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Title: On the theory of light and colours

Author: Thomas Young


        
Release date: September 1, 2026 [eBook #79490]

Language: English

Original publication: London: The Royal Society, 1802

Other information and formats: www.gutenberg.org/ebooks/79490

Credits: Thanks to the Internet Archive for providing the scans used in the production of this eBook.


*** START OF THE PROJECT GUTENBERG EBOOK ON THE THEORY OF LIGHT AND COLOURS ***
Transcriber’s notes:

This e-book comprises the text of a lecture delivered on November 12,
1801 and subsequently published in the Philosophical Transactions of
the Royal Society, Issue 92 (1802), pp. 12–48.

The transcription has mostly been preserved in its original form,
including minor punctuation inconsistencies. However, near the end
of the book (preceding a table) there are four expressions that have
had to be displayed as pseudo fractions because of the limitations
of a plain text transcription. In the original, the 'numerators'
and 'denominators' are positioned one above the other but without a
separating line (vinculum).

Following the table there is a quotation derived from Hooke’s
_Micrographia_. It is not a true verbatim quotation, however, but
rather a selection of phrases and sentences extracted from Hooke’s
text, and it contains several seemingly random quotation marks that
were not present in the latter; they have been omitted from this
transcription.




II. _The Bakerian Lecture. On the Theory of Light and Colours.
By_ Thomas Young, _M.D. F.R.S. Professor of Natural Philosophy in the
Royal Institution._


Read November 12, 1801.

Although the invention of plausible hypotheses, independent of any
connection with experimental observations, can be of very little use
in the promotion of natural knowledge; yet the discovery of simple and
uniform principles, by which a great number of apparently heterogeneous
phenomena are reduced to coherent and universal laws, must ever be
allowed to be of considerable importance towards the improvement of the
human intellect.

The object of the present dissertation is not so much to propose any
opinions which are absolutely new, as to refer some theories, which
have been already advanced, to their original inventors, to support
them by additional evidence, and to apply them to a great number of
diversified facts, which have hitherto been buried in obscurity.
Nor is it absolutely necessary in this instance to produce a single
new experiment; for of experiments there is already an ample store,
which are so much the more unexceptionable, as they must have been
conducted without the least partiality for the system by which they
will be explained; yet some facts, hitherto unobserved, will be brought
forwards, in order to show the perfect agreement of that system with
the multifarious phenomena of nature.

The optical observations of NEWTON are yet unrivalled; and, excepting
some casual inaccuracies, they only rise in our estimation, as we
compare them with later attempts to improve on them. A further
consideration of the colours of thin plates, as they are described in
the second book of NEWTON’s optics, has converted that prepossession
which I before entertained for the undulatory system of light, into a
very strong conviction of its truth and sufficiency; a conviction which
has been since most strikingly confirmed, by an analysis of the colours
of striated substances. The phenomena of thin plates are indeed so
singular, that their general complexion is not without great difficulty
reconcileable to any theory, however complicated, that has hitherto
been applied to them; and some of the principal circumstances have
never been explained by the most gratuitous assumptions; but it will
appear, that the minutest particulars of these phenomena, are not only
perfectly consistent with the theory which will now be detailed, but
that they are all the necessary consequences of that theory, without
any auxiliary suppositions; and this by inferences so simple, that
they become particular corollaries, which scarcely require a distinct
enumeration.

A more extensive examination of NEWTON’s various writings has shown
me, that he was in reality the first that suggested such a theory as
I shall endeavour to maintain; that his own opinions varied less from
this theory than is now almost universally supposed; and that a variety
of arguments have been advanced, as if to confute him, which may be
found nearly in a similar form in his own works; and this by no less a
mathematician than LEONARD EULER, whose system of light, as far as it
is worthy of notice, either was, or might have been, wholly borrowed
from NEWTON, HOOKE, HUYGENS, and MALEBRANCHE.

Those who are attached, as they may be with the greatest justice, to
every doctrine which is stamped with the NEWTONIAN approbation, will
probably be disposed to bestow on these considerations so much the
more of their attention, as they appear to coincide more nearly with
NEWTON’s own opinions. For this reason, after having briefly stated
each particular position of my theory, I shall collect, from NEWTON’s
various writings, such passages as seem to be the most favourable to
its admission; and, although I shall quote some papers which may be
thought to have been partly retracted at the publication of the optics,
yet I shall borrow nothing from them that can be supposed to militate
against his maturer judgment.


HYPOTHESIS I.

_A luminiferous Ether pervades the Universe, rare and elastic in a high
degree._

_Passages from_ NEWTON.

“The hypothesis certainly has a much greater affinity with his own,”
that is, Dr. HOOKE’s, “hypothesis, than he seems to be aware of; the
vibrations of the ether being as useful and necessary in this, as in
his.” (Phil. Trans. Vol. VII. p. 5087. Abr. Vol. I. p. 145. Nov. 1672.)

