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TREATISE ON LIGHT

In which are explained
The causes of that which occurs
In REFLEXION, & in REFRACTION

And particularly
In the strange REFRACTION
OF ICELAND CRYSTAL

by

CHRISTIAAN HUYGENS

Rendered into English by

SILVANUS P. THOMPSON

University of Chicago Press







PREFACE


I wrote this Treatise during my sojourn in France twelve years ago,
and I communicated it in the year 1678 to the learned persons who then
composed the Royal Academy of Science, to the membership of which the
King had done me the honour of calling, me. Several of that body who
are still alive will remember having been present when I read it, and
above the rest those amongst them who applied themselves particularly
to the study of Mathematics; of whom I cannot cite more than the
celebrated gentlemen Cassini, Römer, and De la Hire. And, although I
have since corrected and changed some parts, the copies which I had
made of it at that time may serve for proof that I have yet added
nothing to it save some conjectures touching the formation of Iceland
Crystal, and a novel observation on the refraction of Rock Crystal. I
have desired to relate these particulars to make known how long I have
meditated the things which now I publish, and not for the purpose of
detracting from the merit of those who, without having seen anything
that I have written, may be found to have treated of like matters: as
has in fact occurred to two eminent Geometricians, Messieurs Newton
and Leibnitz, with respect to the Problem of the figure of glasses for
collecting rays when one of the surfaces is given.

One may ask why I have so long delayed to bring this work to the
light. The reason is that I wrote it rather carelessly in the Language
in which it appears, with the intention of translating it into Latin,
so doing in order to obtain greater attention to the thing. After
which I proposed to myself to give it out along with another Treatise
on Dioptrics, in which I explain the effects of Telescopes and those
things which belong more to that Science. But the pleasure of novelty
being past, I have put off from time to time the execution of this
design, and I know not when I shall ever come to an end if it, being
often turned aside either by business or by some new study.
Considering which I have finally judged that it was better worth while
to publish this writing, such as it is, than to let it run the risk,
by waiting longer, of remaining lost.

There will be seen in it demonstrations of those kinds which do not
produce as great a certitude as those of Geometry, and which even
differ much therefrom, since whereas the Geometers prove their
Propositions by fixed and incontestable Principles, here the
Principles are verified by the conclusions to be drawn from them; the
nature of these things not allowing of this being done otherwise.

It is always possible to attain thereby to a degree of probability
which very often is scarcely less than complete proof. To wit, when
things which have been demonstrated by the Principles that have been
assumed correspond perfectly to the phenomena which experiment has
brought under observation; especially when there are a great number of
them, and further, principally, when one can imagine and foresee new
phenomena which ought to follow from the hypotheses which one employs,
and when one finds that therein the fact corresponds to our prevision.
But if all these proofs of probability are met with in that which I
propose to discuss, as it seems to me they are, this ought to be a
very strong confirmation of the success of my inquiry; and it must be
ill if the facts are not pretty much as I represent them. I would
believe then that those who love to know the Causes of things and who
are able to admire the marvels of Light, will find some satisfaction
in these various speculations regarding it, and in the new explanation
of its famous property which is the main foundation of the
construction of our eyes and of those great inventions which extend so
vastly the use of them.

I hope also that there will be some who by following these beginnings
will penetrate much further into this question than I have been able
to do, since the subject must be far from being exhausted. This
appears from the passages which I have indicated where I leave certain
difficulties without having resolved them, and still more from matters
which I have not touched at all, such as Luminous Bodies of several
sorts, and all that concerns Colours; in which no one until now can
boast of having succeeded. Finally, there remains much more to be
investigated touching the nature of Light which I do not pretend to
have disclosed, and I shall owe much in return to him who shall be
able to supplement that which is here lacking to me in knowledge. The
Hague. The 8 January 1690.




NOTE BY THE TRANSLATOR


Considering the great influence which this Treatise has exercised in
the development of the Science of Optics, it seems strange that two
centuries should have passed before an English edition of the work
appeared. Perhaps the circumstance is due to the mistaken zeal with
which formerly everything that conflicted with the cherished ideas of
Newton was denounced by his followers. The Treatise on Light of
Huygens has, however, withstood the test of time: and even now the
exquisite skill with which he applied his conception of the
propagation of waves of light to unravel the intricacies of the
phenomena of the double refraction of crystals, and of the refraction
of the atmosphere, will excite the admiration of the student of
Optics. It is true that his wave theory was far from the complete
doctrine as subsequently developed by Thomas Young and Augustin
Fresnel, and belonged rather to geometrical than to physical Optics.
If Huygens had no conception of transverse vibrations, of the
principle of interference, or of the existence of the ordered sequence
of waves in trains, he nevertheless attained to a remarkably clear
understanding of the principles of wave-propagation; and his
exposition of the subject marks an epoch in the treatment of Optical
problems. It has been needful in preparing this translation to
exercise care lest one should import into the author's text ideas of
subsequent date, by using words that have come to imply modern
conceptions. Hence the adoption of as literal a rendering as possible.
A few of the author's terms need explanation. He uses the word
"refraction," for example, both for the phenomenon or process usually
so denoted, and for the result of that process: thus the refracted ray
he habitually terms "the refraction" of the incident ray. When a
wave-front, or, as he terms it, a "wave," has passed from some initial
position to a subsequent one, he terms the wave-front in its
subsequent position "the continuation" of the wave. He also speaks of
the envelope of a set of elementary waves, formed by coalescence of
those elementary wave-fronts, as "the termination" of the wave; and
the elementary wave-fronts he terms "particular" waves. Owing to the
circumstance that the French word _rayon_ possesses the double
signification of ray of light and radius of a circle, he avoids its
use in the latter sense and speaks always of the semi-diameter, not of
the radius. His speculations as to the ether, his suggestive views of
the structure of crystalline bodies, and his explanation of opacity,
slight as they are, will possibly surprise the reader by their seeming
modernness. And none can read his investigation of the phenomena found
in Iceland spar without marvelling at his insight and sagacity.

S.P.T.

June, 1912.




TABLE OF MATTERS

Contained in this Treatise


CHAPTER I.
On Rays Propagated in Straight Lines.

  That Light is produced by a certain movement.

  That no substance passes from the luminous object to the eyes.

  That Light spreads spherically, almost as Sound does.

  Whether Light takes time to spread.

  Experience seeming to prove that it passes instantaneously.

  Experience proving that it takes time.

  How much its speed is greater than that of Sound.

  In what the emission of Light differs from that of Sound.

  That it is not the same medium which serves for Light and Sound.

  How Sound is propagated.

  How Light is propagated.

  Detailed Remarks on the propagation of Light.

  Why Rays are propagated only in straight lines.

  How Light coming in different directions can cross itself.

CHAPTER II.
On Reflexion.

  Demonstration of equality of angles of incidence and reflexion.

  Why the incident and reflected rays are in the same plane
  perpendicular to the reflecting surface.

  That it is not needful for the reflecting surface to be perfectly
  flat to attain equality of the angles of incidence and reflexion.

CHAPTER III.
On Refraction.

  That bodies may be transparent without any substance passing through
  them.

  Proof that the ethereal matter passes through transparent bodies.

  How this matter passing through can render them transparent.

  That the most solid bodies in appearance are of a very loose texture.

  That Light spreads more slowly in water and in glass than in air.

  Third hypothesis to explain transparency, and the retardation which
  Light suffers.

  On that which makes bodies opaque.

  Demonstration why Refraction obeys the known proportion of Sines.

  Why the incident and refracted Rays produce one another reciprocally.

  Why Reflexion within a triangular glass prism is suddenly augmented
  when the Light can no longer penetrate.

  That bodies which cause greater Refraction also cause stronger
  Reflexion.

  Demonstration of the Theorem of Mr. Fermat.

CHAPTER IV.
On the Refraction of the Air.

  That the emanations of Light in the air are not spherical.

  How consequently some objects appear higher than they are.

  How the Sun may appear on the Horizon before he has risen.

  That the rays of light become curved in the Air of the Atmosphere,
  and what effects this produces.

CHAPTER V.
On the Strange Refraction of Iceland Crystal.

  That this Crystal grows also in other countries.

  Who first-wrote about it.

  Description of Iceland Crystal; its substance, shape, and properties.

  That it has two different Refractions.

  That the ray perpendicular to the surface suffers refraction, and
  that some rays inclined to the surface pass without suffering
  refraction.

  Observation of the refractions in this Crystal.

  That there is a Regular and an Irregular Refraction.

  The way of measuring the two Refractions of Iceland Crystal.

  Remarkable properties of the Irregular Refraction.

  Hypothesis to explain the double Refraction.

  That Rock Crystal has also a double Refraction.

  Hypothesis of emanations of Light, within Iceland Crystal, of
  spheroidal form, for the Irregular Refraction.

  How a perpendicular ray can suffer Refraction.

  How the position and form of the spheroidal emanations in this
  Crystal can be defined.

  Explanation of the Irregular Refraction by these spheroidal
  emanations.

  Easy way to find the Irregular Refraction of each incident ray.

  Demonstration of the oblique ray which traverses the Crystal without
  being refracted.

  Other irregularities of Refraction explained.

  That an object placed beneath the Crystal appears double, in two
  images of different heights.

  Why the apparent heights of one of the images change on changing the
  position of the eyes above the Crystal.

  Of the different sections of this Crystal which produce yet other
  refractions, and confirm all this Theory.

  Particular way of polishing the surfaces after it has been cut.

  Surprising phenomenon touching the rays which pass through two
  separated pieces; the cause of which is not explained.

  Probable conjecture on the internal composition of Iceland Crystal,
  and of what figure its particles are.

  Tests to confirm this conjecture.

  Calculations which have been supposed in this Chapter.

CHAPTER VI.
On the Figures of transparent bodies which serve for Refraction and
for Reflexion.

  General and easy rule to find these Figures.

  Invention of the Ovals of Mr. Des Cartes for Dioptrics.

  How he was able to find these Lines.

  Way of finding the surface of a glass for perfect refraction, when
  the other surface is given.

  Remark on what happens to rays refracted at a spherical surface.

  Remark on the curved line which is formed by reflexion in a spherical
  concave mirror.





CHAPTER I

ON RAYS PROPAGATED IN STRAIGHT LINES


As happens in all the sciences in which Geometry is applied to matter,
the demonstrations concerning Optics are founded on truths drawn from
experience. Such are that the rays of light are propagated in straight
lines; that the angles of reflexion and of incidence are equal; and
that in refraction the ray is bent according to the law of sines, now
so well known, and which is no less certain than the preceding laws.

The majority of those who have written touching the various parts of
Optics have contented themselves with presuming these truths. But
some, more inquiring, have desired to investigate the origin and the
causes, considering these to be in themselves wonderful effects of
Nature. In which they advanced some ingenious things, but not however
such that the most intelligent folk do not wish for better and more
satisfactory explanations. Wherefore I here desire to propound what I
have meditated on the subject, so as to contribute as much as I can
to the explanation of this department of Natural Science, which, not
without reason, is reputed to be one of its most difficult parts. I
recognize myself to be much indebted to those who were the first to
begin to dissipate the strange obscurity in which these things were
enveloped, and to give us hope that they might be explained by
intelligible reasoning. But, on the other hand I am astonished also
that even here these have often been willing to offer, as assured and
demonstrative, reasonings which were far from conclusive. For I do not
find that any one has yet given a probable explanation of the first
and most notable phenomena of light, namely why it is not propagated
except in straight lines, and how visible rays, coming from an
infinitude of diverse places, cross one another without hindering one
another in any way.

I shall therefore essay in this book, to give, in accordance with the
principles accepted in the Philosophy of the present day, some clearer
and more probable reasons, firstly of these properties of light
propagated rectilinearly; secondly of light which is reflected on
meeting other bodies. Then I shall explain the phenomena of those rays
which are said to suffer refraction on passing through transparent
bodies of different sorts; and in this part I shall also explain the
effects of the refraction of the air by the different densities of the
Atmosphere.

Thereafter I shall examine the causes of the strange refraction of a
certain kind of Crystal which is brought from Iceland. And finally I
shall treat of the various shapes of transparent and reflecting bodies
by which rays are collected at a point or are turned aside in various
ways. From this it will be seen with what facility, following our new
Theory, we find not only the Ellipses, Hyperbolas, and other curves
which Mr. Des Cartes has ingeniously invented for this purpose; but
also those which the surface of a glass lens ought to possess when its
other surface is given as spherical or plane, or of any other figure
that may be.

It is inconceivable to doubt that light consists in the motion of some
sort of matter. For whether one considers its production, one sees
that here upon the Earth it is chiefly engendered by fire and flame
which contain without doubt bodies that are in rapid motion, since
they dissolve and melt many other bodies, even the most solid; or
whether one considers its effects, one sees that when light is
collected, as by concave mirrors, it has the property of burning as a
fire does, that is to say it disunites the particles of bodies. This
is assuredly the mark of motion, at least in the true Philosophy, in
which one conceives the causes of all natural effects in terms of
mechanical motions. This, in my opinion, we must necessarily do, or
else renounce all hopes of ever comprehending anything in Physics.

And as, according to this Philosophy, one holds as certain that the
sensation of sight is excited only by the impression of some movement
of a kind of matter which acts on the nerves at the back of our eyes,
there is here yet one reason more for believing that light consists in
a movement of the matter which exists between us and the luminous
body.

Further, when one considers the extreme speed with which light spreads
on every side, and how, when it comes from different regions, even
from those directly opposite, the rays traverse one another without
hindrance, one may well understand that when we see a luminous object,
it cannot be by any transport of matter coming to us from this object,
in the way in which a shot or an arrow traverses the air; for
assuredly that would too greatly impugn these two properties of light,
especially the second of them. It is then in some other way that light
spreads; and that which can lead us to comprehend it is the knowledge
which we have of the spreading of Sound in the air.

We know that by means of the air, which is an invisible and impalpable
body, Sound spreads around the spot where it has been produced, by a
movement which is passed on successively from one part of the air to
another; and that the spreading of this movement, taking place equally
rapidly on all sides, ought to form spherical surfaces ever enlarging
and which strike our ears. Now there is no doubt at all that light
also comes from the luminous body to our eyes by some movement
impressed on the matter which is between the two; since, as we have
already seen, it cannot be by the transport of a body which passes
from one to the other. If, in addition, light takes time for its
passage--which we are now going to examine--it will follow that this
movement, impressed on the intervening matter, is successive; and
consequently it spreads, as Sound does, by spherical surfaces and
waves: for I call them waves from their resemblance to those which are
seen to be formed in water when a stone is thrown into it, and which
present a successive spreading as circles, though these arise from
another cause, and are only in a flat surface.

To see then whether the spreading of light takes time, let us consider
first whether there are any facts of experience which can convince us
to the contrary. As to those which can be made here on the Earth, by
striking lights at great distances, although they prove that light
takes no sensible time to pass over these distances, one may say with
good reason that they are too small, and that the only conclusion to
be drawn from them is that the passage of light is extremely rapid.
Mr. Des Cartes, who was of opinion that it is instantaneous, founded
his views, not without reason, upon a better basis of experience,
drawn from the Eclipses of the Moon; which, nevertheless, as I shall
show, is not at all convincing. I will set it forth, in a way a little
different from his, in order to make the conclusion more
comprehensible.

[Illustration]

Let A be the place of the sun, BD a part of the orbit or annual path
of the Earth: ABC a straight line which I suppose to meet the orbit of
the Moon, which is represented by the circle CD, at C.

Now if light requires time, for example one hour, to traverse the
space which is between the Earth and the Moon, it will follow that the
Earth having arrived at B, the shadow which it casts, or the
interruption of the light, will not yet have arrived at the point C,
but will only arrive there an hour after. It will then be one hour
after, reckoning from the moment when the Earth was at B, that the
Moon, arriving at C, will be obscured: but this obscuration or
interruption of the light will not reach the Earth till after another
hour. Let us suppose that the Earth in these two hours will have
arrived at E. The Earth then, being at E, will see the Eclipsed Moon
at C, which it left an hour before, and at the same time will see the
sun at A. For it being immovable, as I suppose with Copernicus, and
the light moving always in straight lines, it must always appear where
it is. But one has always observed, we are told, that the eclipsed
Moon appears at the point of the Ecliptic opposite to the Sun; and yet
here it would appear in arrear of that point by an amount equal to the
angle GEC, the supplement of AEC. This, however, is contrary to
experience, since the angle GEC would be very sensible, and about 33
degrees. Now according to our computation, which is given in the
Treatise on the causes of the phenomena of Saturn, the distance BA
between the Earth and the Sun is about twelve thousand diameters of
the Earth, and hence four hundred times greater than BC the distance
of the Moon, which is 30 diameters. Then the angle ECB will be nearly
four hundred times greater than BAE, which is five minutes; namely,
the path which the earth travels in two hours along its orbit; and
thus the angle BCE will be nearly 33 degrees; and likewise the angle
CEG, which is greater by five minutes.

But it must be noted that the speed of light in this argument has been
assumed such that it takes a time of one hour to make the passage from
here to the Moon. If one supposes that for this it requires only one
minute of time, then it is manifest that the angle CEG will only be 33
minutes; and if it requires only ten seconds of time, the angle will
be less than six minutes. And then it will not be easy to perceive
anything of it in observations of the Eclipse; nor, consequently, will
it be permissible to deduce from it that the movement of light is
instantaneous.

It is true that we are here supposing a strange velocity that would be
a hundred thousand times greater than that of Sound. For Sound,
according to what I have observed, travels about 180 Toises in the
time of one Second, or in about one beat of the pulse. But this
supposition ought not to seem to be an impossibility; since it is not
a question of the transport of a body with so great a speed, but of a
successive movement which is passed on from some bodies to others. I
have then made no difficulty, in meditating on these things, in
supposing that the emanation of light is accomplished with time,
seeing that in this way all its phenomena can be explained, and that
in following the contrary opinion everything is incomprehensible. For
it has always seemed tome that even Mr. Des Cartes, whose aim has been
to treat all the subjects of Physics intelligibly, and who assuredly
has succeeded in this better than any one before him, has said nothing
that is not full of difficulties, or even inconceivable, in dealing
with Light and its properties.

But that which I employed only as a hypothesis, has recently received
great seemingness as an established truth by the ingenious proof of
Mr. Römer which I am going here to relate, expecting him himself to
give all that is needed for its confirmation. It is founded as is the
preceding argument upon celestial observations, and proves not only
that Light takes time for its passage, but also demonstrates how much
time it takes, and that its velocity is even at least six times
greater than that which I have just stated.

For this he makes use of the Eclipses suffered by the little planets
which revolve around Jupiter, and which often enter his shadow: and
see what is his reasoning. Let A be the Sun, BCDE the annual orbit of
the Earth, F Jupiter, GN the orbit of the nearest of his Satellites,
for it is this one which is more apt for this investigation than any
of the other three, because of the quickness of its revolution. Let G
be this Satellite entering into the shadow of Jupiter, H the same
Satellite emerging from the shadow.

[Illustration]

Let it be then supposed, the Earth being at B some time before the
last quadrature, that one has seen the said Satellite emerge from the
shadow; it must needs be, if the Earth remains at the same place,
that, after 42-1/2 hours, one would again see a similar emergence,
because that is the time in which it makes the round of its orbit, and
when it would come again into opposition to the Sun. And if the Earth,
for instance, were to remain always at B during 30 revolutions of this
Satellite, one would see it again emerge from the shadow after 30
times 42-1/2 hours. But the Earth having been carried along during
this time to C, increasing thus its distance from Jupiter, it follows
that if Light requires time for its passage the illumination of the
little planet will be perceived later at C than it would have been at
B, and that there must be added to this time of 30 times 42-1/2 hours
that which the Light has required to traverse the space MC, the
difference of the spaces CH, BH. Similarly at the other quadrature
when the earth has come to E from D while approaching toward Jupiter,
the immersions of the Satellite ought to be observed at E earlier than
they would have been seen if the Earth had remained at D.

Now in quantities of observations of these Eclipses, made during ten
consecutive years, these differences have been found to be very
considerable, such as ten minutes and more; and from them it has been
concluded that in order to traverse the whole diameter of the annual
orbit KL, which is double the distance from here to the sun, Light
requires about 22 minutes of time.

The movement of Jupiter in his orbit while the Earth passed from B to
C, or from D to E, is included in this calculation; and this makes it
evident that one cannot attribute the retardation of these
illuminations or the anticipation of the eclipses, either to any
irregularity occurring in the movement of the little planet or to its
eccentricity.

