2·86  |   94·2
     37   |   5128  |      7·1     |    37·5    |   2·66  |   91·6
     36   |   5085  |      7·4     |    33·5    |   2·48  |   87·8
     35   |   5043  |      7·7     |    30·0    |   2·31  |   84·4
     34   |   5002  |      8·0     |    26·5    |   2·12  |   81·2
     33   |   4963  |      8·4     |    24·0    |   2·02  |   77·5
     32   |   4924  |      8·8     |    21·0    |   1·85  |   73·8
     31   |   4885  |      9·4     |    18·5    |   1·74  |   69·2
     30   |   4848  |     10·0     |    16·5    |   1·65  |   65·0
     29   |   4812  |     10·7     |    14·5    |   1·55  |   60·6
     28   |   4776  |     11·5     |    13·0    |   1·49  |   56·5
     27   |   4742  |     13·0     |    11·5    |   1·49  |   50·0
     26   |   4707  |     14·5     |    10·5    |   1·52  |   44·8
     24   |   4639  |     18·5     |     8·2    |   1·52  |   34·1
     22   |   4578  |     23·0     |     6·3    |   1·45  |   28·3
     20   |   4517  |     30·0     |     5·0    |   1·50  |   21·7
     18   |   4459  |     39·0     |     4·0    |   1·56  |   16·7
     16   |   4404  |     51       |     3·1    |   1·59  |   12·3
     14   |   4349  |     66       |     2·3    |   1·52  |    9·85
     12   |   4296  |     80       |     1·9    |   1·52  |    8·12
     10   |   4245  |    110       |     1·4    |   1·54  |    5·91
      8   |   4197  |    154       |     1·0    |   1·54  |    4·22
      6   |   4151  |    204       |      ·75   |   1·54  |    3·18
      4   |   4106  |    307       |      ·5    |   1·54  |    2·11
      2   |   4063  |    513       |      ·3    |   1·54  |    1·26
      0   |   4020  |    770       |      ·2    |   1·54  |     ·84
  --------+---------+--------------+------------+---------+------------

From 38 to 64 the extinction is the same as with the central part of
the eye.


TABLE VIII.--P.’S CURVES[B] (see Fig. 31).

  -------+--------+------------+------------+-------------+-----------+------------
   I.    | II.    |  III.      |  IV.       |  V.         | VI.       | VII.
  -------+--------+------------+------------+-------------+-----------+------------
  Scale  | Wave-  | Mean       | Adopted    | Persistency | P.’s      | Absolute
  number.| length.| reading of | reading in | curve       | luminosity| luminosity
         |        | extinction | millionths |  (680 /     | curve.    | of
         |        | in         | of         |ad. reading).|           | extinction.
         |        | millionths | original   |             |           | (IV.×VI.) /
         |        | of         | luminosity.|             |           |    14
         |        | original   |            |             |           |
         |        | luminosity.|            |             |           |
  -------+--------+------------+------------+-------------+-----------+------------
     52  |  5996  |    68      |    68      |     10      |     7     |   34
     50  |  5850  |    35      |    35      |     19·4    |    19     |   47·5
     48  |  5720  |    17      |    17      |     40      |    39     |   47·3
     46  |  5596  |    10·2    |    10      |     68      |    65     |   46·4
     45  |  5538  |     9·3    |     9·0    |     76      |    76     |   48·8
     44  |  5481  |     8·0    |     8·1    |     84      |    90     |   52·8
     42  |  5373  |     7·2    |     7·2    |     94·5    |    98     |   50·3
     40  |  5270  |     6·7    |     6·8    |    100      |    99     |   48·1
     38  |  5172  |     7·2    |     7·0    |     97      |    97·5   |   48·7
     36  |  5085  |     8·05   |     7·7    |     90      |    90     |   49·5
     34  |  5002  |     8·05   |     8·4    |     81      |    80     |   47·9
     32  |  4924  |     9·9    |     9·8    |     69      |    65     |   45·5
     30  |  4848  |    13·2    |    12·5    |     54      |    50     |   44·6
     28  |  4776  |    13·9    |    15·0    |     45·3    |    36     |   38·6
     27  |  4742  |    16·8    |    17·0    |     40      |    31·5   |   38·2
     26  |  4707  |    21·6    |    20·5    |     32      |    26·5   |   38·8
     24  |  4639  |    30      |    27      |     25      |    19·5   |   37·6
     22  |  4578  |    36      |    35      |     19      |    14     |   35
     20  |  4517  |    42      |    45      |     15·5    |    10     |   32·2
     16  |  4404  |    79      |    79      |      8·5    |     5·5   |   31·2
     10  |  4245  |   180      |   190      |      3·6    |     2·5   |   32·2
      6  |  4151  |   270      |   270      |      2·7    |           |
  -------+--------+------------+------------+-------------+-----------+------------

    [B] In this and the next two Tables the intensity of the
        illumination of the D ray before reduction is equal to that
        of an amyl-acetate lamp at one foot from a screen. The
        figures in Col. VII. are in millionths of the illumination
        of an amyl-acetate lamp at one foot distant, every ray
        being made of that intensity.