“To proceed to the hypothesis: first, it is to be supposed therein,
that there is an ethereal medium, much of the same constitution with
air, but far rarer, subtler, and more strongly elastic.—It is not to
be supposed, that this medium is one uniform matter, but compounded,
partly of the main phlegmatic body of ether, partly of other various
ethereal spirits, much after the manner that air is compounded of
the phlegmatic body of air, intermixed with various vapours and
exhalations: for the electric and magnetic effluvia, and gravitating
principle, seem to argue such variety.” (BIRCH. Hist. of R. S. Vol.
III. p. 249. Dec. 1675.)

“Is not the heat (of the warm room) conveyed through the vacuum by the
vibrations of a much subtiler medium than air?—And is not this medium
the same with that medium by which light is refracted and reflected,
and by whose vibrations light communicates heat to bodies, and is put
into fits of easy reflection, and easy transmission? And do not the
vibrations of this medium in hot bodies, contribute to the intenseness
and duration of their heat? And do not hot bodies communicate their
heat to contiguous cold ones, by the vibrations of this medium
propagated from them into the cold ones? And is not this medium
exceedingly more rare and subtile than the air, and exceedingly more
elastic and active? And doth it not readily pervade all bodies? And is
it not, by its elastic force, expanded through all the heavens?—May
not planets and comets, and all gross bodies, perform their motions in
this ethereal medium?—And may not its resistance be so small, as to be
inconsiderable? For instance, if this ether (for so I will call it)
should be supposed 700,000 times more elastic than our air, and above
700,000 times more rare, its resistance would be about 600,000000 times
less than that of water. And so small a resistance would scarce make
any sensible alteration in the motions of the planets, in ten thousand
years. If any one would ask how a medium can be so rare, let him tell
me—how an electric body can by friction emit an exhalation so rare
and subtile, and yet so potent?—And how the effluvia of a magnet can
pass through a plate of glass, without out resistance, and yet turn a
magnetic needle beyond the glass?” (Optics, Qu. 18, 22.)


HYPOTHESIS II.

_Undulations are excited in this Ether whenever a Body becomes
luminous._

_Scholium_. I use the word undulation, in preference to vibration,
because vibration is generally understood as implying a motion which is
continued alternately backwards and forwards, by a combination of the
momentum of the body with an accelerating force, and which is naturally
more or less permanent; but an undulation is supposed to consist in a
vibratory motion, transmitted successively through different parts of a
medium, without any tendency in each particle to continue its motion,
except in consequence of the transmission of succeeding undulations,
from a distinct vibrating body; as, in the air, the vibrations of a
chord produce the undulations constituting sound.

_Passages from_ NEWTON.

“Were I to assume an hypothesis, it should be this, if propounded more
generally, so as not to determine what light is, further than that it
is something or other capable of exciting vibrations in the ether; for
thus it will become so general and comprehensive of other hypotheses,
as to leave little room for new ones to be invented.” (BIRCH. Vol. III.
p. 249. Dec. 1675.)

“In the second place, it is to be supposed, that the ether is a
vibrating medium like air, only the vibrations far more swift and
minute; those of air, made by a man’s ordinary voice, succeeding one
another at more than half a foot, or a foot distance; but those of
ether at a less distance than the hundred thousandth part of an inch.
And, as in air the vibrations are some larger than others, but yet all
equally swift, (for in a ring of bells the sound of every tone is heard
at two or three miles distance, in the same order that the bells are
struck,) so, I suppose, the ethereal vibrations differ in bigness, but
not in swiftness. Now, these vibrations, beside their use in reflection
and refraction, may be supposed the chief means by which the parts of
fermenting or putrifying substances, fluid liquors, or melted, burning,
or other hot bodies, continue in motion.” (BIRCH Vol. III. p. 251. Dec.
1675.)

“When a ray of light falls upon the surface of any pellucid body,
and is there refracted or reflected, may not waves of vibrations, or
tremors, be thereby excited in the refracting or reflecting medium?—And
are not these vibrations propagated from the point of incidence to
great distances? And do they not overtake the rays of light, and by
overtaking them successively, do not they put them into the fits of
easy reflection and easy transmission described above?” (Optics, Qu.
17.)

“Light is in fits of easy reflection and easy transmission, before its
incidence on transparent bodies. And probably it is put into such fits
at its first emission from luminous bodies and continues in them during
all its progress.” (Optics. Second Book. Part III. Prop. 13.)


HYPOTHESIS III.

_The Sensation of different Colours depends on the different frequency
of Vibrations, excited by Light in the Retina._

_Passages from_ NEWTON.