If one considers the vast size of the diameter KL, which according to
me is some 24 thousand diameters of the Earth, one will acknowledge
the extreme velocity of Light. For, supposing that KL is no more than
22 thousand of these diameters, it appears that being traversed in 22
minutes this makes the speed a thousand diameters in one minute, that
is 16-2/3 diameters in one second or in one beat of the pulse, which
makes more than 11 hundred times a hundred thousand toises; since the
diameter of the Earth contains 2,865 leagues, reckoned at 25 to the
degree, and each each league is 2,282 Toises, according to the exact
measurement which Mr. Picard made by order of the King in 1669. But
Sound, as I have said above, only travels 180 toises in the same time
of one second: hence the velocity of Light is more than six hundred
thousand times greater than that of Sound. This, however, is quite
another thing from being instantaneous, since there is all the
difference between a finite thing and an infinite. Now the successive
movement of Light being confirmed in this way, it follows, as I have
said, that it spreads by spherical waves, like the movement of Sound.

But if the one resembles the other in this respect, they differ in
many other things; to wit, in the first production of the movement
which causes them; in the matter in which the movement spreads; and in
the manner in which it is propagated. As to that which occurs in the
production of Sound, one knows that it is occasioned by the agitation
undergone by an entire body, or by a considerable part of one, which
shakes all the contiguous air. But the movement of the Light must
originate as from each point of the luminous object, else we should
not be able to perceive all the different parts of that object, as
will be more evident in that which follows. And I do not believe that
this movement can be better explained than by supposing that all those
of the luminous bodies which are liquid, such as flames, and
apparently the sun and the stars, are composed of particles which
float in a much more subtle medium which agitates them with great
rapidity, and makes them strike against the particles of the ether
which surrounds them, and which are much smaller than they. But I hold
also that in luminous solids such as charcoal or metal made red hot in
the fire, this same movement is caused by the violent agitation of
the particles of the metal or of the wood; those of them which are on
the surface striking similarly against the ethereal matter. The
agitation, moreover, of the particles which engender the light ought
to be much more prompt and more rapid than is that of the bodies which
cause sound, since we do not see that the tremors of a body which is
giving out a sound are capable of giving rise to Light, even as the
movement of the hand in the air is not capable of producing Sound.

Now if one examines what this matter may be in which the movement
coming from the luminous body is propagated, which I call Ethereal
matter, one will see that it is not the same that serves for the
propagation of Sound. For one finds that the latter is really that
which we feel and which we breathe, and which being removed from any
place still leaves there the other kind of matter that serves to
convey Light. This may be proved by shutting up a sounding body in a
glass vessel from which the air is withdrawn by the machine which Mr.
Boyle has given us, and with which he has performed so many beautiful
experiments. But in doing this of which I speak, care must be taken to
place the sounding body on cotton or on feathers, in such a way that
it cannot communicate its tremors either to the glass vessel which
encloses it, or to the machine; a precaution which has hitherto been
neglected. For then after having exhausted all the air one hears no
Sound from the metal, though it is struck.

One sees here not only that our air, which does not penetrate through
glass, is the matter by which Sound spreads; but also that it is not
the same air but another kind of matter in which Light spreads; since
if the air is removed from the vessel the Light does not cease to
traverse it as before.

And this last point is demonstrated even more clearly by the
celebrated experiment of Torricelli, in which the tube of glass from
which the quicksilver has withdrawn itself, remaining void of air,
transmits Light just the same as when air is in it. For this proves
that a matter different from air exists in this tube, and that this
matter must have penetrated the glass or the quicksilver, either one
or the other, though they are both impenetrable to the air. And when,
in the same experiment, one makes the vacuum after putting a little
water above the quicksilver, one concludes equally that the said
matter passes through glass or water, or through both.

As regards the different modes in which I have said the movements of
Sound and of Light are communicated, one may sufficiently comprehend
how this occurs in the case of Sound if one considers that the air is
of such a nature that it can be compressed and reduced to a much
smaller space than that which it ordinarily occupies. And in
proportion as it is compressed the more does it exert an effort to
regain its volume; for this property along with its penetrability,
which remains notwithstanding its compression, seems to prove that it
is made up of small bodies which float about and which are agitated
very rapidly in the ethereal matter composed of much smaller parts. So
that the cause of the spreading of Sound is the effort which these
little bodies make in collisions with one another, to regain freedom
when they are a little more squeezed together in the circuit of these
waves than elsewhere.

But the extreme velocity of Light, and other properties which it has,
cannot admit of such a propagation of motion, and I am about to show
here the way in which I conceive it must occur. For this, it is
needful to explain the property which hard bodies must possess to
transmit movement from one to another.

When one takes a number of spheres of equal size, made of some very
hard substance, and arranges them in a straight line, so that they
touch one another, one finds, on striking with a similar sphere
against the first of these spheres, that the motion passes as in an
instant to the last of them, which separates itself from the row,
without one's being able to perceive that the others have been
stirred. And even that one which was used to strike remains motionless
with them. Whence one sees that the movement passes with an extreme
velocity which is the greater, the greater the hardness of the
substance of the spheres.

But it is still certain that this progression of motion is not
instantaneous, but successive, and therefore must take time. For if
the movement, or the disposition to movement, if you will have it so,
did not pass successively through all these spheres, they would all
acquire the movement at the same time, and hence would all advance
together; which does not happen. For the last one leaves the whole row
and acquires the speed of the one which was pushed. Moreover there are
experiments which demonstrate that all the bodies which we reckon of
the hardest kind, such as quenched steel, glass, and agate, act as
springs and bend somehow, not only when extended as rods but also when
they are in the form of spheres or of other shapes. That is to say
they yield a little in themselves at the place where they are struck,
and immediately regain their former figure. For I have found that on
striking with a ball of glass or of agate against a large and quite
thick thick piece of the same substance which had a flat surface,
slightly soiled with breath or in some other way, there remained round
marks, of smaller or larger size according as the blow had been weak
or strong. This makes it evident that these substances yield where
they meet, and spring back: and for this time must be required.

Now in applying this kind of movement to that which produces Light
there is nothing to hinder us from estimating the particles of the
ether to be of a substance as nearly approaching to perfect hardness
and possessing a springiness as prompt as we choose. It is not
necessary to examine here the causes of this hardness, or of that
springiness, the consideration of which would lead us too far from our
subject. I will say, however, in passing that we may conceive that the
particles of the ether, notwithstanding their smallness, are in turn
composed of other parts and that their springiness consists in the
very rapid movement of a subtle matter which penetrates them from
every side and constrains their structure to assume such a disposition
as to give to this fluid matter the most overt and easy passage
possible. This accords with the explanation which Mr. Des Cartes gives
for the spring, though I do not, like him, suppose the pores to be in
the form of round hollow canals. And it must not be thought that in
this there is anything absurd or impossible, it being on the contrary
quite credible that it is this infinite series of different sizes of
corpuscles, having different degrees of velocity, of which Nature
makes use to produce so many marvellous effects.

But though we shall ignore the true cause of springiness we still see
that there are many bodies which possess this property; and thus there
is nothing strange in supposing that it exists also in little
invisible bodies like the particles of the Ether. Also if one wishes
to seek for any other way in which the movement of Light is
successively communicated, one will find none which agrees better,
with uniform progression, as seems to be necessary, than the property
of springiness; because if this movement should grow slower in
proportion as it is shared over a greater quantity of matter, in
moving away from the source of the light, it could not conserve this
great velocity over great distances. But by supposing springiness in
the ethereal matter, its particles will have the property of equally
rapid restitution whether they are pushed strongly or feebly; and thus
the propagation of Light will always go on with an equal velocity.

[Illustration]

And it must be known that although the particles of the ether are not
ranged thus in straight lines, as in our row of spheres, but
confusedly, so that one of them touches several others, this does not
hinder them from transmitting their movement and from spreading it
always forward. As to this it is to be remarked that there is a law of
motion serving for this propagation, and verifiable by experiment. It
is that when a sphere, such as A here, touches several other similar
spheres CCC, if it is struck by another sphere B in such a way as to
exert an impulse against all the spheres CCC which touch it, it
transmits to them the whole of its movement, and remains after that
motionless like the sphere B. And without supposing that the ethereal
particles are of spherical form (for I see indeed no need to suppose
them so) one may well understand that this property of communicating
an impulse does not fail to contribute to the aforesaid propagation
of movement.

Equality of size seems to be more necessary, because otherwise there
ought to be some reflexion of movement backwards when it passes from a
smaller particle to a larger one, according to the Laws of Percussion
which I published some years ago.

However, one will see hereafter that we have to suppose such an
equality not so much as a necessity for the propagation of light as
for rendering that propagation easier and more powerful; for it is not
beyond the limits of probability that the particles of the ether have
been made equal for a purpose so important as that of light, at least
in that vast space which is beyond the region of atmosphere and which
seems to serve only to transmit the light of the Sun and the Stars.

I have then shown in what manner one may conceive Light to spread
successively, by spherical waves, and how it is possible that this
spreading is accomplished with as great a velocity as that which
experiments and celestial observations demand. Whence it may be
further remarked that although the particles are supposed to be in
continual movement (for there are many reasons for this) the
successive propagation of the waves cannot be hindered by this;
because the propagation consists nowise in the transport of those
particles but merely in a small agitation which they cannot help
communicating to those surrounding, notwithstanding any movement which
may act on them causing them to be changing positions amongst
themselves.

But we must consider still more particularly the origin of these
waves, and the manner in which they spread. And, first, it follows
from what has been said on the production of Light, that each little
region of a luminous body, such as the Sun, a candle, or a burning
coal, generates its own waves of which that region is the centre. Thus
in the flame of a candle, having distinguished the points A, B, C,
concentric circles described about each of these points represent the
waves which come from them. And one must imagine the same about every
point of the surface and of the part within the flame.

[Illustration]

But as the percussions at the centres of these waves possess no
regular succession, it must not be supposed that the waves themselves
follow one another at equal distances: and if the distances marked in
the figure appear to be such, it is rather to mark the progression of
one and the same wave at equal intervals of time than to represent
several of them issuing from one and the same centre.

After all, this prodigious quantity of waves which traverse one
another without confusion and without effacing one another must not be
deemed inconceivable; it being certain that one and the same particle
of matter can serve for many waves coming from different sides or even
from contrary directions, not only if it is struck by blows which
follow one another closely but even for those which act on it at the
same instant. It can do so because the spreading of the movement is
successive. This may be proved by the row of equal spheres of hard
matter, spoken of above. If against this row there are pushed from two
opposite sides at the same time two similar spheres A and D, one will
see each of them rebound with the same velocity which it had in
striking, yet the whole row will remain in its place, although the
movement has passed along its whole length twice over. And if these
contrary movements happen to meet one another at the middle sphere, B,
or at some other such as C, that sphere will yield and act as a spring
at both sides, and so will serve at the same instant to transmit these
two movements.

[Illustration]

But what may at first appear full strange and even incredible is that
the undulations produced by such small movements and corpuscles,
should spread to such immense distances; as for example from the Sun
or from the Stars to us. For the force of these waves must grow feeble
in proportion as they move away from their origin, so that the action
of each one in particular will without doubt become incapable of
making itself felt to our sight. But one will cease to be astonished
by considering how at a great distance from the luminous body an
infinitude of waves, though they have issued from different points of
this body, unite together in such a way that they sensibly compose one
single wave only, which, consequently, ought to have enough force to
make itself felt. Thus this infinite number of waves which originate
at the same instant from all points of a fixed star, big it may be as
the Sun, make practically only one single wave which may well have
force enough to produce an impression on our eyes. Moreover from each
luminous point there may come many thousands of waves in the smallest
imaginable time, by the frequent percussion of the corpuscles which
strike the Ether at these points: which further contributes to
rendering their action more sensible.

[Illustration]

There is the further consideration in the emanation of these waves,
that each particle of matter in which a wave spreads, ought not to
communicate its motion only to the next particle which is in the
straight line drawn from the luminous point, but that it also imparts
some of it necessarily to all the others which touch it and which
oppose themselves to its movement. So it arises that around each
particle there is made a wave of which that particle is the centre.
Thus if DCF is a wave emanating from the luminous point A, which is
its centre, the particle B, one of those comprised within the sphere
DCF, will have made its particular or partial wave KCL, which will
touch the wave DCF at C at the same moment that the principal wave
emanating from the point A has arrived at DCF; and it is clear that it
will be only the region C of the wave KCL which will touch the wave
DCF, to wit, that which is in the straight line drawn through AB.
Similarly the other particles of the sphere DCF, such as _bb_, _dd_,
etc., will each make its own wave. But each of these waves can be
infinitely feeble only as compared with the wave DCF, to the
composition of which all the others contribute by the part of their
surface which is most distant from the centre A.

One sees, in addition, that the wave DCF is determined by the
distance attained in a certain space of time by the movement which
started from the point A; there being no movement beyond this wave,
though there will be in the space which it encloses, namely in parts
of the particular waves, those parts which do not touch the sphere
DCF. And all this ought not to seem fraught with too much minuteness
or subtlety, since we shall see in the sequel that all the properties
of Light, and everything pertaining to its reflexion and its
refraction, can be explained in principle by this means. This is a
matter which has been quite unknown to those who hitherto have begun
to consider the waves of light, amongst whom are Mr. Hooke in his
_Micrographia_, and Father Pardies, who, in a treatise of which he let
me see a portion, and which he was unable to complete as he died
shortly afterward, had undertaken to prove by these waves the effects
of reflexion and refraction. But the chief foundation, which consists
in the remark I have just made, was lacking in his demonstrations; and
for the rest he had opinions very different from mine, as may be will
appear some day if his writing has been preserved.

To come to the properties of Light. We remark first that each portion
of a wave ought to spread in such a way that its extremities lie
always between the same straight lines drawn from the luminous point.
Thus the portion BG of the wave, having the luminous point A as its
centre, will spread into the arc CE bounded by the straight lines ABC,
AGE. For although the particular waves produced by the particles
comprised within the space CAE spread also outside this space, they
yet do not concur at the same instant to compose a wave which
terminates the movement, as they do precisely at the circumference
CE, which is their common tangent.

And hence one sees the reason why light, at least if its rays are not
reflected or broken, spreads only by straight lines, so that it
illuminates no object except when the path from its source to that
object is open along such lines.

For if, for example, there were an opening BG, limited by opaque
bodies BH, GI, the wave of light which issues from the point A will
always be terminated by the straight lines AC, AE, as has just been
shown; the parts of the partial waves which spread outside the space
ACE being too feeble to produce light there.

Now, however small we make the opening BG, there is always the same
reason causing the light there to pass between straight lines; since
this opening is always large enough to contain a great number of
particles of the ethereal matter, which are of an inconceivable
smallness; so that it appears that each little portion of the wave
necessarily advances following the straight line which comes from the
luminous point. Thus then we may take the rays of light as if they
were straight lines.

It appears, moreover, by what has been remarked touching the
feebleness of the particular waves, that it is not needful that all
the particles of the Ether should be equal amongst themselves, though
equality is more apt for the propagation of the movement. For it is
true that inequality will cause a particle by pushing against another
larger one to strive to recoil with a part of its movement; but it
will thereby merely generate backwards towards the luminous point some
partial waves incapable of causing light, and not a wave compounded of
many as CE was.

Another property of waves of light, and one of the most marvellous,
is that when some of them come from different or even from opposing
sides, they produce their effect across one another without any
hindrance. Whence also it comes about that a number of spectators may
view different objects at the same time through the same opening, and
that two persons can at the same time see one another's eyes. Now
according to the explanation which has been given of the action of
light, how the waves do not destroy nor interrupt one another when
they cross one another, these effects which I have just mentioned are
easily conceived. But in my judgement they are not at all easy to
explain according to the views of Mr. Des Cartes, who makes Light to
consist in a continuous pressure merely tending to movement. For this
pressure not being able to act from two opposite sides at the same
time, against bodies which have no inclination to approach one
another, it is impossible so to understand what I have been saying
about two persons mutually seeing one another's eyes, or how two
torches can illuminate one another.




CHAPTER II

ON REFLEXION


Having explained the effects of waves of light which spread in a
homogeneous matter, we will examine next that which happens to them on
encountering other bodies. We will first make evident how the
Reflexion of light is explained by these same waves, and why it
preserves equality of angles.

Let there be a surface AB; plane and polished, of some metal, glass,
or other body, which at first I will consider as perfectly uniform
(reserving to myself to deal at the end of this demonstration with the
inequalities from which it cannot be exempt), and let a line AC,
inclined to AD, represent a portion of a wave of light, the centre of
which is so distant that this portion AC may be considered as a
straight line; for I consider all this as in one plane, imagining to
myself that the plane in which this figure is, cuts the sphere of the
wave through its centre and intersects the plane AB at right angles.
This explanation will suffice once for all.

[Illustration]

The piece C of the wave AC, will in a certain space of time advance as
far as the plane AB at B, following the straight line CB, which may be
supposed to come from the luminous centre, and which in consequence is
perpendicular to AC. Now in this same space of time the portion A of
the same wave, which has been hindered from communicating its movement
beyond the plane AB, or at least partly so, ought to have continued
its movement in the matter which is above this plane, and this along a
distance equal to CB, making its own partial spherical wave,
according to what has been said above. Which wave is here represented
by the circumference SNR, the centre of which is A, and its
semi-diameter AN equal to CB.

If one considers further the other pieces H of the wave AC, it appears
that they will not only have reached the surface AB by straight lines
HK parallel to CB, but that in addition they will have generated in
the transparent air, from the centres K, K, K, particular spherical
waves, represented here by circumferences the semi-diameters of which
are equal to KM, that is to say to the continuations of HK as far as
the line BG parallel to AC. But all these circumferences have as a
common tangent the straight line BN, namely the same which is drawn
from B as a tangent to the first of the circles, of which A is the
centre, and AN the semi-diameter equal to BC, as is easy to see.

It is then the line BN (comprised between B and the point N where the
perpendicular from the point A falls) which is as it were formed by
all these circumferences, and which terminates the movement which is
made by the reflexion of the wave AC; and it is also the place where
the movement occurs in much greater quantity than anywhere else.
Wherefore, according to that which has been explained, BN is the
propagation of the wave AC at the moment when the piece C of it has
arrived at B. For there is no other line which like BN is a common
tangent to all the aforesaid circles, except BG below the plane AB;
which line BG would be the propagation of the wave if the movement
could have spread in a medium homogeneous with that which is above the
plane. And if one wishes to see how the wave AC has come successively
to BN, one has only to draw in the same figure the straight lines KO
parallel to BN, and the straight lines KL parallel to AC. Thus one
will see that the straight wave AC has become broken up into all the
OKL parts successively, and that it has become straight again at NB.

Now it is apparent here that the angle of reflexion is made equal to
the angle of incidence. For the triangles ACB, BNA being rectangular
and having the side AB common, and the side CB equal to NA, it follows
that the angles opposite to these sides will be equal, and therefore
also the angles CBA, NAB. But as CB, perpendicular to CA, marks the
direction of the incident ray, so AN, perpendicular to the wave BN,
marks the direction of the reflected ray; hence these rays are equally
inclined to the plane AB.

But in considering the preceding demonstration, one might aver that it
is indeed true that BN is the common tangent of the circular waves in
the plane of this figure, but that these waves, being in truth
spherical, have still an infinitude of similar tangents, namely all
the straight lines which are drawn from the point B in the surface
generated by the straight line BN about the axis BA. It remains,
therefore, to demonstrate that there is no difficulty herein: and by
the same argument one will see why the incident ray and the reflected
ray are always in one and the same plane perpendicular to the
reflecting plane. I say then that the wave AC, being regarded only as
a line, produces no light. For a visible ray of light, however narrow
it may be, has always some width, and consequently it is necessary, in
representing the wave whose progression constitutes the ray, to put
instead of a line AC some plane figure such as the circle HC in the
following figure, by supposing, as we have done, the luminous point to
be infinitely distant. Now it is easy to see, following the preceding
demonstration, that each small piece of this wave HC having arrived at
the plane AB, and there generating each one its particular wave, these
will all have, when C arrives at B, a common plane which will touch
them, namely a circle BN similar to CH; and this will be intersected
at its middle and at right angles by the same plane which likewise
intersects the circle CH and the ellipse AB.

[Illustration]

One sees also that the said spheres of the partial waves cannot have
any common tangent plane other than the circle BN; so that it will be
this plane where there will be more reflected movement than anywhere
else, and which will therefore carry on the light in continuance from
the wave CH.

I have also stated in the preceding demonstration that the movement of
the piece A of the incident wave is not able to communicate itself
beyond the plane AB, or at least not wholly. Whence it is to be
remarked that though the movement of the ethereal matter might
communicate itself partly to that of the reflecting body, this could
in nothing alter the velocity of progression of the waves, on which
the angle of reflexion depends. For a slight percussion ought to
generate waves as rapid as strong percussion in the same matter. This
comes about from the property of bodies which act as springs, of which
we have spoken above; namely that whether compressed little or much
they recoil in equal times. Equally so in every reflexion of the
light, against whatever body it may be, the angles of reflexion and
incidence ought to be equal notwithstanding that the body might be of
such a nature that it takes away a portion of the movement made by the
incident light. And experiment shows that in fact there is no polished
body the reflexion of which does not follow this rule.