TABLE IX.--H. R.’S CURVES (see Fig. 32).

  -------+--------+------------+------------+-------------+-----------+------------
   I.    | II.    |  III.      |  IV.       |  V.         | VI.       | VII.
  -------+--------+------------+------------+-------------+-----------+------------
  Scale  | Wave-  | Mean       | Adopted    | Persistency | Luminosity| Absolute
  number.| length.| reading of | reading in | curve       | curve.    | luminosity
         |        | extinction | millionths |  (590 /     |           | of
         |        | in         | of         |ad. reading).|           | extinction
         |        | millionths | original   |             |           | (IV.×VI.) /
         |        | of         | luminosity.|             |           |    48
         |        | original   |            |             |           |
         |        | luminosity.|            |             |           |
  -------+--------+------------+------------+-------------+-----------+------------
    57   |  6423  |   1200     |   1200     |      ·49    |     5     |   125
    56   |  6330  |    900     |    850     |      ·69    |     7     |   124
    55   |  6242  |    500     |    550     |     1·07    |    10     |   115
    54   |  6152  |    250     |    250     |     2·36    |    17     |    88
    53   |  6074  |     ..     |    150     |     3·93    |    25     |    78
    52   |  5996  |     90     |     90     |     6·56    |    35     |    66
    51   |  5919  |     60     |     45     |    13·1     |    47     |    44
    50   |  5850  |     27     |     27     |    21·8     |    57     |    32
    48   |  5720  |     18     |     15     |    39·3     |    66     |    21
    46   |  5596  |     10     |     10     |    59       |    69     |    14
    44   |  5481  |      9·3   |      8     |    73·8     |    64     |    11
    42   |  5373  |      6·5   |      6·2   |    95·1     |    56·5   |     7
    40   |  5270  |      5·9   |      5·9   |   100       |    45     |     5·5
    38   |  5172  |      6     |      6     |    98·3     |    32     |     4
    36   |  5085  |     ..     |      6·6   |    89·4     |    20     |     2·7
    35   |  5043  |      7     |      7·2   |    81·9     |    16     |     2·4
    34   |  5002  |     ..     |      8     |    73·8     |    12·5   |     2·1
    32   |  4924  |     10     |      9·6   |    61·5     |     8     |     1·6
    30   |  4848  |     11·5   |     12     |    49·2     |     6     |     1·5
    28   |  4776  |     14·5   |     14·5   |    40·7     |     5     |     1·5
    26   |  4707  |     20     |     17·5   |    33·7     |     4     |     1·5
    24   |  4639  |     20     |     22     |    26·8     |     3     |     1·4
    22   |  4578  |     ..     |     30     |    19·7     |     2·4   |     1·5
    18   |  4459  |     55     |     57     |    10·4     |     1·3   |     1·5
    14   |  4349  |    115     |    115     |     5·1     |      ·7   |     1·7
    10   |  4245  |     ..     |    160     |     3·7     |      ·5   |     1·7
     6   |  4151  |    200     |    200     |     2·9     |      ·4   |     1·7
  -------+--------+------------+------------+-------------+-----------+------------


TABLE X.--V. H.’S CURVES (see Fig. 33).

  -------+--------+------------+------------+-------------+-----------+------------
   I.    | II.    |  III.      |  IV.       |  V.         | VI.       | VII.
  -------+--------+------------+------------+-------------+-----------+------------
  Scale  | Wave-  | Mean       | Adopted    | Persistency | Luminosity| Absolute
  number.| length.| reading of | reading in | curve       | curve.    | luminosity
         |        | extinction | millionths | (530 /      |           | of
         |        | in         | of         |ad. reading).|           | extinction
         |        | millionths | original   |             |           | (IV.×VI.) /
         |        | of         | luminosity.|             |           |    75.
         |        | original   |            |             |           |
         |        | luminosity.|            |             |           |
  -------+--------+------------+------------+-------------+-----------+------------
    57   |  6423  |     500    |     500    |     1·1     |    31     |  206
    56   |  6330  |     350    |     350    |     1·5     |    43     |  200
    54   |  6152  |     200    |     180    |     2·9     |    61     |  146·4
    52   |  5996  |     100    |     100    |     5·3     |    70     |   93·3
    50   |  5850  |      40    |      40    |    13·3     |    73     |   38·9
    48   |  5720  |      ..    |      25    |    21·2     |    69     |   23
    46   |  5596  |      10    |      10    |    53·0     |    63     |    8·4
    45   |  5538  |     6·5    |     6·5    |    81·6     |    58     |    5·0
    44   |  5481  |     6·0    |     5·7    |    93       |    54     |    4·1
    42   |  5373  |     5·5    |     5·3    |   100       |    46     |    3·3
    40   |  5270  |     5·5    |     5·4    |    98·2     |    36     |    2·6
    38   |  5172  |     5·7    |     5·7    |    93       |    24     |    1·8
    36   |  5085  |     6·7    |     6·5    |    81·6     |    15     |    1·3
    34   |  5002  |     7·0    |     7·0    |    75·7     |     9·5   |     ·89
    32   |  4924  |     8·5    |     8·5    |    62·3     |     7·0   |     ·79
    30   |  4848  |    10·7    |    10·5    |    50·5     |     5·0   |     ·70
    28   |  4776  |    16      |    16      |    33·1     |     3·7   |     ·79
    26   |  4707  |     ..     |    22·5    |    23·5     |     2·7   |     ·81
    24   |  4639  |    30      |    31      |    17·1     |     1·82  |     ·75
    22   |  4578  |    42·5    |    42      |    12·6     |     1·4   |     ·78
    20   |  4517  |    55      |    55      |     9·6     |     1·0   |     ·73
    16   |  4404  |   105      |   100      |     5·3     |     ·7    |     ·93
    12   |  4296  |   175      |   170      |     3·1     |     ·45   |    1·02
    10   |  4245  |   200      |   200      |     2·7     |     ·34   |     ·91
  -------+--------+------------+------------+-------------+-----------+------------


TABLE XI.--B. C.’S CURVES (see Fig. 34).

  --------+--------+-------------+------------+-----------+-------------
          |        |             |            |           |
    I.    | II.    | III.        |  IV.       |  V.       |  VI.
  --------+--------+-------------+------------+-----------+-------------
  Scale   | Wave-  | Adopted     | Persistency| Luminosity| Absolute
  Number. | length.| reading in  | curve      | of        | luminosity
          |        | hundred     | 12,500     | original  | of
          |        | thousandths.| readings   | beam.     | extinction
          |        |             | in V.      |           | III. and V.
  --------+--------+-------------+------------+-----------+-------------
     61   |  6839  |    7500     |     1·6    |           |
     60   |  6728  |    5500     |     2·3    |      ·5   |    27·5
     59   |  6622  |    4000     |     3·1    |     1     |    40
     58   |  6520  |    2800     |     4·5    |     2     |    56
     57   |  6423  |    2000     |     6·2    |     4     |    80
     56   |  6330  |    1500     |     8·3    |     6     |    90
     55   |  6242  |    1150     |    10·8    |     8     |    92
     54   |  6152  |     950     |    13·1    |    11·5   |   109·2
     53   |  6074  |     750     |    16·6    |    16     |   120
     52   |  5996  |     580     |    21·6    |    21·5   |   125
     51   |  5919  |     430     |    29      |    28·5   |   122·5
     50   |  5850  |     350     |    36      |    37     |   129·5
     49   |  5783  |     275     |    45·5    |    47     |   129·2
     48   |  5720  |     215     |    58      |    60     |   129
     47   |  5658  |     170     |    73·4    |    76     |   129·2
     46   |  5596  |     140     |    89·3    |    92     |   129
     45   |  5538  |     125     |   100      |    98     |   122·5
     44   |  5481  |     125     |   100      |   100     |   125
     43   |  5427  |     130     |    96·1    |    97     |   126
     42   |  5373  |     150     |    83      |    85     |   127·5
     41   |  5321  |     180     |    69·4    |    65     |   117
     40   |  5270  |     215     |    59      |    45     |    96·7
     39   |  5221  |     250     |    50      |    30     |    75
     38   |  5172  |     290     |    43      |    1·5    |   723·2
     37   |  5128  |     335     |    37      |   16      |    53·6
     36   |  5055  |     380     |    33      |   11·5    |    43·7
     34   |  5002  |     500     |    25      |    7      |    35
     32   |  4994  |     650     |    19      |    4      |    26
     30   |  4848  |     850     |    14      |    2·5    |    23·3
     28   |  4776  |    1100     |    11·4    |    2      |    22
     26   |  4707  |    1500     |     8·3    |    1·5    |    22
     24   |  4639  |    2000     |     6·2    |    1      |    20
     22   |  4578  |    2700     |     4·6    |    5      |    13·5
     18   |  4459  |    4750     |            |           |
     14   |  4349  |    7500     |            |           |
     10   |  4245  |   11000     |            |           |
  --------+--------+-------------+------------+-----------+-----------


TABLE XII.--M.’S LUMINOSITY CURVE COMPARED WITH THE NORMAL (see Fig.
30).