But the thing to be above all remarked in our demonstration is that it
does not require that the reflecting surface should be considered as a
uniform plane, as has been supposed by all those who have tried to
explain the effects of reflexion; but only an evenness such as may be
attained by the particles of the matter of the reflecting body being
set near to one another; which particles are larger than those of the
ethereal matter, as will appear by what we shall say in treating of
the transparency and opacity of bodies. For the surface consisting
thus of particles put together, and the ethereal particles being
above, and smaller, it is evident that one could not demonstrate the
equality of the angles of incidence and reflexion by similitude to
that which happens to a ball thrown against a wall, of which writers
have always made use. In our way, on the other hand, the thing is
explained without difficulty. For the smallness of the particles of
quicksilver, for example, being such that one must conceive millions
of them, in the smallest visible surface proposed, arranged like a
heap of grains of sand which has been flattened as much as it is
capable of being, this surface then becomes for our purpose as even
as a polished glass is: and, although it always remains rough with
respect to the particles of the Ether it is evident that the centres
of all the particular spheres of reflexion, of which we have spoken,
are almost in one uniform plane, and that thus the common tangent can
fit to them as perfectly as is requisite for the production of light.
And this alone is requisite, in our method of demonstration, to cause
equality of the said angles without the remainder of the movement
reflected from all parts being able to produce any contrary effect.




CHAPTER III

ON REFRACTION


In the same way as the effects of Reflexion have been explained by
waves of light reflected at the surface of polished bodies, we will
explain transparency and the phenomena of refraction by waves which
spread within and across diaphanous bodies, both solids, such as
glass, and liquids, such as water, oils, etc. But in order that it may
not seem strange to suppose this passage of waves in the interior of
these bodies, I will first show that one may conceive it possible in
more than one mode.

First, then, if the ethereal matter cannot penetrate transparent
bodies at all, their own particles would be able to communicate
successively the movement of the waves, the same as do those of the
Ether, supposing that, like those, they are of a nature to act as a
spring. And this is easy to conceive as regards water and other
transparent liquids, they being composed of detached particles. But it
may seem more difficult as regards glass and other transparent and
hard bodies, because their solidity does not seem to permit them to
receive movement except in their whole mass at the same time. This,
however, is not necessary because this solidity is not such as it
appears to us, it being probable rather that these bodies are composed
of particles merely placed close to one another and held together by
some pressure from without of some other matter, and by the
irregularity of their shapes. For primarily their rarity is shown by
the facility with which there passes through them the matter of the
vortices of the magnet, and that which causes gravity. Further, one
cannot say that these bodies are of a texture similar to that of a
sponge or of light bread, because the heat of the fire makes them flow
and thereby changes the situation of the particles amongst themselves.
It remains then that they are, as has been said, assemblages of
particles which touch one another without constituting a continuous
solid. This being so, the movement which these particles receive to
carry on the waves of light, being merely communicated from some of
them to others, without their going for that purpose out of their
places or without derangement, it may very well produce its effect
without prejudicing in any way the apparent solidity of the compound.

By pressure from without, of which I have spoken, must not be
understood that of the air, which would not be sufficient, but that of
some other more subtle matter, a pressure which I chanced upon by
experiment long ago, namely in the case of water freed from air, which
remains suspended in a tube open at its lower end, notwithstanding
that the air has been removed from the vessel in which this tube is
enclosed.

One can then in this way conceive of transparency in a solid without
any necessity that the ethereal matter which serves for light should
pass through it, or that it should find pores in which to insinuate
itself. But the truth is that this matter not only passes through
solids, but does so even with great facility; of which the experiment
of Torricelli, above cited, is already a proof. Because on the
quicksilver and the water quitting the upper part of the glass tube,
it appears that it is immediately filled with ethereal matter, since
light passes across it. But here is another argument which proves this
ready penetrability, not only in transparent bodies but also in all
others.

When light passes across a hollow sphere of glass, closed on all
sides, it is certain that it is full of ethereal matter, as much as
the spaces outside the sphere. And this ethereal matter, as has been
shown above, consists of particles which just touch one another. If
then it were enclosed in the sphere in such a way that it could not
get out through the pores of the glass, it would be obliged to follow
the movement of the sphere when one changes its place: and it would
require consequently almost the same force to impress a certain
velocity on this sphere, when placed on a horizontal plane, as if it
were full of water or perhaps of quicksilver: because every body
resists the velocity of the motion which one would give to it, in
proportion to the quantity of matter which it contains, and which is
obliged to follow this motion. But on the contrary one finds that the
sphere resists the impress of movement only in proportion to the
quantity of matter of the glass of which it is made. Then it must be
that the ethereal matter which is inside is not shut up, but flows
through it with very great freedom. We shall demonstrate hereafter
that by this process the same penetrability may be inferred also as
relating to opaque bodies.

The second mode then of explaining transparency, and one which appears
more probably true, is by saying that the waves of light are carried
on in the ethereal matter, which continuously occupies the interstices
or pores of transparent bodies. For since it passes through them
continuously and freely, it follows that they are always full of it.
And one may even show that these interstices occupy much more space
than the coherent particles which constitute the bodies. For if what
we have just said is true: that force is required to impress a certain
horizontal velocity on bodies in proportion as they contain coherent
matter; and if the proportion of this force follows the law of
weights, as is confirmed by experiment, then the quantity of the
constituent matter of bodies also follows the proportion of their
weights. Now we see that water weighs only one fourteenth part as much
as an equal portion of quicksilver: therefore the matter of the water
does not occupy the fourteenth part of the space which its mass
obtains. It must even occupy much less of it, since quicksilver is
less heavy than gold, and the matter of gold is by no means dense, as
follows from the fact that the matter of the vortices of the magnet
and of that which is the cause of gravity pass very freely through it.

But it may be objected here that if water is a body of so great
rarity, and if its particles occupy so small a portion of the space of
its apparent bulk, it is very strange how it yet resists Compression
so strongly without permitting itself to be condensed by any force
which one has hitherto essayed to employ, preserving even its entire
liquidity while subjected to this pressure.

This is no small difficulty. It may, however, be resolved by saying
that the very violent and rapid motion of the subtle matter which
renders water liquid, by agitating the particles of which it is
composed, maintains this liquidity in spite of the pressure which
hitherto any one has been minded to apply to it.

The rarity of transparent bodies being then such as we have said, one
easily conceives that the waves might be carried on in the ethereal
matter which fills the interstices of the particles. And, moreover,
one may believe that the progression of these waves ought to be a
little slower in the interior of bodies, by reason of the small
detours which the same particles cause. In which different velocity of
light I shall show the cause of refraction to consist.

Before doing so, I will indicate the third and last mode in which
transparency may be conceived; which is by supposing that the movement
of the waves of light is transmitted indifferently both in the
particles of the ethereal matter which occupy the interstices of
bodies, and in the particles which compose them, so that the movement
passes from one to the other. And it will be seen hereafter that this
hypothesis serves excellently to explain the double refraction of
certain transparent bodies.

Should it be objected that if the particles of the ether are smaller
than those of transparent bodies (since they pass through their
intervals), it would follow that they can communicate to them but
little of their movement, it may be replied that the particles of
these bodies are in turn composed of still smaller particles, and so
it will be these secondary particles which will receive the movement
from those of the ether.

Furthermore, if the particles of transparent bodies have a recoil a
little less prompt than that of the ethereal particles, which nothing
hinders us from supposing, it will again follow that the progression
of the waves of light will be slower in the interior of such bodies
than it is outside in the ethereal matter.

All this I have found as most probable for the mode in which the waves
of light pass across transparent bodies. To which it must further be
added in what respect these bodies differ from those which are opaque;
and the more so since it might seem because of the easy penetration of
bodies by the ethereal matter, of which mention has been made, that
there would not be any body that was not transparent. For by the same
reasoning about the hollow sphere which I have employed to prove the
smallness of the density of glass and its easy penetrability by the
ethereal matter, one might also prove that the same penetrability
obtains for metals and for every other sort of body. For this sphere
being for example of silver, it is certain that it contains some of
the ethereal matter which serves for light, since this was there as
well as in the air when the opening of the sphere was closed. Yet,
being closed and placed upon a horizontal plane, it resists the
movement which one wishes to give to it, merely according to the
quantity of silver of which it is made; so that one must conclude, as
above, that the ethereal matter which is enclosed does not follow the
movement of the sphere; and that therefore silver, as well as glass,
is very easily penetrated by this matter. Some of it is therefore
present continuously and in quantities between the particles of silver
and of all other opaque bodies: and since it serves for the
propagation of light it would seem that these bodies ought also to be
transparent, which however is not the case.

Whence then, one will say, does their opacity come? Is it because the
particles which compose them are soft; that is to say, these particles
being composed of others that are smaller, are they capable of
changing their figure on receiving the pressure of the ethereal
particles, the motion of which they thereby damp, and so hinder the
continuance of the waves of light? That cannot be: for if the
particles of the metals are soft, how is it that polished silver and
mercury reflect light so strongly? What I find to be most probable
herein, is to say that metallic bodies, which are almost the only
really opaque ones, have mixed amongst their hard particles some soft
ones; so that some serve to cause reflexion and the others to hinder
transparency; while, on the other hand, transparent bodies contain
only hard particles which have the faculty of recoil, and serve
together with those of the ethereal matter for the propagation of the
waves of light, as has been said.

[Illustration]

Let us pass now to the explanation of the effects of Refraction,
assuming, as we have done, the passage of waves of light through
transparent bodies, and the diminution of velocity which these same
waves suffer in them.

The chief property of Refraction is that a ray of light, such as AB,
being in the air, and falling obliquely upon the polished surface of a
transparent body, such as FG, is broken at the point of incidence B,
in such a way that with the straight line DBE which cuts the surface
perpendicularly it makes an angle CBE less than ABD which it made with
the same perpendicular when in the air. And the measure of these
angles is found by describing, about the point B, a circle which cuts
the radii AB, BC. For the perpendiculars AD, CE, let fall from the
points of intersection upon the straight line DE, which are called the
Sines of the angles ABD, CBE, have a certain ratio between themselves;
which ratio is always the same for all inclinations of the incident
ray, at least for a given transparent body. This ratio is, in glass,
very nearly as 3 to 2; and in water very nearly as 4 to 3; and is
likewise different in other diaphanous bodies.

Another property, similar to this, is that the refractions are
reciprocal between the rays entering into a transparent body and those
which are leaving it. That is to say that if the ray AB in entering
the transparent body is refracted into BC, then likewise CB being
taken as a ray in the interior of this body will be refracted, on
passing out, into BA.

[Illustration]

To explain then the reasons of these phenomena according to our
principles, let AB be the straight line which represents a plane
surface bounding the transparent substances which lie towards C and
towards N. When I say plane, that does not signify a perfect evenness,
but such as has been understood in treating of reflexion, and for the
same reason. Let the line AC represent a portion of a wave of light,
the centre of which is supposed so distant that this portion may be
considered as a straight line. The piece C, then, of the wave AC, will
in a certain space of time have advanced as far as the plane AB
following the straight line CB, which may be imagined as coming from
the luminous centre, and which consequently will cut AC at right
angles. Now in the same time the piece A would have come to G along
the straight line AG, equal and parallel to CB; and all the portion of
wave AC would be at GB if the matter of the transparent body
transmitted the movement of the wave as quickly as the matter of the
Ether. But let us suppose that it transmits this movement less
quickly, by one-third, for instance. Movement will then be spread from
the point A, in the matter of the transparent body through a distance
equal to two-thirds of CB, making its own particular spherical wave
according to what has been said before. This wave is then represented
by the circumference SNR, the centre of which is A, and its
semi-diameter equal to two-thirds of CB. Then if one considers in
order the other pieces H of the wave AC, it appears that in the same
time that the piece C reaches B they will not only have arrived at the
surface AB along the straight lines HK parallel to CB, but that, in
addition, they will have generated in the diaphanous substance from
the centres K, partial waves, represented here by circumferences the
semi-diameters of which are equal to two-thirds of the lines KM, that
is to say, to two-thirds of the prolongations of HK down to the
straight line BG; for these semi-diameters would have been equal to
entire lengths of KM if the two transparent substances had been of the
same penetrability.

Now all these circumferences have for a common tangent the straight
line BN; namely the same line which is drawn as a tangent from the
point B to the circumference SNR which we considered first. For it is
easy to see that all the other circumferences will touch the same BN,
from B up to the point of contact N, which is the same point where AN
falls perpendicularly on BN.

It is then BN, which is formed by small arcs of these circumferences,
which terminates the movement that the wave AC has communicated within
the transparent body, and where this movement occurs in much greater
amount than anywhere else. And for that reason this line, in
accordance with what has been said more than once, is the propagation
of the wave AC at the moment when its piece C has reached B. For there
is no other line below the plane AB which is, like BN, a common
tangent to all these partial waves. And if one would know how the wave
AC has come progressively to BN, it is necessary only to draw in the
same figure the straight lines KO parallel to BN, and all the lines KL
parallel to AC. Thus one will see that the wave CA, from being a
straight line, has become broken in all the positions LKO
successively, and that it has again become a straight line at BN. This
being evident by what has already been demonstrated, there is no need
to explain it further.

Now, in the same figure, if one draws EAF, which cuts the plane AB at
right angles at the point A, since AD is perpendicular to the wave AC,
it will be DA which will mark the ray of incident light, and AN which
was perpendicular to BN, the refracted ray: since the rays are nothing
else than the straight lines along which the portions of the waves
advance.

Whence it is easy to recognize this chief property of refraction,
namely that the Sine of the angle DAE has always the same ratio to the
Sine of the angle NAF, whatever be the inclination of the ray DA: and
that this ratio is the same as that of the velocity of the waves in
the transparent substance which is towards AE to their velocity in the
transparent substance towards AF. For, considering AB as the radius of
a circle, the Sine of the angle BAC is BC, and the Sine of the angle
ABN is AN. But the angle BAC is equal to DAE, since each of them added
to CAE makes a right angle. And the angle ABN is equal to NAF, since
each of them with BAN makes a right angle. Then also the Sine of the
angle DAE is to the Sine of NAF as BC is to AN. But the ratio of BC to
AN was the same as that of the velocities of light in the substance
which is towards AE and in that which is towards AF; therefore also
the Sine of the angle DAE will be to the Sine of the angle NAF the
same as the said velocities of light.

To see, consequently, what the refraction will be when the waves of
light pass into a substance in which the movement travels more quickly
than in that from which they emerge (let us again assume the ratio of
3 to 2), it is only necessary to repeat all the same construction and
demonstration which we have just used, merely substituting everywhere
3/2 instead of 2/3. And it will be found by the same reasoning, in
this other figure, that when the piece C of the wave AC shall have
reached the surface AB at B, all the portions of the wave AC will
have advanced as far as BN, so that BC the perpendicular on AC is to
AN the perpendicular on BN as 2 to 3. And there will finally be this
same ratio of 2 to 3 between the Sine of the angle BAD and the Sine of
the angle FAN.

Hence one sees the reciprocal relation of the refractions of the ray
on entering and on leaving one and the same transparent body: namely
that if NA falling on the external surface AB is refracted into the
direction AD, so the ray AD will be refracted on leaving the
transparent body into the direction AN.

[Illustration]

One sees also the reason for a noteworthy accident which happens in
this refraction: which is this, that after a certain obliquity of the
incident ray DA, it begins to be quite unable to penetrate into the
other transparent substance. For if the angle DAQ or CBA is such that
in the triangle ACB, CB is equal to 2/3 of AB, or is greater, then AN
cannot form one side of the triangle ANB, since it becomes equal to or
greater than AB: so that the portion of wave BN cannot be found
anywhere, neither consequently can AN, which ought to be perpendicular
to it. And thus the incident ray DA does not then pierce the surface
AB.

When the ratio of the velocities of the waves is as two to three, as
in our example, which is that which obtains for glass and air, the
angle DAQ must be more than 48 degrees 11 minutes in order that the
ray DA may be able to pass by refraction. And when the ratio of the
velocities is as 3 to 4, as it is very nearly in water and air, this
angle DAQ must exceed 41 degrees 24 minutes. And this accords
perfectly with experiment.

But it might here be asked: since the meeting of the wave AC against
the surface AB ought to produce movement in the matter which is on the
other side, why does no light pass there? To which the reply is easy
if one remembers what has been said before. For although it generates
an infinitude of partial waves in the matter which is at the other
side of AB, these waves never have a common tangent line (either
straight or curved) at the same moment; and so there is no line
terminating the propagation of the wave AC beyond the plane AB, nor
any place where the movement is gathered together in sufficiently
great quantity to produce light. And one will easily see the truth of
this, namely that CB being larger than 2/3 of AB, the waves excited
beyond the plane AB will have no common tangent if about the centres K
one then draws circles having radii equal to 3/2 of the lengths LB to
which they correspond. For all these circles will be enclosed in one
another and will all pass beyond the point B.

Now it is to be remarked that from the moment when the angle DAQ is
smaller than is requisite to permit the refracted ray DA to pass into
the other transparent substance, one finds that the interior reflexion
which occurs at the surface AB is much augmented in brightness, as is
easy to realize by experiment with a triangular prism; and for this
our theory can afford this reason. When the angle DAQ is still large
enough to enable the ray DA to pass, it is evident that the light from
the portion AC of the wave is collected in a minimum space when it
reaches BN. It appears also that the wave BN becomes so much the
smaller as the angle CBA or DAQ is made less; until when the latter is
diminished to the limit indicated a little previously, this wave BN is
collected together always at one point. That is to say, that when the
piece C of the wave AC has then arrived at B, the wave BN which is the
propagation of AC is entirely reduced to the same point B. Similarly
when the piece H has reached K, the part AH is entirely reduced to the
same point K. This makes it evident that in proportion as the wave CA
comes to meet the surface AB, there occurs a great quantity of
movement along that surface; which movement ought also to spread
within the transparent body and ought to have much re-enforced the
partial waves which produce the interior reflexion against the surface
AB, according to the laws of reflexion previously explained.

And because a slight diminution of the angle of incidence DAQ causes
the wave BN, however great it was, to be reduced to zero, (for this
angle being 49 degrees 11 minutes in the glass, the angle BAN is still
11 degrees 21 minutes, and the same angle being reduced by one degree
only the angle BAN is reduced to zero, and so the wave BN reduced to a
point) thence it comes about that the interior reflexion from being
obscure becomes suddenly bright, so soon as the angle of incidence is
such that it no longer gives passage to the refraction.

Now as concerns ordinary external reflexion, that is to say which
occurs when the angle of incidence DAQ is still large enough to enable
the refracted ray to penetrate beyond the surface AB, this reflexion
should occur against the particles of the substance which touches the
transparent body on its outside. And it apparently occurs against the
particles of the air or others mingled with the ethereal particles and
larger than they. So on the other hand the external reflexion of these
bodies occurs against the particles which compose them, and which are
also larger than those of the ethereal matter, since the latter flows
in their interstices. It is true that there remains here some
difficulty in those experiments in which this interior reflexion
occurs without the particles of air being able to contribute to it, as
in vessels or tubes from which the air has been extracted.

Experience, moreover, teaches us that these two reflexions are of
nearly equal force, and that in different transparent bodies they are
so much the stronger as the refraction of these bodies is the greater.
Thus one sees manifestly that the reflexion of glass is stronger than
that of water, and that of diamond stronger than that of glass.

I will finish this theory of refraction by demonstrating a remarkable
proposition which depends on it; namely, that a ray of light in order
to go from one point to another, when these points are in different
media, is refracted in such wise at the plane surface which joins
these two media that it employs the least possible time: and exactly
the same happens in the case of reflexion against a plane surface. Mr.
Fermat was the first to propound this property of refraction, holding
with us, and directly counter to the opinion of Mr. Des Cartes, that
light passes more slowly through glass and water than through air.
But he assumed besides this a constant ratio of Sines, which we have
just proved by these different degrees of velocity alone: or rather,
what is equivalent, he assumed not only that the velocities were
different but that the light took the least time possible for its
passage, and thence deduced the constant ratio of the Sines. His
demonstration, which may be seen in his printed works, and in the
volume of letters of Mr. Des Cartes, is very long; wherefore I give
here another which is simpler and easier.