  -------+--------+--------+-------+----------+-----------+----------
    I.   |  II.   |  III.  |  IV.  |    V.    |    VI.    |   VII.
  -------+--------+--------+-------+----------+-----------+----------
  Scale  | Wave-  |  Mean  |  Mean |  Normal  |Difference |
  number.| length.|reading.|reading|luminosity|of last two|Difference
         |        |        |× 1·8. |  curve,  | columns.  |  × 5·15.
         |        |        |       |centre of |           |
         |        |        |       |   eye.   |           |
  -------+--------+--------+-------+----------+-----------+----------
    61   |  6839  |   2    |  3·6  |    4     |    ·4     |   2·57
    59   |  6621  |   7    | 12·6  |   12·5   |   -·1     |    ·51
    57   |  6423  |  18    | 32·4  |   33     |   +·6     |   3·09
    55   |  6242  |  36    | 64·8  |   65     |    ·2     |   1·03
    53   |  6074  |  49    | 88·2  |   89·5   |   1·3     |   6·71
    52   |  5996  |  52    | 95·4  |   96·5   |   1·1     |   5·66
    51   |  5919  |  54    | 97·2  |   99·5   |   2·3     |  11·8
    50   |  5850  |  54    | 97·2  |  100     |   2·8     |  14·4
    49   |  5782  |  52·5  | 94·5  |   99·5   |   5·0     |  25·7
    48   |  5720  |  50    | 90    |   97     |   7·0     |  36·0
    47   |  5658  |  46    | 82·8  |   92·5   |   9·7     |  49·9
    46   |  5596  |  41    | 73·8  |   87     |  13·2     |  68·0
    44   |  5481  |  32    | 57·6  |   75     |  17·4     |  89
    42   |  5373  |  23    | 43·2  |   62.5   |  19·3     |  99
    40   |  5270  |  17    | 30·6  |   50     |  19·4     | 100
    38   |  5172  |  10    | 17·5  |   35·5   |  18       |  93
    36   |  5085  |   4    |  7·2  |   24     |  16·8     |  86·5
    34   |  5002  |   1·0  |  1·8  |   14·5   |  12·7     |  65·5
    31   |  4885  |    ·5  |   ·7  |    6·5   |   5·8     |  37·7
    28   |  4776  |   0    |  0    |    4     |   4       |  20·6
  -------+--------+--------+-------+----------+-----------+----------


TABLE XIII.--MISS W.’S CURVES (see Fig. 39).

  --------+---------+-----------+--------------+------------
  Scale   | Wave-   | Readings. | Extinction in| Persistency
  number. | length. |           |   1/100000.  |   curve.
  --------+---------+-----------+--------------+------------
    63    |  7082   |    0      |              |
    62    |  6957   |    1      |              |
    60    |  6728   |    7      |              |
    58    |  6520   |   18      |              |
    57    |  6423   |   28      |              |
    56    |  6330   |   43      |              |
    54    |  6152   |   76      |    900       |   2
    52    |  5996   |   90      |    250       |   7
    50    |  5850   |   95      |    130       |  13·5
    48    |  5720   |   93      |     60       |  29
    46    |  5596   |   83      |     34       |  51
    44    |  5481   |   71      |     22       |  80
    42    |  5321   |   58      |     18·5     |  92
    40    |  5270   |   46      |     17·5     | 100
    38    |  5172   |   32      |     18       |  94
    36    |  5085   |   21      |     19·5     |  90
    34    |  5002   |   12·5    |     22       |  79
    32    |  4924   |    7      |     27       |  65
    30    |  4848   |    4·5    |     34       |  51
    28    |  4776   |    3·0    |     40       |  38·5
    25    |  4675   |    1·5    |     60       |  29
    20    |  4518   |    0·4    |    250       |   7
    19    |  4488   |    0·0    |    350       |   5
    16    |  4404   |   --      |    600       |
  --------+---------+-----------+--------------+------------