[Illustration]

Let KF be the plane surface; A the point in the medium which the light
traverses more easily, as the air; C the point in the other which is
more difficult to penetrate, as water. And suppose that a ray has come
from A, by B, to C, having been refracted at B according to the law
demonstrated a little before; that is to say that, having drawn PBQ,
which cuts the plane at right angles, let the sine of the angle ABP
have to the sine of the angle CBQ the same ratio as the velocity of
light in the medium where A is to the velocity of light in the medium
where C is. It is to be shown that the time of passage of light along
AB and BC taken together, is the shortest that can be. Let us assume
that it may have come by other lines, and, in the first place, along
AF, FC, so that the point of refraction F may be further from B than
the point A; and let AO be a line perpendicular to AB, and FO parallel
to AB; BH perpendicular to FO, and FG to BC.

Since then the angle HBF is equal to PBA, and the angle BFG equal to
QBC, it follows that the sine of the angle HBF will also have the same
ratio to the sine of BFG, as the velocity of light in the medium A is
to its velocity in the medium C. But these sines are the straight
lines HF, BG, if we take BF as the semi-diameter of a circle. Then
these lines HF, BG, will bear to one another the said ratio of the
velocities. And, therefore, the time of the light along HF, supposing
that the ray had been OF, would be equal to the time along BG in the
interior of the medium C. But the time along AB is equal to the time
along OH; therefore the time along OF is equal to the time along AB,
BG. Again the time along FC is greater than that along GC; then the
time along OFC will be longer than that along ABC. But AF is longer
than OF, then the time along AFC will by just so much more exceed the
time along ABC.

Now let us assume that the ray has come from A to C along AK, KC; the
point of refraction K being nearer to A than the point B is; and let
CN be the perpendicular upon BC, KN parallel to BC: BM perpendicular
upon KN, and KL upon BA.

Here BL and KM are the sines of angles BKL, KBM; that is to say, of
the angles PBA, QBC; and therefore they are to one another as the
velocity of light in the medium A is to the velocity in the medium C.
Then the time along LB is equal to the time along KM; and since the
time along BC is equal to the time along MN, the time along LBC will
be equal to the time along KMN. But the time along AK is longer than
that along AL: hence the time along AKN is longer than that along ABC.
And KC being longer than KN, the time along AKC will exceed, by as
much more, the time along ABC. Hence it appears that the time along
ABC is the shortest possible; which was to be proven.




CHAPTER IV

ON THE REFRACTION OF THE AIR


We have shown how the movement which constitutes light spreads by
spherical waves in any homogeneous matter. And it is evident that when
the matter is not homogeneous, but of such a constitution that the
movement is communicated in it more rapidly toward one side than
toward another, these waves cannot be spherical: but that they must
acquire their figure according to the different distances over which
the successive movement passes in equal times.

It is thus that we shall in the first place explain the refractions
which occur in the air, which extends from here to the clouds and
beyond. The effects of which refractions are very remarkable; for by
them we often see objects which the rotundity of the Earth ought
otherwise to hide; such as Islands, and the tops of mountains when one
is at sea. Because also of them the Sun and the Moon appear as risen
before in fact they have, and appear to set later: so that at times
the Moon has been seen eclipsed while the Sun appeared still above the
horizon. And so also the heights of the Sun and of the Moon, and those
of all the Stars always appear a little greater than they are in
reality, because of these same refractions, as Astronomers know. But
there is one experiment which renders this refraction very evident;
which is that of fixing a telescope on some spot so that it views an
object, such as a steeple or a house, at a distance of half a league
or more. If then you look through it at different hours of the day,
leaving it always fixed in the same way, you will see that the same
spots of the object will not always appear at the middle of the
aperture of the telescope, but that generally in the morning and in
the evening, when there are more vapours near the Earth, these objects
seem to rise higher, so that the half or more of them will no longer
be visible; and so that they seem lower toward mid-day when these
vapours are dissipated.

Those who consider refraction to occur only in the surfaces which
separate transparent bodies of different nature, would find it
difficult to give a reason for all that I have just related; but
according to our Theory the thing is quite easy. It is known that the
air which surrounds us, besides the particles which are proper to it
and which float in the ethereal matter as has been explained, is full
also of particles of water which are raised by the action of heat; and
it has been ascertained further by some very definite experiments that
as one mounts up higher the density of air diminishes in proportion.
Now whether the particles of water and those of air take part, by
means of the particles of ethereal matter, in the movement which
constitutes light, but have a less prompt recoil than these, or
whether the encounter and hindrance which these particles of air and
water offer to the propagation of movement of the ethereal progress,
retard the progression, it follows that both kinds of particles flying
amidst the ethereal particles, must render the air, from a great
height down to the Earth, gradually less easy for the spreading of the
waves of light.

[Illustration]

Whence the configuration of the waves ought to become nearly such as
this figure represents: namely, if A is a light, or the visible point
of a steeple, the waves which start from it ought to spread more
widely upwards and less widely downwards, but in other directions more
or less as they approximate to these two extremes. This being so, it
necessarily follows that every line intersecting one of these waves at
right angles will pass above the point A, always excepting the one
line which is perpendicular to the horizon.

[Illustration]

Let BC be the wave which brings the light to the spectator who is at
B, and let BD be the straight line which intersects this wave at right
angles. Now because the ray or straight line by which we judge the
spot where the object appears to us is nothing else than the
perpendicular to the wave that reaches our eye, as will be understood
by what was said above, it is manifest that the point A will be
perceived as being in the line BD, and therefore higher than in fact it
is.

Similarly if the Earth be AB, and the top of the Atmosphere CD, which
probably is not a well defined spherical surface (since we know that
the air becomes rare in proportion as one ascends, for above there is
so much less of it to press down upon it), the waves of light from the
sun coming, for instance, in such a way that so long as they have not
reached the Atmosphere CD the straight line AE intersects them
perpendicularly, they ought, when they enter the Atmosphere, to
advance more quickly in elevated regions than in regions nearer to the
Earth. So that if CA is the wave which brings the light to the
spectator at A, its region C will be the furthest advanced; and the
straight line AF, which intersects this wave at right angles, and
which determines the apparent place of the Sun, will pass above the
real Sun, which will be seen along the line AE. And so it may occur
that when it ought not to be visible in the absence of vapours,
because the line AE encounters the rotundity of the Earth, it will be
perceived in the line AF by refraction. But this angle EAF is scarcely
ever more than half a degree because the attenuation of the vapours
alters the waves of light but little. Furthermore these refractions
are not altogether constant in all weathers, particularly at small
elevations of 2 or 3 degrees; which results from the different
quantity of aqueous vapours rising above the Earth.

And this same thing is the cause why at certain times a distant object
will be hidden behind another less distant one, and yet may at another
time be able to be seen, although the spot from which it is viewed is
always the same. But the reason for this effect will be still more
evident from what we are going to remark touching the curvature of
rays. It appears from the things explained above that the progression
or propagation of a small part of a wave of light is properly what one
calls a ray. Now these rays, instead of being straight as they are in
homogeneous media, ought to be curved in an atmosphere of unequal
penetrability. For they necessarily follow from the object to the eye
the line which intersects at right angles all the progressions of the
waves, as in the first figure the line AEB does, as will be shown
hereafter; and it is this line which determines what interposed bodies
would or would not hinder us from seeing the object. For although the
point of the steeple A appears raised to D, it would yet not appear to
the eye B if the tower H was between the two, because it crosses the
curve AEB. But the tower E, which is beneath this curve, does not
hinder the point A from being seen. Now according as the air near the
Earth exceeds in density that which is higher, the curvature of the
ray AEB becomes greater: so that at certain times it passes above the
summit E, which allows the point A to be perceived by the eye at B;
and at other times it is intercepted by the same tower E which hides A
from this same eye.

[Illustration]

But to demonstrate this curvature of the rays conformably to all our
preceding Theory, let us imagine that AB is a small portion of a wave
of light coming from the side C, which we may consider as a straight
line. Let us also suppose that it is perpendicular to the Horizon, the
portion B being nearer to the Earth than the portion A; and that
because the vapours are less hindering at A than at B, the particular
wave which comes from the point A spreads through a certain space AD
while the particular wave which starts from the point B spreads
through a shorter space BE; AD and BE being parallel to the Horizon.
Further, supposing the straight lines FG, HI, etc., to be drawn from
an infinitude of points in the straight line AB and to terminate on
the line DE (which is straight or may be considered as such), let the
different penetrabilities at the different heights in the air between
A and B be represented by all these lines; so that the particular
wave, originating from the point F, will spread across the space FG,
and that from the point H across the space HI, while that from the
point A spreads across the space AD.

Now if about the centres A, B, one describes the circles DK, EL, which
represent the spreading of the waves which originate from these two
points, and if one draws the straight line KL which touches these two
circles, it is easy to see that this same line will be the common
tangent to all the other circles drawn about the centres F, H, etc.;
and that all the points of contact will fall within that part of this
line which is comprised between the perpendiculars AK, BL. Then it
will be the line KL which will terminate the movement of the
particular waves originating from the points of the wave AB; and this
movement will be stronger between the points KL, than anywhere else at
the same instant, since an infinitude of circumferences concur to form
this straight line; and consequently KL will be the propagation of the
portion of wave AB, as has been said in explaining reflexion and
ordinary refraction. Now it appears that AK and BL dip down toward the
side where the air is less easy to penetrate: for AK being longer than
BL, and parallel to it, it follows that the lines AB and KL, being
prolonged, would meet at the side L. But the angle K is a right angle:
hence KAB is necessarily acute, and consequently less than DAB. If one
investigates in the same way the progression of the portion of the
wave KL, one will find that after a further time it has arrived at MN
in such a manner that the perpendiculars KM, LN, dip down even more
than do AK, BL. And this suffices to show that the ray will continue
along the curved line which intersects all the waves at right angles,
as has been said.




CHAPTER V

ON THE STRANGE REFRACTION OF ICELAND CRYSTAL


1.

There is brought from Iceland, which is an Island in the North Sea, in
the latitude of 66 degrees, a kind of Crystal or transparent stone,
very remarkable for its figure and other qualities, but above all for
its strange refractions. The causes of this have seemed to me to be
worthy of being carefully investigated, the more so because amongst
transparent bodies this one alone does not follow the ordinary rules
with respect to rays of light. I have even been under some necessity
to make this research, because the refractions of this Crystal seemed
to overturn our preceding explanation of regular refraction; which
explanation, on the contrary, they strongly confirm, as will be seen
after they have been brought under the same principle. In Iceland are
found great lumps of this Crystal, some of which I have seen of 4 or 5
pounds. But it occurs also in other countries, for I have had some of
the same sort which had been found in France near the town of Troyes
in Champagne, and some others which came from the Island of Corsica,
though both were less clear and only in little bits, scarcely capable
of letting any effect of refraction be observed.

2. The first knowledge which the public has had about it is due to Mr.
Erasmus Bartholinus, who has given a description of Iceland Crystal
and of its chief phenomena. But here I shall not desist from giving my
own, both for the instruction of those who may not have seen his book,
and because as respects some of these phenomena there is a slight
difference between his observations and those which I have made: for I
have applied myself with great exactitude to examine these properties
of refraction, in order to be quite sure before undertaking to explain
the causes of them.

3. As regards the hardness of this stone, and the property which it
has of being easily split, it must be considered rather as a species
of Talc than of Crystal. For an iron spike effects an entrance into it
as easily as into any other Talc or Alabaster, to which it is equal in
gravity.

[Illustration]

4. The pieces of it which are found have the figure of an oblique
parallelepiped; each of the six faces being a parallelogram; and it
admits of being split in three directions parallel to two of these
opposed faces. Even in such wise, if you will, that all the six faces
are equal and similar rhombuses. The figure here added represents a
piece of this Crystal. The obtuse angles of all the parallelograms, as
C, D, here, are angles of 101 degrees 52 minutes, and consequently
the acute angles, such as A and B, are of 78 degrees 8 minutes.

5. Of the solid angles there are two opposite to one another, such as
C and E, which are each composed of three equal obtuse plane angles.
The other six are composed of two acute angles and one obtuse. All
that I have just said has been likewise remarked by Mr. Bartholinus in
the aforesaid treatise; if we differ it is only slightly about the
values of the angles. He recounts moreover some other properties of
this Crystal; to wit, that when rubbed against cloth it attracts
straws and other light things as do amber, diamond, glass, and Spanish
wax. Let a piece be covered with water for a day or more, the surface
loses its natural polish. When aquafortis is poured on it it produces
ebullition, especially, as I have found, if the Crystal has been
pulverized. I have also found by experiment that it may be heated to
redness in the fire without being in anywise altered or rendered less
transparent; but a very violent fire calcines it nevertheless. Its
transparency is scarcely less than that of water or of Rock Crystal,
and devoid of colour. But rays of light pass through it in another
fashion and produce those marvellous refractions the causes of which I
am now going to try to explain; reserving for the end of this Treatise
the statement of my conjectures touching the formation and
extraordinary configuration of this Crystal.

6. In all other transparent bodies that we know there is but one sole
and simple refraction; but in this substance there are two different
ones. The effect is that objects seen through it, especially such as
are placed right against it, appear double; and that a ray of
sunlight, falling on one of its surfaces, parts itself into two rays
and traverses the Crystal thus.

7. It is again a general law in all other transparent bodies that the
ray which falls perpendicularly on their surface passes straight on
without suffering refraction, and that an oblique ray is always
refracted. But in this Crystal the perpendicular ray suffers
refraction, and there are oblique rays which pass through it quite
straight.

[Illustration]

8. But in order to explain these phenomena more particularly, let
there be, in the first place, a piece ABFE of the same Crystal, and
let the obtuse angle ACB, one of the three which constitute the
equilateral solid angle C, be divided into two equal parts by the
straight line CG, and let it be conceived that the Crystal is
intersected by a plane which passes through this line and through the
side CF, which plane will necessarily be perpendicular to the surface
AB; and its section in the Crystal will form a parallelogram GCFH. We
will call this section the principal section of the Crystal.

9. Now if one covers the surface AB, leaving there only a small
aperture at the point K, situated in the straight line CG, and if one
exposes it to the sun, so that his rays face it perpendicularly above,
then the ray IK will divide itself at the point K into two, one of
which will continue to go on straight by KL, and the other will
separate itself along the straight line KM, which is in the plane
GCFH, and which makes with KL an angle of about 6 degrees 40 minutes,
tending from the side of the solid angle C; and on emerging from the
other side of the Crystal it will turn again parallel to JK, along MZ.
And as, in this extraordinary refraction, the point M is seen by the
refracted ray MKI, which I consider as going to the eye at I, it
necessarily follows that the point L, by virtue of the same
refraction, will be seen by the refracted ray LRI, so that LR will be
parallel to MK if the distance from the eye KI is supposed very great.
The point L appears then as being in the straight line IRS; but the
same point appears also, by ordinary refraction, to be in the straight
line IK, hence it is necessarily judged to be double. And similarly if
L be a small hole in a sheet of paper or other substance which is laid
against the Crystal, it will appear when turned towards daylight as if
there were two holes, which will seem the wider apart from one another
the greater the thickness of the Crystal.

10. Again, if one turns the Crystal in such wise that an incident ray
NO, of sunlight, which I suppose to be in the plane continued from
GCFH, makes with GC an angle of 73 degrees and 20 minutes, and is
consequently nearly parallel to the edge CF, which makes with FH an
angle of 70 degrees 57 minutes, according to the calculation which I
shall put at the end, it will divide itself at the point O into two
rays, one of which will continue along OP in a straight line with NO,
and will similarly pass out of the other side of the crystal without
any refraction; but the other will be refracted and will go along OQ.
And it must be noted that it is special to the plane through GCF and
to those which are parallel to it, that all incident rays which are in
one of these planes continue to be in it after they have entered the
Crystal and have become double; for it is quite otherwise for rays in
all other planes which intersect the Crystal, as we shall see
afterwards.

11. I recognized at first by these experiments and by some others that
of the two refractions which the ray suffers in this Crystal, there is
one which follows the ordinary rules; and it is this to which the rays
KL and OQ belong. This is why I have distinguished this ordinary
refraction from the other; and having measured it by exact
observation, I found that its proportion, considered as to the Sines
of the angles which the incident and refracted rays make with the
perpendicular, was very precisely that of 5 to 3, as was found also by
Mr. Bartholinus, and consequently much greater than that of Rock
Crystal, or of glass, which is nearly 3 to 2.

[Illustration]

12. The mode of making these observations exactly is as follows. Upon
a leaf of paper fixed on a thoroughly flat table there is traced a
black line AB, and two others, CED and KML, which cut it at right
angles and are more or less distant from one another according as it
is desired to examine a ray that is more or less oblique. Then place
the Crystal upon the intersection E so that the line AB concurs with
that which bisects the obtuse angle of the lower surface, or with some
line parallel to it. Then by placing the eye directly above the line
AB it will appear single only; and one will see that the portion
viewed through the Crystal and the portions which appear outside it,
meet together in a straight line: but the line CD will appear double,
and one can distinguish the image which is due to regular refraction
by the circumstance that when one views it with both eyes it seems
raised up more than the other, or again by the circumstance that, when
the Crystal is turned around on the paper, this image remains
stationary, whereas the other image shifts and moves entirely around.
Afterwards let the eye be placed at I (remaining always in the plane
perpendicular through AB) so that it views the image which is formed
by regular refraction of the line CD making a straight line with the
remainder of that line which is outside the Crystal. And then, marking
on the surface of the Crystal the point H where the intersection E
appears, this point will be directly above E. Then draw back the eye
towards O, keeping always in the plane perpendicular through AB, so
that the image of the line CD, which is formed by ordinary refraction,
may appear in a straight line with the line KL viewed without
refraction; and then mark on the Crystal the point N where the point
of intersection E appears.

13. Then one will know the length and position of the lines NH, EM,
and of HE, which is the thickness of the Crystal: which lines being
traced separately upon a plan, and then joining NE and NM which cuts
HE at P, the proportion of the refraction will be that of EN to NP,
because these lines are to one another as the sines of the angles NPH,
NEP, which are equal to those which the incident ray ON and its
refraction NE make with the perpendicular to the surface. This
proportion, as I have said, is sufficiently precisely as 5 to 3, and
is always the same for all inclinations of the incident ray.

14. The same mode of observation has also served me for examining the
extraordinary or irregular refraction of this Crystal. For, the point
H having been found and marked, as aforesaid, directly above the point
E, I observed the appearance of the line CD, which is made by the
extraordinary refraction; and having placed the eye at Q, so that this
appearance made a straight line with the line KL viewed without
refraction, I ascertained the triangles REH, RES, and consequently the
angles RSH, RES, which the incident and the refracted ray make with
the perpendicular.

15. But I found in this refraction that the ratio of FR to RS was not
constant, like the ordinary refraction, but that it varied with the
varying obliquity of the incident ray.

16. I found also that when QRE made a straight line, that is, when the
incident ray entered the Crystal without being refracted (as I
ascertained by the circumstance that then the point E viewed by the
extraordinary refraction appeared in the line CD, as seen without
refraction) I found, I say, then that the angle QRG was 73 degrees 20
minutes, as has been already remarked; and so it is not the ray
parallel to the edge of the Crystal, which crosses it in a straight
line without being refracted, as Mr. Bartholinus believed, since that
inclination is only 70 degrees 57 minutes, as was stated above. And
this is to be noted, in order that no one may search in vain for the
cause of the singular property of this ray in its parallelism to the
edges mentioned.

[Illustration]

17. Finally, continuing my observations to discover the nature of
this refraction, I learned that it obeyed the following remarkable
rule. Let the parallelogram GCFH, made by the principal section of the
Crystal, as previously determined, be traced separately. I found then
that always, when the inclinations of two rays which come from
opposite sides, as VK, SK here, are equal, their refractions KX and KT
meet the bottom line HF in such wise that points X and T are equally
distant from the point M, where the refraction of the perpendicular
ray IK falls; and this occurs also for refractions in other sections
of this Crystal. But before speaking of those, which have also other
particular properties, we will investigate the causes of the phenomena
which I have already reported.

It was after having explained the refraction of ordinary transparent
bodies by means of the spherical emanations of light, as above, that I
resumed my examination of the nature of this Crystal, wherein I had
previously been unable to discover anything.

18. As there were two different refractions, I conceived that there
were also two different emanations of waves of light, and that one
could occur in the ethereal matter extending through the body of the
Crystal. Which matter, being present in much larger quantity than is
that of the particles which compose it, was alone capable of causing
transparency, according to what has been explained heretofore. I
attributed to this emanation of waves the regular refraction which is
observed in this stone, by supposing these waves to be ordinarily of
spherical form, and having a slower progression within the Crystal
than they have outside it; whence proceeds refraction as I have
demonstrated.

19. As to the other emanation which should produce the irregular
refraction, I wished to try what Elliptical waves, or rather
spheroidal waves, would do; and these I supposed would spread
indifferently both in the ethereal matter diffused throughout the
crystal and in the particles of which it is composed, according to the
last mode in which I have explained transparency. It seemed to me that
the disposition or regular arrangement of these particles could
contribute to form spheroidal waves (nothing more being required for
this than that the successive movement of light should spread a little
more quickly in one direction than in the other) and I scarcely
doubted that there were in this crystal such an arrangement of equal
and similar particles, because of its figure and of its angles with
their determinate and invariable measure. Touching which particles,
and their form and disposition, I shall, at the end of this Treatise,
propound my conjectures and some experiments which confirm them.

20. The double emission of waves of light, which I had imagined,
became more probable to me after I had observed a certain phenomenon
in the ordinary [Rock] Crystal, which occurs in hexagonal form, and
which, because of this regularity, seems also to be composed of
particles, of definite figure, and ranged in order. This was, that
this crystal, as well as that from Iceland, has a double refraction,
though less evident. For having had cut from it some well polished
Prisms of different sections, I remarked in all, in viewing through
them the flame of a candle or the lead of window panes, that
everything appeared double, though with images not very distant from
one another. Whence I understood the reason why this substance, though
so transparent, is useless for Telescopes, when they have ever so
little length.

21. Now this double refraction, according to my Theory hereinbefore
established, seemed to demand a double emission of waves of light,
both of them spherical (for both the refractions are regular) and
those of one series a little slower only than the others. For thus the
phenomenon is quite naturally explained, by postulating substances
which serve as vehicle for these waves, as I have done in the case of
Iceland Crystal. I had then less trouble after that in admitting two
emissions of waves in one and the same body. And since it might have
been objected that in composing these two kinds of crystal of equal
particles of a certain figure, regularly piled, the interstices which
these particles leave and which contain the ethereal matter would
scarcely suffice to transmit the waves of light which I have localized
there, I removed this difficulty by regarding these particles as being
of a very rare texture, or rather as composed of other much smaller
particles, between which the ethereal matter passes quite freely.
This, moreover, necessarily follows from that which has been already
demonstrated touching the small quantity of matter of which the bodies
are built up.

22. Supposing then these spheroidal waves besides the spherical ones,
I began to examine whether they could serve to explain the phenomena
of the irregular refraction, and how by these same phenomena I could
determine the figure and position of the spheroids: as to which I
obtained at last the desired success, by proceeding as follows.

[Illustration]

23. I considered first the effect of waves so formed, as respects the
ray which falls perpendicularly on the flat surface of a transparent
body in which they should spread in this manner. I took AB for the
exposed region of the surface. And, since a ray perpendicular to a
plane, and coming from a very distant source of light, is nothing
else, according to the precedent Theory, than the incidence of a
portion of the wave parallel to that plane, I supposed the straight
line RC, parallel and equal to AB, to be a portion of a wave of light,
in which an infinitude of points such as RH_h_C come to meet the
surface AB at the points AK_k_B. Then instead of the hemispherical
partial waves which in a body of ordinary refraction would spread from
each of these last points, as we have above explained in treating of
refraction, these must here be hemi-spheroids. The axes (or rather the
major diameters) of these I supposed to be oblique to the plane AB, as
is AV the semi-axis or semi-major diameter of the spheroid SVT, which
represents the partial wave coming from the point A, after the wave RC
has reached AB. I say axis or major diameter, because the same ellipse
SVT may be considered as the section of a spheroid of which the axis
is AZ perpendicular to AV. But, for the present, without yet deciding
one or other, we will consider these spheroids only in those sections
of them which make ellipses in the plane of this figure. Now taking a
certain space of time during which the wave SVT has spread from A, it
would needs be that from all the other points K_k_B there should
proceed, in the same time, waves similar to SVT and similarly
situated. And the common tangent NQ of all these semi-ellipses would
be the propagation of the wave RC which fell on AB, and would be the
place where this movement occurs in much greater amount than anywhere
else, being made up of arcs of an infinity of ellipses, the centres of
which are along the line AB.

24. Now it appeared that this common tangent NQ was parallel to AB,
and of the same length, but that it was not directly opposite to it,
since it was comprised between the lines AN, BQ, which are diameters
of ellipses having A and B for centres, conjugate with respect to
diameters which are not in the straight line AB. And in this way I
comprehended, a matter which had seemed to me very difficult, how a
ray perpendicular to a surface could suffer refraction on entering a
transparent body; seeing that the wave RC, having come to the aperture
AB, went on forward thence, spreading between the parallel lines AN,
BQ, yet itself remaining always parallel to AB, so that here the light
does not spread along lines perpendicular to its waves, as in ordinary
refraction, but along lines cutting the waves obliquely.

[Illustration]

25. Inquiring subsequently what might be the position and form of
these spheroids in the crystal, I considered that all the six faces
produced precisely the same refractions. Taking, then, the
parallelopiped AFB, of which the obtuse solid angle C is contained
between the three equal plane angles, and imagining in it the three
principal sections, one of which is perpendicular to the face DC and
passes through the edge CF, another perpendicular to the face BF
passing through the edge CA, and the third perpendicular to the face
AF passing through the edge BC; I knew that the refractions of the
incident rays belonging to these three planes were all similar. But
there could be no position of the spheroid which would have the same
relation to these three sections except that in which the axis was
also the axis of the solid angle C. Consequently I saw that the axis
of this angle, that is to say the straight line which traversed the
crystal from the point C with equal inclination to the edges CF, CA,
CB was the line which determined the position of the axis of all the
spheroidal waves which one imagined to originate from some point,
taken within or on the surface of the crystal, since all these
spheroids ought to be alike, and have their axes parallel to one
another.

26. Considering after this the plane of one of these three sections,
namely that through GCF, the angle of which is 109 degrees 3 minutes,
since the angle F was shown above to be 70 degrees 57 minutes; and,
imagining a spheroidal wave about the centre C, I knew, because I have
just explained it, that its axis must be in the same plane, the half
of which axis I have marked CS in the next figure: and seeking by
calculation (which will be given with others at the end of this
discourse) the value of the angle CGS, I found it 45 degrees 20
minutes.

[Illustration]

27. To know from this the form of this spheroid, that is to say the
proportion of the semi-diameters CS, CP, of its elliptical section,
which are perpendicular to one another, I considered that the point M
where the ellipse is touched by the straight line FH, parallel to CG,
ought to be so situated that CM makes with the perpendicular CL an
angle of 6 degrees 40 minutes; since, this being so, this ellipse
satisfies what has been said about the refraction of the ray
perpendicular to the surface CG, which is inclined to the
perpendicular CL by the same angle. This, then, being thus disposed,
and taking CM at 100,000 parts, I found by the calculation which will
be given at the end, the semi-major diameter CP to be 105,032, and the
semi-axis CS to be 93,410, the ratio of which numbers is very nearly 9
to 8; so that the spheroid was of the kind which resembles a
compressed sphere, being generated by the revolution of an ellipse
about its smaller diameter. I found also the value of CG the
semi-diameter parallel to the tangent ML to be 98,779.

[Illustration]

28. Now passing to the investigation of the refractions which
obliquely incident rays must undergo, according to our hypothesis of
spheroidal waves, I saw that these refractions depended on the ratio
between the velocity of movement of the light outside the crystal in
the ether, and that within the crystal. For supposing, for example,
this proportion to be such that while the light in the crystal forms
the spheroid GSP, as I have just said, it forms outside a sphere the
semi-diameter of which is equal to the line N which will be determined
hereafter, the following is the way of finding the refraction of the
incident rays. Let there be such a ray RC falling upon the surface
CK. Make CO perpendicular to RC, and across the angle KCO adjust OK,
equal to N and perpendicular to CO; then draw KI, which touches the
Ellipse GSP, and from the point of contact I join IC, which will be
the required refraction of the ray RC. The demonstration of this is,
it will be seen, entirely similar to that of which we made use in
explaining ordinary refraction. For the refraction of the ray RC is
nothing else than the progression of the portion C of the wave CO,
continued in the crystal. Now the portions H of this wave, during the
time that O came to K, will have arrived at the surface CK along the
straight lines H_x_, and will moreover have produced in the crystal
around the centres _x_ some hemi-spheroidal partial waves similar to
the hemi-spheroidal GSP_g_, and similarly disposed, and of which the
major and minor diameters will bear the same proportions to the lines
_xv_ (the continuations of the lines H_x_ up to KB parallel to CO)
that the diameters of the spheroid GSP_g_ bear to the line CB, or N.
And it is quite easy to see that the common tangent of all these
spheroids, which are here represented by Ellipses, will be the
straight line IK, which consequently will be the propagation of the
wave CO; and the point I will be that of the point C, conformably with
that which has been demonstrated in ordinary refraction.

Now as to finding the point of contact I, it is known that one must
find CD a third proportional to the lines CK, CG, and draw DI parallel
to CM, previously determined, which is the conjugate diameter to CG;
for then, by drawing KI it touches the Ellipse at I.

29. Now as we have found CI the refraction of the ray RC, similarly
one will find C_i_ the refraction of the ray _r_C, which comes from
the opposite side, by making C_o_ perpendicular to _r_C and following
out the rest of the construction as before. Whence one sees that if
the ray _r_C is inclined equally with RC, the line C_d_ will
necessarily be equal to CD, because C_k_ is equal to CK, and C_g_ to
CG. And in consequence I_i_ will be cut at E into equal parts by the
line CM, to which DI and _di_ are parallel. And because CM is the
conjugate diameter to CG, it follows that _i_I will be parallel to
_g_G. Therefore if one prolongs the refracted rays CI, C_i_, until
they meet the tangent ML at T and _t_, the distances MT, M_t_, will
also be equal. And so, by our hypothesis, we explain perfectly the
phenomenon mentioned above; to wit, that when there are two rays
equally inclined, but coming from opposite sides, as here the rays RC,
_rc_, their refractions diverge equally from the line followed by the
refraction of the ray perpendicular to the surface, by considering
these divergences in the direction parallel to the surface of the
crystal.

30. To find the length of the line N, in proportion to CP, CS, CG, it
must be determined by observations of the irregular refraction which
occurs in this section of the crystal; and I find thus that the ratio
of N to GC is just a little less than 8 to 5. And having regard to
some other observations and phenomena of which I shall speak
afterwards, I put N at 156,962 parts, of which the semi-diameter CG is
found to contain 98,779, making this ratio 8 to 5-1/29. Now this
proportion, which there is between the line N and CG, may be called
the Proportion of the Refraction; similarly as in glass that of 3 to
2, as will be manifest when I shall have explained a short process in
the preceding way to find the irregular refractions.

31. Supposing then, in the next figure, as previously, the surface of
the crystal _g_G, the Ellipse GP_g_, and the line N; and CM the
refraction of the perpendicular ray FC, from which it diverges by 6
degrees 40 minutes. Now let there be some other ray RC, the refraction
of which must be found.

About the centre C, with semi-diameter CG, let the circumference _g_RG
be described, cutting the ray RC at R; and let RV be the perpendicular
on CG. Then as the line N is to CG let CV be to CD, and let DI be
drawn parallel to CM, cutting the Ellipse _g_MG at I; then joining CI,
this will be the required refraction of the ray RC. Which is
demonstrated thus.

[Illustration]

Let CO be perpendicular to CR, and across the angle OCG let OK be
adjusted, equal to N and perpendicular to CO, and let there be drawn
the straight line KI, which if it is demonstrated to be a tangent to
the Ellipse at I, it will be evident by the things heretofore
explained that CI is the refraction of the ray RC. Now since the angle
RCO is a right angle, it is easy to see that the right-angled
triangles RCV, KCO, are similar. As then, CK is to KO, so also is RC
to CV. But KO is equal to N, and RC to CG: then as CK is to N so will
CG be to CV. But as N is to CG, so, by construction, is CV to CD. Then
as CK is to CG so is CG to CD. And because DI is parallel to CM, the
conjugate diameter to CG, it follows that KI touches the Ellipse at I;
which remained to be shown.

32. One sees then that as there is in the refraction of ordinary
media a certain constant proportion between the sines of the angles
which the incident ray and the refracted ray make with the
perpendicular, so here there is such a proportion between CV and CD or
IE; that is to say between the Sine of the angle which the incident
ray makes with the perpendicular, and the horizontal intercept, in the
Ellipse, between the refraction of this ray and the diameter CM. For
the ratio of CV to CD is, as has been said, the same as that of N to
the semi-diameter CG.

33. I will add here, before passing away, that in comparing together
the regular and irregular refraction of this crystal, there is this
remarkable fact, that if ABPS be the spheroid by which light spreads
in the Crystal in a certain space of time (which spreading, as has
been said, serves for the irregular refraction), then the inscribed
sphere BVST is the extension in the same space of time of the light
which serves for the regular refraction.

[Illustration]

For we have stated before this, that the line N being the radius of a
spherical wave of light in air, while in the crystal it spread through
the spheroid ABPS, the ratio of N to CS will be 156,962 to 93,410. But
it has also been stated that the proportion of the regular refraction
was 5 to 3; that is to say, that N being the radius of a spherical
wave of light in air, its extension in the crystal would, in the same
space of time, form a sphere the radius of which would be to N as 3 to
5. Now 156,962 is to 93,410 as 5 to 3 less 1/41. So that it is
sufficiently nearly, and may be exactly, the sphere BVST, which the
light describes for the regular refraction in the crystal, while it
describes the spheroid BPSA for the irregular refraction, and while it
describes the sphere of radius N in air outside the crystal.

Although then there are, according to what we have supposed, two
different propagations of light within the crystal, it appears that it
is only in directions perpendicular to the axis BS of the spheroid
that one of these propagations occurs more rapidly than the other; but
that they have an equal velocity in the other direction, namely, in
that parallel to the same axis BS, which is also the axis of the
obtuse angle of the crystal.

[Illustration]

34. The proportion of the refraction being what we have just seen, I
will now show that there necessarily follows thence that notable
property of the ray which falling obliquely on the surface of the
crystal enters it without suffering refraction. For supposing the same
things as before, and that the ray makes with the same surface _g_G
the angle RCG of 73 degrees 20 minutes, inclining to the same side as
the crystal (of which ray mention has been made above); if one
investigates, by the process above explained, the refraction CI, one
will find that it makes exactly a straight line with RC, and that thus
this ray is not deviated at all, conformably with experiment. This is
proved as follows by calculation.

CG or CR being, as precedently, 98,779; CM being 100,000; and the
angle RCV 73 degrees 20 minutes, CV will be 28,330. But because CI is
the refraction of the ray RC, the proportion of CV to CD is 156,962 to
98,779, namely, that of N to CG; then CD is 17,828.

Now the rectangle _g_DC is to the square of DI as the square of CG is
to the square of CM; hence DI or CE will be 98,353. But as CE is to
EI, so will CM be to MT, which will then be 18,127. And being added to
ML, which is 11,609 (namely the sine of the angle LCM, which is 6
degrees 40 minutes, taking CM 100,000 as radius) we get LT 27,936; and
this is to LC 99,324 as CV to VR, that is to say, as 29,938, the
tangent of the complement of the angle RCV, which is 73 degrees 20
minutes, is to the radius of the Tables. Whence it appears that RCIT
is a straight line; which was to be proved.

35. Further it will be seen that the ray CI in emerging through the
opposite surface of the crystal, ought to pass out quite straight,
according to the following demonstration, which proves that the
reciprocal relation of refraction obtains in this crystal the same as
in other transparent bodies; that is to say, that if a ray RC in
meeting the surface of the crystal CG is refracted as CI, the ray CI
emerging through the opposite parallel surface of the crystal, which
I suppose to be IB, will have its refraction IA parallel to the ray
RC.

[Illustration]

Let the same things be supposed as before; that is to say, let CO,
perpendicular to CR, represent a portion of a wave the continuation of
which in the crystal is IK, so that the piece C will be continued on
along the straight line CI, while O comes to K. Now if one takes a
second period of time equal to the first, the piece K of the wave IK
will, in this second period, have advanced along the straight line KB,
equal and parallel to CI, because every piece of the wave CO, on
arriving at the surface CK, ought to go on in the crystal the same as
the piece C; and in this same time there will be formed in the air
from the point I a partial spherical wave having a semi-diameter IA
equal to KO, since KO has been traversed in an equal time. Similarly,
if one considers some other point of the wave IK, such as _h_, it will
go along _hm_, parallel to CI, to meet the surface IB, while the point
K traverses K_l_ equal to _hm_; and while this accomplishes the
remainder _l_B, there will start from the point _m_ a partial wave the
semi-diameter of which, _mn_, will have the same ratio to _l_B as IA
to KB. Whence it is evident that this wave of semi-diameter _mn_, and
the other of semi-diameter IA will have the same tangent BA. And
similarly for all the partial spherical waves which will be formed
outside the crystal by the impact of all the points of the wave IK
against the surface of the Ether IB. It is then precisely the tangent
BA which will be the continuation of the wave IK, outside the crystal,
when the piece K has reached B. And in consequence IA, which is
perpendicular to BA, will be the refraction of the ray CI on emerging
from the crystal. Now it is clear that IA is parallel to the incident
ray RC, since IB is equal to CK, and IA equal to KO, and the angles A
and O are right angles.

It is seen then that, according to our hypothesis, the reciprocal
relation of refraction holds good in this crystal as well as in
ordinary transparent bodies; as is thus in fact found by observation.

36. I pass now to the consideration of other sections of the crystal,
and of the refractions there produced, on which, as will be seen, some
other very remarkable phenomena depend.

Let ABH be a parallelepiped of crystal, and let the top surface AEHF
be a perfect rhombus, the obtuse angles of which are equally divided
by the straight line EF, and the acute angles by the straight line AH
perpendicular to FE.

The section which we have hitherto considered is that which passes
through the lines EF, EB, and which at the same time cuts the plane
AEHF at right angles. Refractions in this section have this in common
with the refractions in ordinary media that the plane which is drawn
through the incident ray and which also intersects the surface of the
crystal at right angles, is that in which the refracted ray also is
found. But the refractions which appertain to every other section of
this crystal have this strange property that the refracted ray always
quits the plane of the incident ray perpendicular to the surface, and
turns away towards the side of the slope of the crystal. For which
fact we shall show the reason, in the first place, for the section
through AH; and we shall show at the same time how one can determine
the refraction, according to our hypothesis. Let there be, then, in
the plane which passes through AH, and which is perpendicular to the
plane AFHE, the incident ray RC; it is required to find its refraction
in the crystal.

[Illustration]

37. About the centre C, which I suppose to be in the intersection of
AH and FE, let there be imagined a hemi-spheroid QG_qg_M, such as the
light would form in spreading in the crystal, and let its section by
the plane AEHF form the Ellipse QG_qg_, the major diameter of which
Q_q_, which is in the line AH, will necessarily be one of the major
diameters of the spheroid; because the axis of the spheroid being in
the plane through FEB, to which QC is perpendicular, it follows that
QC is also perpendicular to the axis of the spheroid, and consequently
QC_q_ one of its major diameters. But the minor diameter of this
Ellipse, G_g_, will bear to Q_q_ the proportion which has been defined
previously, Article 27, between CG and the major semi-diameter of the
spheroid, CP, namely, that of 98,779 to 105,032.

Let the line N be the length of the travel of light in air during the
time in which, within the crystal, it makes, from the centre C, the
spheroid QC_qg_M. Then having drawn CO perpendicular to the ray CR and
situate in the plane through CR and AH, let there be adjusted, across
the angle ACO, the straight line OK equal to N and perpendicular to
CO, and let it meet the straight line AH at K. Supposing consequently
that CL is perpendicular to the surface of the crystal AEHF, and that
CM is the refraction of the ray which falls perpendicularly on this
same surface, let there be drawn a plane through the line CM and
through KCH, making in the spheroid the semi-ellipse QM_q_, which will
be given, since the angle MCL is given of value 6 degrees 40 minutes.
And it is certain, according to what has been explained above, Article
27, that a plane which would touch the spheroid at the point M, where
I suppose the straight line CM to meet the surface, would be parallel
to the plane QG_q_. If then through the point K one now draws KS
parallel to G_g_, which will be parallel also to QX, the tangent to
the Ellipse QG_q_ at Q; and if one conceives a plane passing through
KS and touching the spheroid, the point of contact will necessarily be
in the Ellipse QM_q_, because this plane through KS, as well as the
plane which touches the spheroid at the point M, are parallel to QX,
the tangent of the spheroid: for this consequence will be demonstrated
at the end of this Treatise. Let this point of contact be at I, then
making KC, QC, DC proportionals, draw DI parallel to CM; also join CI.
I say that CI will be the required refraction of the ray RC. This will
be manifest if, in considering CO, which is perpendicular to the ray
RC, as a portion of the wave of light, we can demonstrate that the
continuation of its piece C will be found in the crystal at I, when O
has arrived at K.