INDEX


                                                                    PAGE

  Absorption by the Yellow Spot                                       90

  Artificial Spectrum                                                 33


  Cases of Defective Colour Vision unrecognised                       67

  Clerk Maxwell’s Colour-Box                                          42

  Clerk Maxwell’s Colour Curves                                       47

  Colour, and the Sensations required to produce it                   50

  Colour Blindness due to Disease                                    137

  Colour-Blind Persons see a Grey in the Spectrum                     65

  Colour Discs                                                        32

  Colour Fields                                                       13

  Colour Matches made by the Colour Blind                             70

  Colour Patch Apparatus                                              18

  Colour Patch Apparatus, Original Form of                            19

  Comparison of the Young and Hering Theory                          189

  Complex Colours matched by Simple Colours                           22

  Contrast Colours                                                   187

  Curious Case of Congenital Colour Blindness, A                     164


  Dalton Colour Blindness                                             58

  Daltonism, or Colour Blindness                                      57

  Defective Form Vision connected with Colour Deficiency due to
        Disease                                                      138

  Definition at different parts of the Retina                         11


  Enfeebled Spectrum Luminosity                                       98

  Exhibiting Colour Blindness by Colour Discs                         74

  Extinction and Persistency Curves of Green-Blind Persons           127

  Extinction and Persistency Curves of Monochromatic Vision          125

  Extinction and Persistency Curves of Red-Blind Persons             127

  Extinction of Colour                                               105

  Extinction of Light by the Centre and Periphery of the Eye         114

  Extinction of Colour of equal Luminosity                           110

  Extinction of Light in the Spectrum                                109

  Eye: Explanation of its Functions                                    3


  Fatigue of the Retina                                            6, 30

  Field of View                                                       10

  Fovea Centralis                                                      4

  Fundamental Light                                                   34


  Green-Blind Person’s Description of the Spectrum, A                 64

  Green Monochromatic Vision                                         131


  Helmholtz Diagram of Sensations                                     38

  Heredity in Colour Blindness                                        58

  Hering’s Colour Vision Theory                                       52

  Hering’s Theory not tri-chromic                                     57

  Holmgren’s Colour Tests                                            169


  Kœnig’s Colour Sensation Curves                                     49


  Lissajou’s Figures                                                  37

  Luminosity of the Spectrum to the Centre of the Eye, the Fovea
        Centralis, and outside the Yellow Spot                        88

  Luminosity of the Spectrum to partially Colour Blind                86

  Luminosity of the Spectrum to the Colour Blind                      81

  Luminosity of the Spectrum to the Normal Eyed                       78


  Malingerers, Detection of                                          185

  Matching Colours by Mixtures of Simple Colours                      26

  Maxwell’s Colour Equations                                         202

  Maxwell’s Curves for Red Blindness                                  69

  Measurement of Colour Fields                                       207

  Monochromatic Vision and the Spectrum                               66


  Number of Cones in the Eye                                           8


  Optograms                                                            9


  Pellet Tests                                                       146

  Pendulum Experiments                                                36

  Persistency Curves                                                 119

  Primary Colours                                                     25

  Primary Pigment Colours                                             27

  Progressive Atrophy of the Optic Nerve                             153

  Purkinje’s Figures                                                   7

  Purples                                                             24


  Red and Green matched                                               72

  Red-Blind Person’s Description of the Spectrum, A                   63

  Retina, Structure of                                                 6

  Retinal Fatigue                                                  6, 30

  Rods and Cones                                                       8


  Seat of Visual Sensation                                             7

  Sensation Curves in Terms of Luminosity                             93

  Sensitiveness of the Eye                                           121

  Simple Colours                                                      17

  Simulation of Red and Green Blindness                              175

  Spectrum described by the Tobacco Blind, The                       143

  Spectrum Test for Colour Blindness                                 181


  Table of Wave-Lengths                                               17

  Tables                                                             211

  Tobacco Ambyopia                                                   140

  Tobacco Blindness, Examples of                                     148


  Violet Blindness                                                    73

  Visibility of an Object in light of different Colours              123

  Visual Purple                                                        9


  White Monochromatic Vision                                         158

  Wool Test, The                                                     170


  Yellow Spot                                                          4

  Yellow Spot and Colour Mixtures, The                                28

  Young’s Theory, Modification of                                    196




WORKS ON PHOTOGRAPHY

BY

CAPT. W. DE W. ABNEY, C.B., D.C.L., F.R.S., _Late Royal Engineers_.


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Transcriber’s Notes


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

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

Illustrations in this eBook have been positioned between paragraphs.

The index was not checked for proper alphabetization or correct page
references.