38. Now as in the Chapter on Reflexion, in demonstrating that the
incident and reflected rays are always in the same plane perpendicular
to the reflecting surface, we considered the breadth of the wave of
light, so, similarly, we must here consider the breadth of the wave CO
in the diameter G_g_. Taking then the breadth C_c_ on the side toward
the angle E, let the parallelogram CO_oc_ be taken as a portion of a
wave, and let us complete the parallelograms CK_kc_, CI_ic_, Kl_ik_,
OK_ko_. In the time then that the line O_o_ arrives at the surface of
the crystal at K_k_, all the points of the wave CO_oc_ will have
arrived at the rectangle K_c_ along lines parallel to OK; and from the
points of their incidences there will originate, beyond that, in the
crystal partial hemi-spheroids, similar to the hemi-spheroid QM_q_,
and similarly disposed. These hemi-spheroids will necessarily all
touch the plane of the parallelogram KI_ik_ at the same instant that
O_o_ has reached K_k_. Which is easy to comprehend, since, of these
hemi-spheroids, all those which have their centres along the line CK,
touch this plane in the line KI (for this is to be shown in the same
way as we have demonstrated the refraction of the oblique ray in the
principal section through EF) and all those which have their centres
in the line C_c_ will touch the same plane KI in the line I_i_; all
these being similar to the hemi-spheroid QM_q_. Since then the
parallelogram K_i_ is that which touches all these spheroids, this
same parallelogram will be precisely the continuation of the wave
CO_oc_ in the crystal, when O_o_ has arrived at K_k_, because it forms
the termination of the movement and because of the quantity of
movement which occurs more there than anywhere else: and thus it
appears that the piece C of the wave CO_oc_ has its continuation at I;
that is to say, that the ray RC is refracted as CI.

From this it is to be noted that the proportion of the refraction for
this section of the crystal is that of the line N to the semi-diameter
CQ; by which one will easily find the refractions of all incident
rays, in the same way as we have shown previously for the case of the
section through FE; and the demonstration will be the same. But it
appears that the said proportion of the refraction is less here than
in the section through FEB; for it was there the same as the ratio of
N to CG, that is to say, as 156,962 to 98,779, very nearly as 8 to 5;
and here it is the ratio of N to CQ the major semi-diameter of the
spheroid, that is to say, as 156,962 to 105,032, very nearly as 3 to
2, but just a little less. Which still agrees perfectly with what one
finds by observation.

39. For the rest, this diversity of proportion of refraction produces
a very singular effect in this Crystal; which is that when it is
placed upon a sheet of paper on which there are letters or anything
else marked, if one views it from above with the two eyes situated in
the plane of the section through EF, one sees the letters raised up by
this irregular refraction more than when one puts one's eyes in the
plane of section through AH: and the difference of these elevations
appears by comparison with the other ordinary refraction of the
crystal, the proportion of which is as 5 to 3, and which always raises
the letters equally, and higher than the irregular refraction does.
For one sees the letters and the paper on which they are written, as
on two different stages at the same time; and in the first position of
the eyes, namely, when they are in the plane through AH these two
stages are four times more distant from one another than when the eyes
are in the plane through EF.

We will show that this effect follows from the refractions; and it
will enable us at the same time to ascertain the apparent place of a
point of an object placed immediately under the crystal, according to
the different situation of the eyes.

40. Let us see first by how much the irregular refraction of the plane
through AH ought to lift the bottom of the crystal. Let the plane of
this figure represent separately the section through Q_q_ and CL, in
which section there is also the ray RC, and let the semi-elliptic
plane through Q_q_ and CM be inclined to the former, as previously, by
an angle of 6 degrees 40 minutes; and in this plane CI is then the
refraction of the ray RC.

[Illustration]

If now one considers the point I as at the bottom of the crystal, and
that it is viewed by the rays ICR, _Icr_, refracted equally at the
points C_c_, which should be equally distant from D, and that these
rays meet the two eyes at R_r_; it is certain that the point I will
appear raised to S where the straight lines RC, _rc_, meet; which
point S is in DP, perpendicular to Q_q_. And if upon DP there is drawn
the perpendicular IP, which will lie at the bottom of the crystal, the
length SP will be the apparent elevation of the point I above the
bottom.

Let there be described on Q_q_ a semicircle cutting the ray CR at B,
from which BV is drawn perpendicular to Q_q_; and let the proportion
of the refraction for this section be, as before, that of the line N
to the semi-diameter CQ.

Then as N is to CQ so is VC to CD, as appears by the method of finding
the refraction which we have shown above, Article 31; but as VC is to
CD, so is VB to DS. Then as N is to CQ, so is VB to DS. Let ML be
perpendicular to CL. And because I suppose the eyes R_r_ to be distant
about a foot or so from the crystal, and consequently the angle RS_r_
very small, VB may be considered as equal to the semi-diameter CQ, and
DP as equal to CL; then as N is to CQ so is CQ to DS. But N is valued
at 156,962 parts, of which CM contains 100,000 and CQ 105,032. Then DS
will have 70,283. But CL is 99,324, being the sine of the complement
of the angle MCL which is 6 degrees 40 minutes; CM being supposed as
radius. Then DP, considered as equal to CL, will be to DS as 99,324 to
70,283. And so the elevation of the point I by the refraction of this
section is known.

[Illustration]

41. Now let there be represented the other section through EF in the
figure before the preceding one; and let CM_g_ be the semi-ellipse,
considered in Articles 27 and 28, which is made by cutting a
spheroidal wave having centre C. Let the point I, taken in this
ellipse, be imagined again at the bottom of the Crystal; and let it be
viewed by the refracted rays ICR, I_cr_, which go to the two eyes; CR
and _cr_ being equally inclined to the surface of the crystal G_g_.
This being so, if one draws ID parallel to CM, which I suppose to be
the refraction of the perpendicular ray incident at the point C, the
distances DC, D_c_, will be equal, as is easy to see by that which has
been demonstrated in Article 28. Now it is certain that the point I
should appear at S where the straight lines RC, _rc_, meet when
prolonged; and that this point will fall in the line DP perpendicular
to G_g_. If one draws IP perpendicular to this DP, it will be the
distance PS which will mark the apparent elevation of the point I. Let
there be described on G_g_ a semicircle cutting CR at B, from which
let BV be drawn perpendicular to G_g_; and let N to GC be the
proportion of the refraction in this section, as in Article 28. Since
then CI is the refraction of the radius BC, and DI is parallel to CM,
VC must be to CD as N to GC, according to what has been demonstrated
in Article 31. But as VC is to CD so is BV to DS. Let ML be drawn
perpendicular to CL. And because I consider, again, the eyes to be
distant above the crystal, BV is deemed equal to the semi-diameter CG;
and hence DS will be a third proportional to the lines N and CG: also
DP will be deemed equal to CL. Now CG consisting of 98,778 parts, of
which CM contains 100,000, N is taken as 156,962. Then DS will be
62,163. But CL is also determined, and contains 99,324 parts, as has
been said in Articles 34 and 40. Then the ratio of PD to DS will be as
99,324 to 62,163. And thus one knows the elevation of the point at the
bottom I by the refraction of this section; and it appears that this
elevation is greater than that by the refraction of the preceding
section, since the ratio of PD to DS was there as 99,324 to 70,283.

[Illustration]

But by the regular refraction of the crystal, of which we have above
said that the proportion is 5 to 3, the elevation of the point I, or
P, from the bottom, will be 2/5 of the height DP; as appears by this
figure, where the point P being viewed by the rays PCR, P_cr_,
refracted equally at the surface C_c_, this point must needs appear
to be at S, in the perpendicular PD where the lines RC, _rc_, meet
when prolonged: and one knows that the line PC is to CS as 5 to 3,
since they are to one another as the sine of the angle CSP or DSC is
to the sine of the angle SPC. And because the ratio of PD to DS is
deemed the same as that of PC to CS, the two eyes Rr being supposed
very far above the crystal, the elevation PS will thus be 2/5 of PD.

[Illustration]

42. If one takes a straight line AB for the thickness of the crystal,
its point B being at the bottom, and if one divides it at the points
C, D, E, according to the proportions of the elevations found, making
AE 3/5 of AB, AB to AC as 99,324 to 70,283, and AB to AD as 99,324 to
62,163, these points will divide AB as in this figure. And it will be
found that this agrees perfectly with experiment; that is to say by
placing the eyes above in the plane which cuts the crystal according
to the shorter diameter of the rhombus, the regular refraction will
lift up the letters to E; and one will see the bottom, and the letters
over which it is placed, lifted up to D by the irregular refraction.
But by placing the eyes above in the plane which cuts the crystal
according to the longer diameter of the rhombus, the regular
refraction will lift the letters to E as before; but the irregular
refraction will make them, at the same time, appear lifted up only to
C; and in such a way that the interval CE will be quadruple the
interval ED, which one previously saw.


43. I have only to make the remark here that in both the positions of
the eyes the images caused by the irregular refraction do not appear
directly below those which proceed from the regular refraction, but
they are separated from them by being more distant from the
equilateral solid angle of the Crystal. That follows, indeed, from all
that has been hitherto demonstrated about the irregular refraction;
and it is particularly shown by these last demonstrations, from which
one sees that the point I appears by irregular refraction at S in the
perpendicular line DP, in which line also the image of the point P
ought to appear by regular refraction, but not the image of the point
I, which will be almost directly above the same point, and higher than
S.

But as to the apparent elevation of the point I in other positions of
the eyes above the crystal, besides the two positions which we have
just examined, the image of that point by the irregular refraction
will always appear between the two heights of D and C, passing from
one to the other as one turns one's self around about the immovable
crystal, while looking down from above. And all this is still found
conformable to our hypothesis, as any one can assure himself after I
shall have shown here the way of finding the irregular refractions
which appear in all other sections of the crystal, besides the two
which we have considered. Let us suppose one of the faces of the
crystal, in which let there be the Ellipse HDE, the centre C of which
is also the centre of the spheroid HME in which the light spreads, and
of which the said Ellipse is the section. And let the incident ray be
RC, the refraction of which it is required to find.

Let there be taken a plane passing through the ray RC and which is
perpendicular to the plane of the ellipse HDE, cutting it along the
straight line BCK; and having in the same plane through RC made CO
perpendicular to CR, let OK be adjusted across the angle OCK, so as
to be perpendicular to OC and equal to the line N, which I suppose to
measure the travel of the light in air during the time that it spreads
in the crystal through the spheroid HDEM. Then in the plane of the
Ellipse HDE let KT be drawn, through the point K, perpendicular to
BCK. Now if one conceives a plane drawn through the straight line KT
and touching the spheroid HME at I, the straight line CI will be the
refraction of the ray RC, as is easy to deduce from that which has
been demonstrated in Article 36.

[Illustration]

But it must be shown how one can determine the point of contact I. Let
there be drawn parallel to the line KT a line HF which touches the
Ellipse HDE, and let this point of contact be at H. And having drawn a
straight line along CH to meet KT at T, let there be imagined a plane
passing through the same CH and through CM (which I suppose to be the
refraction of the perpendicular ray), which makes in the spheroid the
elliptical section HME. It is certain that the plane which will pass
through the straight line KT, and which will touch the spheroid, will
touch it at a point in the Ellipse HME, according to the Lemma which
will be demonstrated at the end of the Chapter. Now this point is
necessarily the point I which is sought, since the plane drawn through
TK can touch the spheroid at one point only. And this point I is easy
to determine, since it is needful only to draw from the point T, which
is in the plane of this Ellipse, the tangent TI, in the way shown
previously. For the Ellipse HME is given, and its conjugate
semi-diameters are CH and CM; because a straight line drawn through M,
parallel to HE, touches the Ellipse HME, as follows from the fact that
a plane taken through M, and parallel to the plane HDE, touches the
spheroid at that point M, as is seen from Articles 27 and 23. For the
rest, the position of this ellipse, with respect to the plane through
the ray RC and through CK, is also given; from which it will be easy
to find the position of CI, the refraction corresponding to the ray
RC.

Now it must be noted that the same ellipse HME serves to find the
refractions of any other ray which may be in the plane through RC and
CK. Because every plane, parallel to the straight line HF, or TK,
which will touch the spheroid, will touch it in this ellipse,
according to the Lemma quoted a little before.

I have investigated thus, in minute detail, the properties of the
irregular refraction of this Crystal, in order to see whether each
phenomenon that is deduced from our hypothesis accords with that which
is observed in fact. And this being so it affords no slight proof of
the truth of our suppositions and principles. But what I am going to
add here confirms them again marvellously. It is this: that there are
different sections of this Crystal, the surfaces of which, thereby
produced, give rise to refractions precisely such as they ought to be,
and as I had foreseen them, according to the preceding Theory.

In order to explain what these sections are, let ABKF _be_ the
principal section through the axis of the crystal ACK, in which there
will also be the axis SS of a spheroidal wave of light spreading in
the crystal from the centre C; and the straight line which cuts SS
through the middle and at right angles, namely PP, will be one of the
major diameters.

[Illustration: {Section ABKF}]

Now as in the natural section of the crystal, made by a plane parallel
to two opposite faces, which plane is here represented by the line GG,
the refraction of the surfaces which are produced by it will be
governed by the hemi-spheroids GNG, according to what has been
explained in the preceding Theory. Similarly, cutting the Crystal
through NN, by a plane perpendicular to the parallelogram ABKF, the
refraction of the surfaces will be governed by the hemi-spheroids NGN.
And if one cuts it through PP, perpendicularly to the said
parallelogram, the refraction of the surfaces ought to be governed by
the hemi-spheroids PSP, and so for others. But I saw that if the plane
NN was almost perpendicular to the plane GG, making the angle NCG,
which is on the side A, an angle of 90 degrees 40 minutes, the
hemi-spheroids NGN would become similar to the hemi-spheroids GNG,
since the planes NN and GG were equally inclined by an angle of 45
degrees 20 minutes to the axis SS. In consequence it must needs be, if
our theory is true, that the surfaces which the section through NN
produces should effect the same refractions as the surfaces of the
section through GG. And not only the surfaces of the section NN but
all other sections produced by planes which might be inclined to the
axis at an angle equal to 45 degrees 20 minutes. So that there are an
infinitude of planes which ought to produce precisely the same
refractions as the natural surfaces of the crystal, or as the section
parallel to any one of those surfaces which are made by cleavage.

I saw also that by cutting it by a plane taken through PP, and
perpendicular to the axis SS, the refraction of the surfaces ought to
be such that the perpendicular ray should suffer thereby no deviation;
and that for oblique rays there would always be an irregular
refraction, differing from the regular, and by which objects placed
beneath the crystal would be less elevated than by that other
refraction.

That, similarly, by cutting the crystal by any plane through the axis
SS, such as the plane of the figure is, the perpendicular ray ought to
suffer no refraction; and that for oblique rays there were different
measures for the irregular refraction according to the situation of
the plane in which the incident ray was.

Now these things were found in fact so; and, after that, I could not
doubt that a similar success could be met with everywhere. Whence I
concluded that one might form from this crystal solids similar to
those which are its natural forms, which should produce, at all their
surfaces, the same regular and irregular refractions as the natural
surfaces, and which nevertheless would cleave in quite other ways, and
not in directions parallel to any of their faces. That out of it one
would be able to fashion pyramids, having their base square,
pentagonal, hexagonal, or with as many sides as one desired, all the
surfaces of which should have the same refractions as the natural
surfaces of the crystal, except the base, which will not refract the
perpendicular ray. These surfaces will each make an angle of 45
degrees 20 minutes with the axis of the crystal, and the base will be
the section perpendicular to the axis.

That, finally, one could also fashion out of it triangular prisms, or
prisms with as many sides as one would, of which neither the sides nor
the bases would refract the perpendicular ray, although they would yet
all cause double refraction for oblique rays. The cube is included
amongst these prisms, the bases of which are sections perpendicular to
the axis of the crystal, and the sides are sections parallel to the
same axis.

From all this it further appears that it is not at all in the
disposition of the layers of which this crystal seems to be composed,
and according to which it splits in three different senses, that the
cause resides of its irregular refraction; and that it would be in
vain to wish to seek it there.

But in order that any one who has some of this stone may be able to
find, by his own experience, the truth of what I have just advanced, I
will state here the process of which I have made use to cut it, and to
polish it. Cutting is easy by the slicing wheels of lapidaries, or in
the way in which marble is sawn: but polishing is very difficult, and
by employing the ordinary means one more often depolishes the surfaces
than makes them lucent.

After many trials, I have at last found that for this service no plate
of metal must be used, but a piece of mirror glass made matt and
depolished. Upon this, with fine sand and water, one smoothes the
crystal little by little, in the same way as spectacle glasses, and
polishes it simply by continuing the work, but ever reducing the
material. I have not, however, been able to give it perfect clarity
and transparency; but the evenness which the surfaces acquire enables
one to observe in them the effects of refraction better than in those
made by cleaving the stone, which always have some inequality.

Even when the surface is only moderately smoothed, if one rubs it over
with a little oil or white of egg, it becomes quite transparent, so
that the refraction is discerned in it quite distinctly. And this aid
is specially necessary when it is wished to polish the natural
surfaces to remove the inequalities; because one cannot render them
lucent equally with the surfaces of other sections, which take a
polish so much the better the less nearly they approximate to these
natural planes.

Before finishing the treatise on this Crystal, I will add one more
marvellous phenomenon which I discovered after having written all the
foregoing. For though I have not been able till now to find its cause,
I do not for that reason wish to desist from describing it, in order
to give opportunity to others to investigate it. It seems that it will
be necessary to make still further suppositions besides those which I
have made; but these will not for all that cease to keep their
probability after having been confirmed by so many tests.

[Illustration]

The phenomenon is, that by taking two pieces of this crystal and
applying them one over the other, or rather holding them with a space
between the two, if all the sides of one are parallel to those of the
other, then a ray of light, such as AB, is divided into two in the
first piece, namely into BD and BC, following the two refractions,
regular and irregular. On penetrating thence into the other piece
each ray will pass there without further dividing itself in two; but
that one which underwent the regular refraction, as here DG, will
undergo again only a regular refraction at GH; and the other, CE, an
irregular refraction at EF. And the same thing occurs not only in this
disposition, but also in all those cases in which the principal
section of each of the pieces is situated in one and the same plane,
without it being needful for the two neighbouring surfaces to be
parallel. Now it is marvellous why the rays CE and DG, incident from
the air on the lower crystal, do not divide themselves the same as the
first ray AB. One would say that it must be that the ray DG in passing
through the upper piece has lost something which is necessary to move
the matter which serves for the irregular refraction; and that
likewise CE has lost that which was necessary to move the matter
which serves for regular refraction: but there is yet another thing
which upsets this reasoning. It is that when one disposes the two
crystals in such a way that the planes which constitute the principal
sections intersect one another at right angles, whether the
neighbouring surfaces are parallel or not, then the ray which has come
by the regular refraction, as DG, undergoes only an irregular
refraction in the lower piece; and on the contrary the ray which has
come by the irregular refraction, as CE, undergoes only a regular
refraction.

But in all the infinite other positions, besides those which I have
just stated, the rays DG, CE, divide themselves anew each one into
two, by refraction in the lower crystal so that from the single ray AB
there are four, sometimes of equal brightness, sometimes some much
less bright than others, according to the varying agreement in the
positions of the crystals: but they do not appear to have all together
more light than the single ray AB.

When one considers here how, while the rays CE, DG, remain the same,
it depends on the position that one gives to the lower piece, whether
it divides them both in two, or whether it does not divide them, and
yet how the ray AB above is always divided, it seems that one is
obliged to conclude that the waves of light, after having passed
through the first crystal, acquire a certain form or disposition in
virtue of which, when meeting the texture of the second crystal, in
certain positions, they can move the two different kinds of matter
which serve for the two species of refraction; and when meeting the
second crystal in another position are able to move only one of these
kinds of matter. But to tell how this occurs, I have hitherto found
nothing which satisfies me.

Leaving then to others this research, I pass to what I have to say
touching the cause of the extraordinary figure of this crystal, and
why it cleaves easily in three different senses, parallel to any one
of its surfaces.

There are many bodies, vegetable, mineral, and congealed salts, which
are formed with certain regular angles and figures. Thus among flowers
there are many which have their leaves disposed in ordered polygons,
to the number of 3, 4, 5, or 6 sides, but not more. This well deserves
to be investigated, both as to the polygonal figure, and as to why it
does not exceed the number 6.

Rock Crystal grows ordinarily in hexagonal bars, and diamonds are
found which occur with a square point and polished surfaces. There is
a species of small flat stones, piled up directly upon one another,
which are all of pentagonal figure with rounded angles, and the sides
a little folded inwards. The grains of gray salt which are formed from
sea water affect the figure, or at least the angle, of the cube; and
in the congelations of other salts, and in that of sugar, there are
found other solid angles with perfectly flat faces. Small snowflakes
almost always fall in little stars with 6 points, and sometimes in
hexagons with straight sides. And I have often observed, in water
which is beginning to freeze, a kind of flat and thin foliage of ice,
the middle ray of which throws out branches inclined at an angle of 60
degrees. All these things are worthy of being carefully investigated
to ascertain how and by what artifice nature there operates. But it is
not now my intention to treat fully of this matter. It seems that in
general the regularity which occurs in these productions comes from
the arrangement of the small invisible equal particles of which they
are composed. And, coming to our Iceland Crystal, I say that if there
were a pyramid such as ABCD, composed of small rounded corpuscles, not
spherical but flattened spheroids, such as would be made by the
rotation of the ellipse GH around its lesser diameter EF (of which the
ratio to the greater diameter is very nearly that of 1 to the square
root of 8)--I say that then the solid angle of the point D would be
equal to the obtuse and equilateral angle of this Crystal. I say,
further, that if these corpuscles were lightly stuck together, on
breaking this pyramid it would break along faces parallel to those
that make its point: and by this means, as it is easy to see, it would
produce prisms similar to those of the same crystal as this other
figure represents. The reason is that when broken in this fashion a
whole layer separates easily from its neighbouring layer since each
spheroid has to be detached only from the three spheroids of the next
layer; of which three there is but one which touches it on its
flattened surface, and the other two at the edges. And the reason why
the surfaces separate sharp and polished is that if any spheroid of
the neighbouring surface would come out by attaching itself to the
surface which is being separated, it would be needful for it to detach
itself from six other spheroids which hold it locked, and four of
which press it by these flattened surfaces. Since then not only the
angles of our crystal but also the manner in which it splits agree
precisely with what is observed in the assemblage composed of such
spheroids, there is great reason to believe that the particles are
shaped and ranged in the same way.

[Illustration: {Pyramid and section of spheroids}]

There is even probability enough that the prisms of this crystal are
produced by the breaking up of pyramids, since Mr. Bartholinus relates
that he occasionally found some pieces of triangularly pyramidal
figure. But when a mass is composed interiorly only of these little
spheroids thus piled up, whatever form it may have exteriorly, it is
certain, by the same reasoning which I have just explained, that if
broken it would produce similar prisms. It remains to be seen whether
there are other reasons which confirm our conjecture, and whether
there are none which are repugnant to it.

[Illustration: {paralleloid arrangement of spheroids with planes of
potential cleavage}]

It may be objected that this crystal, being so composed, might be
capable of cleavage in yet two more fashions; one of which would be
along planes parallel to the base of the pyramid, that is to say to
the triangle ABC; the other would be parallel to a plane the trace of
which is marked by the lines GH, HK, KL. To which I say that both the
one and the other, though practicable, are more difficult than those
which were parallel to any one of the three planes of the pyramid; and
that therefore, when striking on the crystal in order to break it, it
ought always to split rather along these three planes than along the
two others. When one has a number of spheroids of the form above
described, and ranges them in a pyramid, one sees why the two methods
of division are more difficult. For in the case of that division which
would be parallel to the base, each spheroid would be obliged to
detach itself from three others which it touches upon their flattened
surfaces, which hold more strongly than the contacts at the edges. And
besides that, this division will not occur along entire layers,
because each of the spheroids of a layer is scarcely held at all by
the 6 of the same layer that surround it, since they only touch it at
the edges; so that it adheres readily to the neighbouring layer, and
the others to it, for the same reason; and this causes uneven
surfaces. Also one sees by experiment that when grinding down the
crystal on a rather rough stone, directly on the equilateral solid
angle, one verily finds much facility in reducing it in this
direction, but much difficulty afterwards in polishing the surface
which has been flattened in this manner.

As for the other method of division along the plane GHKL, it will be
seen that each spheroid would have to detach itself from four of the
neighbouring layer, two of which touch it on the flattened surfaces,
and two at the edges. So that this division is likewise more difficult
than that which is made parallel to one of the surfaces of the
crystal; where, as we have said, each spheroid is detached from only
three of the neighbouring layer: of which three there is one only
which touches it on the flattened surface, and the other two at the
edges only.

However, that which has made me know that in the crystal there are
layers in this last fashion, is that in a piece weighing half a pound
which I possess, one sees that it is split along its length, as is the
above-mentioned prism by the plane GHKL; as appears by colours of the
Iris extending throughout this whole plane although the two pieces
still hold together. All this proves then that the composition of the
crystal is such as we have stated. To which I again add this
experiment; that if one passes a knife scraping along any one of the
natural surfaces, and downwards as it were from the equilateral obtuse
angle, that is to say from the apex of the pyramid, one finds it quite
hard; but by scraping in the opposite sense an incision is easily
made. This follows manifestly from the situation of the small
spheroids; over which, in the first manner, the knife glides; but in
the other manner it seizes them from beneath almost as if they were
the scales of a fish.

I will not undertake to say anything touching the way in which so many
corpuscles all equal and similar are generated, nor how they are set
in such beautiful order; whether they are formed first and then
assembled, or whether they arrange themselves thus in coming into
being and as fast as they are produced, which seems to me more
probable. To develop truths so recondite there would be needed a
knowledge of nature much greater than that which we have. I will add
only that these little spheroids could well contribute to form the
spheroids of the waves of light, here above supposed, these as well as
those being similarly situated, and with their axes parallel.


_Calculations which have been supposed in this Chapter_.

Mr. Bartholinus, in his treatise of this Crystal, puts at 101 degrees
the obtuse angles of the faces, which I have stated to be 101 degrees
52 minutes. He states that he measured these angles directly on the
crystal, which is difficult to do with ultimate exactitude, because
the edges such as CA, CB, in this figure, are generally worn, and not
quite straight. For more certainty, therefore, I preferred to measure
actually the obtuse angle by which the faces CBDA, CBVF, are inclined
to one another, namely the angle OCN formed by drawing CN
perpendicular to FV, and CO perpendicular to DA. This angle OCN I
found to be 105 degrees; and its supplement CNP, to be 75 degrees, as
it should be.

[Illustration]

To find from this the obtuse angle BCA, I imagined a sphere having its
centre at C, and on its surface a spherical triangle, formed by the
intersection of three planes which enclose the solid angle C. In this
equilateral triangle, which is ABF in this other figure, I see that
each of the angles should be 105 degrees, namely equal to the angle
OCN; and that each of the sides should be of as many degrees as the
angle ACB, or ACF, or BCF. Having then drawn the arc FQ perpendicular
to the side AB, which it divides equally at Q, the triangle FQA has a
right angle at Q, the angle A 105 degrees, and F half as much, namely
52 degrees 30 minutes; whence the hypotenuse AF is found to be 101
degrees 52 minutes. And this arc AF is the measure of the angle ACF in
the figure of the crystal.

[Illustration]

In the same figure, if the plane CGHF cuts the crystal so that it
divides the obtuse angles ACB, MHV, in the middle, it is stated, in
Article 10, that the angle CFH is 70 degrees 57 minutes. This again is
easily shown in the same spherical triangle ABF, in which it appears
that the arc FQ is as many degrees as the angle GCF in the crystal,
the supplement of which is the angle CFH. Now the arc FQ is found to
be 109 degrees 3 minutes. Then its supplement, 70 degrees 57 minutes,
is the angle CFH.

It was stated, in Article 26, that the straight line CS, which in the
preceding figure is CH, being the axis of the crystal, that is to say
being equally inclined to the three sides CA, CB, CF, the angle GCH is
45 degrees 20 minutes. This is also easily calculated by the same
spherical triangle. For by drawing the other arc AD which cuts BF
equally, and intersects FQ at S, this point will be the centre of the
triangle. And it is easy to see that the arc SQ is the measure of the
angle GCH in the figure which represents the crystal. Now in the
triangle QAS, which is right-angled, one knows also the angle A, which
is 52 degrees 30 minutes, and the side AQ 50 degrees 56 minutes;
whence the side SQ is found to be 45 degrees 20 minutes.

In Article 27 it was required to show that PMS being an ellipse the
centre of which is C, and which touches the straight line MD at M so
that the angle MCL which CM makes with CL, perpendicular on DM, is 6
degrees 40 minutes, and its semi-minor axis CS making with CG (which
is parallel to MD) an angle GCS of 45 degrees 20 minutes, it was
required to show, I say, that, CM being 100,000 parts, PC the
semi-major diameter of this ellipse is 105,032 parts, and CS, the
semi-minor diameter, 93,410.

Let CP and CS be prolonged and meet the tangent DM at D and Z; and
from the point of contact M let MN and MO be drawn as perpendiculars
to CP and CS. Now because the angles SCP, GCL, are right angles, the
angle PCL will be equal to GCS which was 45 degrees 20 minutes. And
deducting the angle LCM, which is 6 degrees 40 minutes, from LCP,
which is 45 degrees 20 minutes, there remains MCP, 38 degrees 40
minutes. Considering then CM as a radius of 100,000 parts, MN, the
sine of 38 degrees 40 minutes, will be 62,479. And in the right-angled
triangle MND, MN will be to ND as the radius of the Tables is to the
tangent of 45 degrees 20 minutes (because the angle NMD is equal to
DCL, or GCS); that is to say as 100,000 to 101,170: whence results ND
63,210. But NC is 78,079 of the same parts, CM being 100,000, because
NC is the sine of the complement of the angle MCP, which was 38
degrees 40 minutes. Then the whole line DC is 141,289; and CP, which
is a mean proportional between DC and CN, since MD touches the
Ellipse, will be 105,032.

[Illustration]

Similarly, because the angle OMZ is equal to CDZ, or LCZ, which is 44
degrees 40 minutes, being the complement of GCS, it follows that, as
the radius of the Tables is to the tangent of 44 degrees 40 minutes,
so will OM 78,079 be to OZ 77,176. But OC is 62,479 of these same
parts of which CM is 100,000, because it is equal to MN, the sine of
the angle MCP, which is 38 degrees 40 minutes. Then the whole line CZ
is 139,655; and CS, which is a mean proportional between CZ and CO
will be 93,410.

At the same place it was stated that GC was found to be 98,779 parts.
To prove this, let PE be drawn in the same figure parallel to DM, and
meeting CM at E. In the right-angled triangle CLD the side CL is
99,324 (CM being 100,000), because CL is the sine of the complement of
the angle LCM, which is 6 degrees 40 minutes. And since the angle LCD
is 45 degrees 20 minutes, being equal to GCS, the side LD is found to
be 100,486: whence deducting ML 11,609 there will remain MD 88,877.
Now as CD (which was 141,289) is to DM 88,877, so will CP 105,032 be
to PE 66,070. But as the rectangle MEH (or rather the difference of
the squares on CM and CE) is to the square on MC, so is the square on
PE to the square on C_g_; then also as the difference of the squares
on DC and CP to the square on CD, so also is the square on PE to the
square on _g_C. But DP, CP, and PE are known; hence also one knows GC,
which is 98,779.


_Lemma which has been supposed_.

If a spheroid is touched by a straight line, and also by two or more
planes which are parallel to this line, though not parallel to one
another, all the points of contact of the line, as well as of the
planes, will be in one and the same ellipse made by a plane which
passes through the centre of the spheroid.

Let LED be the spheroid touched by the line BM at the point B, and
also by the planes parallel to this line at the points O and A. It is
required to demonstrate that the points B, O, and A are in one and the
same Ellipse made in the spheroid by a plane which passes through its
centre.

[Illustration]

Through the line BM, and through the points O and A, let there be
drawn planes parallel to one another, which, in cutting the spheroid
make the ellipses LBD, POP, QAQ; which will all be similar and
similarly disposed, and will have their centres K, N, R, in one and
the same diameter of the spheroid, which will also be the diameter of
the ellipse made by the section of the plane that passes through the
centre of the spheroid, and which cuts the planes of the three said
Ellipses at right angles: for all this is manifest by proposition 15
of the book of Conoids and Spheroids of Archimedes. Further, the two
latter planes, which are drawn through the points O and A, will also,
by cutting the planes which touch the spheroid in these same points,
generate straight lines, as OH and AS, which will, as is easy to see,
be parallel to BM; and all three, BM, OH, AS, will touch the Ellipses
LBD, POP, QAQ in these points, B, O, A; since they are in the planes
of these ellipses, and at the same time in the planes which touch the
spheroid. If now from these points B, O, A, there are drawn the
straight lines BK, ON, AR, through the centres of the same ellipses,
and if through these centres there are drawn also the diameters LD,
PP, QQ, parallel to the tangents BM, OH, AS; these will be conjugate
to the aforesaid BK, ON, AR. And because the three ellipses are
similar and similarly disposed, and have their diameters LD, PP, QQ
parallel, it is certain that their conjugate diameters BK, ON, AR,
will also be parallel. And the centres K, N, R being, as has been
stated, in one and the same diameter of the spheroid, these parallels
BK, ON, AR will necessarily be in one and the same plane, which passes
through this diameter of the spheroid, and, in consequence, the points
R, O, A are in one and the same ellipse made by the intersection of
this plane. Which was to be proved. And it is manifest that the
demonstration would be the same if, besides the points O, A, there had
been others in which the spheroid had been touched by planes parallel
to the straight line BM.




CHAPTER VI

ON THE FIGURES OF THE TRANSPARENT BODIES

Which serve for Refraction and for Reflexion


After having explained how the properties of reflexion and refraction
follow from what we have supposed concerning the nature of light, and
of opaque bodies, and of transparent media, I will here set forth a
very easy and natural way of deducing, from the same principles, the
true figures which serve, either by reflexion or by refraction, to
collect or disperse the rays of light, as may be desired. For though I
do not see yet that there are means of making use of these figures, so
far as relates to Refraction, not only because of the difficulty of
shaping the glasses of Telescopes with the requisite exactitude
according to these figures, but also because there exists in
refraction itself a property which hinders the perfect concurrence of
the rays, as Mr. Newton has very well proved by experiment, I will yet
not desist from relating the invention, since it offers itself, so to
speak, of itself, and because it further confirms our Theory of
refraction, by the agreement which here is found between the refracted
ray and the reflected ray. Besides, it may occur that some one in the
future will discover in it utilities which at present are not seen.

[Illustration]

To proceed then to these figures, let us suppose first that it is
desired to find a surface CDE which shall reassemble at a point B rays
coming from another point A; and that the summit of the surface shall
be the given point D in the straight line AB. I say that, whether by
reflexion or by refraction, it is only necessary to make this surface
such that the path of the light from the point A to all points of the
curved line CDE, and from these to the point of concurrence (as here
the path along the straight lines AC, CB, along AL, LB, and along AD,
DB), shall be everywhere traversed in equal times: by which principle
the finding of these curves becomes very easy.

[Illustration]

So far as relates to the reflecting surface, since the sum of the
lines AC, CB ought to be equal to that of AD, DB, it appears that DCE
ought to be an ellipse; and for refraction, the ratio of the
velocities of waves of light in the media A and B being supposed to be
known, for example that of 3 to 2 (which is the same, as we have
shown, as the ratio of the Sines in the refraction), it is only
necessary to make DH equal to 3/2 of DB; and having after that
described from the centre A some arc FC, cutting DB at F, then
describe another from centre B with its semi-diameter BX equal to 2/3
of FH; and the point of intersection of the two arcs will be one of
the points required, through which the curve should pass. For this
point, having been found in this fashion, it is easy forthwith to
demonstrate that the time along AC, CB, will be equal to the time
along AD, DB.

For assuming that the line AD represents the time which the light
takes to traverse this same distance AD in air, it is evident that DH,
equal to 3/2 of DB, will represent the time of the light along DB in
the medium, because it needs here more time in proportion as its speed
is slower. Therefore the whole line AH will represent the time along
AD, DB. Similarly the line AC or AF will represent the time along AC;
and FH being by construction equal to 3/2 of CB, it will represent the
time along CB in the medium; and in consequence the whole line AH will
represent also the time along AC, CB. Whence it appears that the time
along AC, CB, is equal to the time along AD, DB. And similarly it can
be shown if L and K are other points in the curve CDE, that the times
along AL, LB, and along AK, KB, are always represented by the line AH,
and therefore equal to the said time along AD, DB.

In order to show further that the surfaces, which these curves will
generate by revolution, will direct all the rays which reach them from
the point A in such wise that they tend towards B, let there be
supposed a point K in the curve, farther from D than C is, but such
that the straight line AK falls from outside upon the curve which
serves for the refraction; and from the centre B let the arc KS be
described, cutting BD at S, and the straight line CB at R; and from
the centre A describe the arc DN meeting AK at N.

Since the sums of the times along AK, KB, and along AC, CB are equal,
if from the former sum one deducts the time along KB, and if from the
other one deducts the time along RB, there will remain the time along
AK as equal to the time along the two parts AC, CR. Consequently in
the time that the light has come along AK it will also have come along
AC and will in addition have made, in the medium from the centre C, a
partial spherical wave, having a semi-diameter equal to CR. And this
wave will necessarily touch the circumference KS at R, since CB cuts
this circumference at right angles. Similarly, having taken any other
point L in the curve, one can show that in the same time as the light
passes along AL it will also have come along AL and in addition will
have made a partial wave, from the centre L, which will touch the same
circumference KS. And so with all other points of the curve CDE. Then
at the moment that the light reaches K the arc KRS will be the
termination of the movement, which has spread from A through DCK. And
thus this same arc will constitute in the medium the propagation of
the wave emanating from A; which wave may be represented by the arc
DN, or by any other nearer the centre A. But all the pieces of the arc
KRS are propagated successively along straight lines which are
perpendicular to them, that is to say, which tend to the centre B (for
that can be demonstrated in the same way as we have proved above that
the pieces of spherical waves are propagated along the straight lines
coming from their centre), and these progressions of the pieces of the
waves constitute the rays themselves of light. It appears then that
all these rays tend here towards the point B.

One might also determine the point C, and all the others, in this
curve which serves for the refraction, by dividing DA at G in such a
way that DG is 2/3 of DA, and describing from the centre B any arc CX
which cuts BD at N, and another from the centre A with its
semi-diameter AF equal to 3/2 of GX; or rather, having described, as
before, the arc CX, it is only necessary to make DF equal to 3/2 of
DX, and from-the centre A to strike the arc FC; for these two
constructions, as may be easily known, come back to the first one
which was shown before. And it is manifest by the last method that
this curve is the same that Mr. Des Cartes has given in his Geometry,
and which he calls the first of his Ovals.

It is only a part of this oval which serves for the refraction,
namely, the part DK, ending at K, if AK is the tangent. As to the,
other part, Des Cartes has remarked that it could serve for
reflexions, if there were some material of a mirror of such a nature
that by its means the force of the rays (or, as we should say, the
velocity of the light, which he could not say, since he held that the
movement of light was instantaneous) could be augmented in the
proportion of 3 to 2. But we have shown that in our way of explaining
reflexion, such a thing could not arise from the matter of the mirror,
and it is entirely impossible.

[Illustration]

[Illustration]

From what has been demonstrated about this oval, it will be easy to
find the figure which serves to collect to a point incident parallel
rays. For by supposing just the same construction, but the point A
infinitely distant, giving parallel rays, our oval becomes a true
Ellipse, the construction of which differs in no way from that of the
oval, except that FC, which previously was an arc of a circle, is here
a straight line, perpendicular to DB. For the wave of light DN, being
likewise represented by a straight line, it will be seen that all the
points of this wave, travelling as far as the surface KD along lines
parallel to DB, will advance subsequently towards the point B, and
will arrive there at the same time. As for the Ellipse which served
for reflexion, it is evident that it will here become a parabola,
since its focus A may be regarded as infinitely distant from the
other, B, which is here the focus of the parabola, towards which all
the reflexions of rays parallel to AB tend. And the demonstration of
these effects is just the same as the preceding.

But that this curved line CDE which serves for refraction is an
Ellipse, and is such that its major diameter is to the distance
between its foci as 3 to 2, which is the proportion of the refraction,
can be easily found by the calculus of Algebra. For DB, which is
given, being called _a_; its undetermined perpendicular DT being
called _x_; and TC _y_; FB will be _a - y_; CB will be sqrt(_xx + aa
-2ay + yy_). But the nature of the curve is such that 2/3 of TC
together with CB is equal to DB, as was stated in the last
construction: then the equation will be between _(2/3)y + sqrt(xx + aa
- 2ay + yy)_ and _a_; which being reduced, gives _(6/5)ay - yy_ equal
to _(9/5)xx_; that is to say that having made DO equal to 6/5 of DB,
the rectangle DFO is equal to 9/5 of the square on FC. Whence it is
seen that DC is an ellipse, of which the axis DO is to the parameter
as 9 to 5; and therefore the square on DO is to the square of the
distance between the foci as 9 to 9 - 5, that is to say 4; and finally
the line DO will be to this distance as 3 to 2.

[Illustration]

Again, if one supposes the point B to be infinitely distant, in lieu
of our first oval we shall find that CDE is a true Hyperbola; which
will make those rays become parallel which come from the point A. And
in consequence also those which are parallel within the transparent
body will be collected outside at the point A. Now it must be remarked
that CX and KS become straight lines perpendicular to BA, because they
represent arcs of circles the centre of which is infinitely distant.
And the intersection of the perpendicular CX with the arc FC will give
the point C, one of those through which the curve ought to pass. And
this operates so that all the parts of the wave of light DN, coming to
meet the surface KDE, will advance thence along parallels to KS and
will arrive at this straight line at the same time; of which the proof
is again the same as that which served for the first oval. Besides one
finds by a calculation as easy as the preceding one, that CDE is here
a hyperbola of which the axis DO is 4/5 of AD, and the parameter
equal to AD. Whence it is easily proved that DO is to the distance
between the foci as 3 to 2.

[Illustration]

These are the two cases in which Conic sections serve for refraction,
and are the same which are explained, in his _Dioptrique_, by Des
Cartes, who first found out the use of these lines in relation to
refraction, as also that of the Ovals the first of which we have
already set forth. The second oval is that which serves for rays that
tend to a given point; in which oval, if the apex of the surface which
receives the rays is D, it will happen that the other apex will be
situated between B and A, or beyond A, according as the ratio of AD to
DB is given of greater or lesser value. And in this latter case it is
the same as that which Des Cartes calls his 3rd oval.

Now the finding and construction of this second oval is the same as
that of the first, and the demonstration of its effect likewise. But
it is worthy of remark that in one case this oval becomes a perfect
circle, namely when the ratio of AD to DB is the same as the ratio of
the refractions, here as 3 to 2, as I observed a long time ago. The
4th oval, serving only for impossible reflexions, there is no need to
set it forth.

[Illustration]

As for the manner in which Mr. Des Cartes discovered these lines,
since he has given no explanation of it, nor any one else since that I
know of, I will say here, in passing, what it seems to me it must have
been. Let it be proposed to find the surface generated by the
revolution of the curve KDE, which, receiving the incident rays coming
to it from the point A, shall deviate them toward the point B. Then
considering this other curve as already known, and that its apex D is
in the straight line AB, let us divide it up into an infinitude of
small pieces by the points G, C, F; and having drawn from each of
these points, straight lines towards A to represent the incident rays,
and other straight lines towards B, let there also be described with
centre A the arcs GL, CM, FN, DO, cutting the rays that come from A at
L, M, N, O; and from the points K, G, C, F, let there be described
the arcs KQ, GR, CS, FT cutting the rays towards B at Q, R, S, T; and
let us suppose that the straight line HKZ cuts the curve at K at
right-angles.

[Illustration]

Then AK being an incident ray, and KB its refraction within the
medium, it needs must be, according to the law of refraction which was
known to Mr. Des Cartes, that the sine of the angle ZKA should be to
the sine of the angle HKB as 3 to 2, supposing that this is the
proportion of the refraction of glass; or rather, that the sine of the
angle KGL should have this same ratio to the sine of the angle GKQ,
considering KG, GL, KQ as straight lines because of their smallness.
But these sines are the lines KL and GQ, if GK is taken as the radius
of the circle. Then LK ought to be to GQ as 3 to 2; and in the same
ratio MG to CR, NC to FS, OF to DT. Then also the sum of all the
antecedents to all the consequents would be as 3 to 2. Now by
prolonging the arc DO until it meets AK at X, KX is the sum of the
antecedents. And by prolonging the arc KQ till it meets AD at Y, the
sum of the consequents is DY. Then KX ought to be to DY as 3 to 2.
Whence it would appear that the curve KDE was of such a nature that
having drawn from some point which had been assumed, such as K, the
straight lines KA, KB, the excess by which AK surpasses AD should be
to the excess of DB over KB, as 3 to 2. For it can similarly be
demonstrated, by taking any other point in the curve, such as G, that
the excess of AG over AD, namely VG, is to the excess of BD over DG,
namely DP, in this same ratio of 3 to 2. And following this principle
Mr. Des Cartes constructed these curves in his _Geometric_; and he
easily recognized that in the case of parallel rays, these curves
became Hyperbolas and Ellipses.

Let us now return to our method and let us see how it leads without
difficulty to the finding of the curves which one side of the glass
requires when the other side is of a given figure; a figure not only
plane or spherical, or made by one of the conic sections (which is the
restriction with which Des Cartes proposed this problem, leaving the
solution to those who should come after him) but generally any figure
whatever: that is to say, one made by the revolution of any given
curved line to which one must merely know how to draw straight lines
as tangents.

Let the given figure be that made by the revolution of some curve such
as AK about the axis AV, and that this side of the glass receives rays
coming from the point L. Furthermore, let the thickness AB of the
middle of the glass be given, and the point F at which one desires the
rays to be all perfectly reunited, whatever be the first refraction
occurring at the surface AK.

I say that for this the sole requirement is that the outline BDK which
constitutes the other surface shall be such that the path of the
light from the point L to the surface AK, and from thence to the
surface BDK, and from thence to the point F, shall be traversed
everywhere in equal times, and in each case in a time equal to that
which the light employs, to pass along the straight line LF of which
the part AB is within the glass.

[Illustration]

Let LG be a ray falling on the arc AK. Its refraction GV will be given
by means of the tangent which will be drawn at the point G. Now in GV
the point D must be found such that FD together with 3/2 of DG and the
straight line GL, may be equal to FB together with 3/2 of BA and the
straight line AL; which, as is clear, make up a given length. Or
rather, by deducting from each the length of LG, which is also given,
it will merely be needful to adjust FD up to the straight line VG in
such a way that FD together with 3/2 of DG is equal to a given
straight line, which is a quite easy plane problem: and the point D
will be one of those through which the curve BDK ought to pass. And
similarly, having drawn another ray LM, and found its refraction MO,
the point N will be found in this line, and so on as many times as one
desires.

To demonstrate the effect of the curve, let there be described about
the centre L the circular arc AH, cutting LG at H; and about the
centre F the arc BP; and in AB let AS be taken equal to 2/3 of HG; and
SE equal to GD. Then considering AH as a wave of light emanating from
the point L, it is certain that during the time in which its piece H
arrives at G the piece A will have advanced within the transparent
body only along AS; for I suppose, as above, the proportion of the
refraction to be as 3 to 2. Now we know that the piece of wave which
is incident on G, advances thence along the line GD, since GV is the
refraction of the ray LG. Then during the time that this piece of wave
has taken from G to D, the other piece which was at S has reached E,
since GD, SE are equal. But while the latter will advance from E to B,
the piece of wave which was at D will have spread into the air its
partial wave, the semi-diameter of which, DC (supposing this wave to
cut the line DF at C), will be 3/2 of EB, since the velocity of light
outside the medium is to that inside as 3 to 2. Now it is easy to show
that this wave will touch the arc BP at this point C. For since, by
construction, FD + 3/2 DG + GL are equal to FB + 3/2 BA + AL; on
deducting the equals LH, LA, there will remain FD + 3/2 DG + GH equal
to FB + 3/2 BA. And, again, deducting from one side GH, and from the
other side 3/2 of AS, which are equal, there will remain FD with 3/2
DG equal to FB with 3/2 of BS. But 3/2 of DG are equal to 3/2 of ES;
then FD is equal to FB with 3/2 of BE. But DC was equal to 3/2 of EB;
then deducting these equal lengths from one side and from the other,
there will remain CF equal to FB. And thus it appears that the wave,
the semi-diameter of which is DC, touches the arc BP at the moment
when the light coming from the point L has arrived at B along the line
LB. It can be demonstrated similarly that at this same moment the
light that has come along any other ray, such as LM, MN, will have
propagated the movement which is terminated at the arc BP. Whence it
follows, as has been often said, that the propagation of the wave AH,
after it has passed through the thickness of the glass, will be the
spherical wave BP, all the pieces of which ought to advance along
straight lines, which are the rays of light, to the centre F. Which
was to be proved. Similarly these curved lines can be found in all the
cases which can be proposed, as will be sufficiently shown by one or
two examples which I will add.

Let there be given the surface of the glass AK, made by the revolution
about the axis BA of the line AK, which may be straight or curved. Let
there be also given in the axis the point L and the thickness BA of
the glass; and let it be required to find the other surface KDB, which
receiving rays that are parallel to AB will direct them in such wise
that after being again refracted at the given surface AK they will all
be reassembled at the point L.

[Illustration]

From the point L let there be drawn to some point of the given line
AK the straight line LG, which, being considered as a ray of light,
its refraction GD will then be found. And this line being then
prolonged at one side or the other will meet the straight line BL, as
here at V. Let there then be erected on AB the perpendicular BC, which
will represent a wave of light coming from the infinitely distant
point F, since we have supposed the rays to be parallel. Then all the
parts of this wave BC must arrive at the same time at the point L; or
rather all the parts of a wave emanating from the point L must arrive
at the same time at the straight line BC. And for that, it is
necessary to find in the line VGD the point D such that having drawn
DC parallel to AB, the sum of CD, plus 3/2 of DG, plus GL may be equal
to 3/2 of AB, plus AL: or rather, on deducting from both sides GL,
which is given, CD plus 3/2 of DG must be equal to a given length;
which is a still easier problem than the preceding construction. The
point D thus found will be one of those through which the curve ought
to pass; and the proof will be the same as before. And by this it will
be proved that the waves which come from the point L, after having
passed through the glass KAKB, will take the form of straight lines,
as BC; which is the same thing as saying that the rays will become
parallel. Whence it follows reciprocally that parallel rays falling on
the surface KDB will be reassembled at the point L.

[Illustration]

Again, let there be given the surface AK, of any desired form,
generated by revolution about the axis AB, and let the thickness of
the glass at the middle be AB. Also let the point L be given in the
axis behind the glass; and let it be supposed that the rays which fall
on the surface AK tend to this point, and that it is required to find
the surface BD, which on their emergence from the glass turns them as
if they came from the point F in front of the glass.

Having taken any point G in the line AK, and drawing the straight line
IGL, its part GI will represent one of the incident rays, the
refraction of which, GV, will then be found: and it is in this line
that we must find the point D, one of those through which the curve DG
ought to pass. Let us suppose that it has been found: and about L as
centre let there be described GT, the arc of a circle cutting the
straight line AB at T, in case the distance LG is greater than LA; for
otherwise the arc AH must be described about the same centre, cutting
the straight line LG at H. This arc GT (or AH, in the other case) will
represent an incident wave of light, the rays of which tend towards
L. Similarly, about the centre F let there be described the circular
arc DQ, which will represent a wave emanating from the point F.

Then the wave TG, after having passed through the glass, must form the
wave QD; and for this I observe that the time taken by the light along
GD in the glass must be equal to that taken along the three, TA, AB,
and BQ, of which AB alone is within the glass. Or rather, having taken
AS equal to 2/3 of AT, I observe that 3/2 of GD ought to be equal to
3/2 of SB, plus BQ; and, deducting both of them from FD or FQ, that FD
less 3/2 of GD ought to be equal to FB less 3/2 of SB. And this last
difference is a given length: and all that is required is to draw the
straight line FD from the given point F to meet VG so that it may be
thus. Which is a problem quite similar to that which served for the
first of these constructions, where FD plus 3/2 of GD had to be equal
to a given length.

In the demonstration it is to be observed that, since the arc BC falls
within the glass, there must be conceived an arc RX, concentric with
it and on the other side of QD. Then after it shall have been shown
that the piece G of the wave GT arrives at D at the same time that the
piece T arrives at Q, which is easily deduced from the construction,
it will be evident as a consequence that the partial wave generated at
the point D will touch the arc RX at the moment when the piece Q shall
have come to R, and that thus this arc will at the same moment be the
termination of the movement that comes from the wave TG; whence all
the rest may be concluded.

Having shown the method of finding these curved lines which serve for
the perfect concurrence of the rays, there remains to be explained a
notable thing touching the uncoordinated refraction of spherical,
plane, and other surfaces: an effect which if ignored might cause some
doubt concerning what we have several times said, that rays of light
are straight lines which intersect at right angles the waves which
travel along them.

[Illustration]

For in the case of rays which, for example, fall parallel upon a
spherical surface AFE, intersecting one another, after refraction, at
different points, as this figure represents; what can the waves of
light be, in this transparent body, which are cut at right angles by
the converging rays? For they can not be spherical. And what will
these waves become after the said rays begin to intersect one another?
It will be seen in the solution of this difficulty that something very
remarkable comes to pass herein, and that the waves do not cease to
persist though they do not continue entire, as when they cross the
glasses designed according to the construction we have seen.

According to what has been shown above, the straight line AD, which
has been drawn at the summit of the sphere, at right angles to the
axis parallel to which the rays come, represents the wave of light;
and in the time taken by its piece D to reach the spherical surface
AGE at E, its other parts will have met the same surface at F, G, H,
etc., and will have also formed spherical partial waves of which these
points are the centres. And the surface EK which all those waves will
touch, will be the continuation of the wave AD in the sphere at the
moment when the piece D has reached E. Now the line EK is not an arc
of a circle, but is a curved line formed as the evolute of another
curve ENC, which touches all the rays HL, GM, FO, etc., that are the
refractions of the parallel rays, if we imagine laid over the
convexity ENC a thread which in unwinding describes at its end E the
said curve EK. For, supposing that this curve has been thus described,
we will show that the said waves formed from the centres F, G, H,
etc., will all touch it.

It is certain that the curve EK and all the others described by the
evolution of the curve ENC, with different lengths of thread, will cut
all the rays HL, GM, FO, etc., at right angles, and in such wise that
the parts of them intercepted between two such curves will all be
equal; for this follows from what has been demonstrated in our
treatise _de Motu Pendulorum_. Now imagining the incident rays as
being infinitely near to one another, if we consider two of them, as
RG, TF, and draw GQ perpendicular to RG, and if we suppose the curve
FS which intersects GM at P to have been described by evolution from
the curve NC, beginning at F, as far as which the thread is supposed
to extend, we may assume the small piece FP as a straight line
perpendicular to the ray GM, and similarly the arc GF as a straight
line. But GM being the refraction of the ray RG, and FP being
perpendicular to it, QF must be to GP as 3 to 2, that is to say in the
proportion of the refraction; as was shown above in explaining the
discovery of Des Cartes. And the same thing occurs in all the small
arcs GH, HA, etc., namely that in the quadrilaterals which enclose
them the side parallel to the axis is to the opposite side as 3 to 2.
Then also as 3 to 2 will the sum of the one set be to the sum of the
other; that is to say, TF to AS, and DE to AK, and BE to SK or DV,
supposing V to be the intersection of the curve EK and the ray FO.
But, making FB perpendicular to DE, the ratio of 3 to 2 is also that
of BE to the semi-diameter of the spherical wave which emanated from
the point F while the light outside the transparent body traversed the
space BE. Then it appears that this wave will intersect the ray FM at
the same point V where it is intersected at right angles by the curve
EK, and consequently that the wave will touch this curve. In the same
way it can be proved that the same will apply to all the other waves
above mentioned, originating at the points G, H, etc.; to wit, that
they will touch the curve EK at the moment when the piece D of the
wave ED shall have reached E.

Now to say what these waves become after the rays have begun to cross
one another: it is that from thence they fold back and are composed of
two contiguous parts, one being a curve formed as evolute of the curve
ENC in one sense, and the other as evolute of the same curve in the
opposite sense. Thus the wave KE, while advancing toward the meeting
place becomes _abc_, whereof the part _ab_ is made by the evolute
_b_C, a portion of the curve ENC, while the end C remains attached;
and the part _bc_ by the evolute of the portion _b_E while the end E
remains attached. Consequently the same wave becomes _def_, then
_ghk_, and finally CY, from whence it subsequently spreads without any
fold, but always along curved lines which are evolutes of the curve
ENC, increased by some straight line at the end C.

There is even, in this curve, a part EN which is straight, N being the
point where the perpendicular from the centre X of the sphere falls
upon the refraction of the ray DE, which I now suppose to touch the
sphere. The folding of the waves of light begins from the point N up
to the end of the curve C, which point is formed by taking AC to CX in
the proportion of the refraction, as here 3 to 2.

As many other points as may be desired in the curve NC are found by a
Theorem which Mr. Barrow has demonstrated in section 12 of his
_Lectiones Opticae_, though for another purpose. And it is to be noted
that a straight line equal in length to this curve can be given. For
since it together with the line NE is equal to the line CK, which is
known, since DE is to AK in the proportion of the refraction, it
appears that by deducting EN from CK the remainder will be equal to
the curve NC.

Similarly the waves that are folded back in reflexion by a concave
spherical mirror can be found. Let ABC be the section, through the
axis, of a hollow hemisphere, the centre of which is D, its axis being
DB, parallel to which I suppose the rays of light to come. All the
reflexions of those rays which fall upon the quarter-circle AB will
touch a curved line AFE, of which line the end E is at the focus of
the hemisphere, that is to say, at the point which divides the
semi-diameter BD into two equal parts. The points through which this
curve ought to pass are found by taking, beyond A, some arc AO, and
making the arc OP double the length of it; then dividing the chord OP
at F in such wise that the part FP is three times the part FO; for
then F is one of the required points.

[Illustration]

And as the parallel rays are merely perpendiculars to the waves which
fall on the concave surface, which waves are parallel to AD, it will
be found that as they come successively to encounter the surface AB,
they form on reflexion folded waves composed of two curves which
originate from two opposite evolutions of the parts of the curve AFE.
So, taking AD as an incident wave, when the part AG shall have met the
surface AI, that is to say when the piece G shall have reached I, it
will be the curves HF, FI, generated as evolutes of the curves FA, FE,
both beginning at F, which together constitute the propagation of the
part AG. And a little afterwards, when the part AK has met the surface
AM, the piece K having come to M, then the curves LN, NM, will
together constitute the propagation of that part. And thus this folded
wave will continue to advance until the point N has reached the focus
E. The curve AFE can be seen in smoke, or in flying dust, when a
concave mirror is held opposite the sun. And it should be known that
it is none other than that curve which is described by the point E on
the circumference of the circle EB, when that circle is made to roll
within another whose semi-diameter is ED and whose centre is D. So
that it is a kind of Cycloid, of which, however, the points can be
found geometrically.

Its length is exactly equal to 3/4 of the diameter of the sphere, as
can be found and demonstrated by means of these waves, nearly in the
same way as the mensuration of the preceding curve; though it may also
be demonstrated in other ways, which I omit as outside the subject.
The area AOBEFA, comprised between the arc of the quarter-circle, the
straight line BE, and the curve EFA, is equal to the fourth part of
the quadrant DAB.





INDEX

Archimedes, 104.

Atmospheric refraction, 45.

Barrow, Isaac, 126.

Bartholinus, Erasmus, 53, 54, 57, 60, 97, 99.

Boyle, Hon. Robert, 11.

Cassini, Jacques, iii.

Caustic Curves, 123.

Crystals, see Iceland Crystal, Rock Crystal.

Crystals, configuration of, 95.

Descartes, Rénê, 3, 5, 7, 14, 22, 42, 43, 109, 113.

Double Refraction, discovery of, 54, 81, 93.

Elasticity, 12, 14.

Ether, the, or Ethereal matter, 11, 14, 16, 28.

Extraordinary refraction, 55, 56.

Fermat, principle of, 42.

Figures of transparent bodies, 105.

Hooke, Robert, 20.

Iceland Crystal, 2, 52 sqq.

Iceland Crystal, Cutting and Polishing of, 91, 92, 98.

Leibnitz, G.W., vi.

Light, nature of, 3.

Light, velocity of, 4, 15.

Molecular texture of bodies, 27, 95.

Newton, Sir Isaac, vi, 106.

Opacity, 34.

Ovals, Cartesian, 107, 113.

Pardies, Rev. Father, 20.

Rays, definition of, 38, 49.

Reflexion, 22.

Refraction, 28, 34.

Rock Crystal, 54, 57, 62, 95.

Römer, Olaf, v, 7.

Roughness of surfaces, 27.

Sines, law of, 1, 35, 38, 43.

Spheres, elasticity of, 15.

Spheroidal waves in crystals, 63.

Spheroids, lemma about, 103.

Sound, speed of, 7, 10, 12.

Telescopes, lenses for, 62, 105.

Torricelli's experiment, 12, 30.

Transparency, explanation of, 28, 31, 32.

Waves, no regular succession of, 17.

Waves, principle of wave envelopes, 19, 24.

Waves, principle of elementary wave fronts, 19.

Waves, propagation of light as, 16, 63.