“The objector’s hypothesis, as to the fundamental part of it, is not
against me. That fundamental supposition is, that the parts of bodies,
when briskly agitated, do excite vibrations in the ether, which are
propagated every way from those bodies in straight lines, and cause a
sensation of light by beating and dashing against the bottom of the
eye, something after the manner that vibrations in the air cause a
sensation of sound by beating against the organs of hearing. Now, the
most free and natural application of this hypothesis to the solution
of phenomena, I take to be this: that the agitated parts of bodies,
according to their several sizes, figures, and motions, do excite
vibrations in the ether of various depths or bignesses, which, being
promiscuously propagated through that medium to our eyes, effect in us
a sensation of light of a white colour; but if by any means those of
unequal bignesses be separated from one another, the largest beget a
sensation of a red colour, the least or shortest of a deep violet, and
the intermediate ones of intermediate colours; much after the manner
that bodies, according to their several sizes, shapes, and motions,
excite vibrations in the air of various bignesses, which, according
to those bignesses, make several tones in sound: that the largest
vibrations are best able to overcome the resistance of a refracting
superficies, and so break through it with least refraction; whence the
vibrations of several bignesses, that is, the rays of several colours,
which are blended together in light, must be parted from one another by
refraction, and so cause the phenomena of prisms, and other refracting
substances; and that it depends on the thickness of a thin transparent
plate or bubble, whether a vibration shall be reflected at its further
superficies, or transmitted; so that, according to the number of
vibrations, interceding the two superficies, they may be reflected or
transmitted for many successive thicknesses. And, since the vibrations
which make blue and violet, are supposed shorter than those which make
red and yellow, they must be reflected at a less thickness of the
plate: which is sufficient to explicate all the ordinary phenomena of
those plates or bubbles, and also of all natural bodies, whose parts
are like so many fragments of such plates. These seem to be the most
plain, genuine, and necessary conditions of this hypothesis. And they
agree so justly with my theory, that if the animadversor think fit to
apply them, he need not, on that account, apprehend a divorce from it.
But yet, how he will defend it from other difficulties, I know not.”
(Phil. Trans. Vol. VII. p. 5088. Abr. Vol. I. p. 145. Nov. 1672.)

“To explain colours, I suppose, that as bodies of various sizes,
densities, or sensations, do by percussion or other action excite
sounds of various tones, and consequently vibrations in the air
of different bigness; so the rays of light, by impinging on the
stiff refracting superficies, excite vibrations in the ether,—of
various bigness; the biggest, strongest, or most potent rays, the
largest vibrations; and others shorter, according to their bigness,
strength, or power: and therefore the ends of the capillamenta of the
optic nerve, which pave or face the retina, being such refracting
superficies, when the rays impinge upon them, they must there excite
these vibrations, which vibrations (like those of sound in a trunk or
trumpet) will run along the aqueous pores or crystalline pith of the
capillamenta, through the optic nerves, into the sensorium;—and there,
I suppose, affect the sense with various colours, according to their
bigness and mixture; the biggest with the strongest colours, reds and
yellows; the least with the weakest, blues and violets; the middle
with green; and a confusion of all with white, much after the manner
that, in the sense of hearing, nature makes use of aerial vibrations of
several bignesses, to generate sounds of divers tones; for the analogy
of nature is to be observed.” (BIRCH Vol. III. p. 262. Dec. 1675.)

“Considering the lastingness of the motions excited in the bottom of
the eye by light, are they not of a vibrating nature?—Do not the most
refrangible rays excite the shortest vibrations,—the least refrangible
the largest? May not the harmony and discord of colours arise from the
proportions of the vibrations propagated through the fibres of the
optic nerve into the brain, as the harmony and discord of sounds arise
from the proportions of the vibrations of the air?” (Optics, Qu. 16,
13, 14.)

_Scholium_. Since, for the reason here assigned by NEWTON, it is
probable that the motion of the retina is rather of a vibratory than of
an undulatory nature, the frequency of the vibrations must be dependent
on the constitution of this substance. Now, as it is almost impossible
to conceive each sensitive point of the retina to contain an infinite
number of particles, each capable of vibrating in perfect unison with
every possible undulation, it becomes necessary to suppose the number
limited, for instance, to the three principal colours, red, yellow,
and blue, of which the undulations are related in magnitude nearly as
the numbers 8, 7, and 6; and that each of the particles is capable of
being put in motion less or more forcibly, by undulations differing
less or more from a perfect unison; for instance, the undulations of
green light being nearly in the ratio of 6-1/2, will affect equally the
particles in unison with yellow and blue, and produce the same effect
as a light composed of those two species: and each sensitive filament
of the nerve may consist of three portions, one for each principal
colour. Allowing this statement, it appears that any attempt to produce
a musical effect from colours, must be unsuccessful, or at least that
nothing more than a very simple melody could be imitated by them; for
the period, which in fact constitutes the harmony of any concord, being
a multiple of the periods of the single undulations, would in this case
be wholly without the limits of sympathy of the retina, and would lose
its effect; in the same manner as the harmony of a third or a fourth is
destroyed, by depressing it to the lowest notes of the audible scale.
In hearing, there seems to be no permanent vibration of any part of the
organ.


HYPOTHESIS IV.

_All material Bodies have an Attraction for the ethereal Medium, by
means of which it is accumulated within their Substance, and for a
small Distance around them, in a State of greater Density, but not of
greater Elasticity._

It has been shewn, that the three former hypotheses, which may be
called essential, are literally parts of the more complicated NEWTONIAN
system. This fourth hypothesis differs perhaps in some degree from
any that have been proposed by former authors, and is diametrically
opposite to that of NEWTON; but, both being in themselves equally
probable, the opposition is merely accidental; and it is only to be
inquired which is the best capable of explaining the phenomena. Other
suppositions might perhaps be substituted for this, and therefore I do
not consider it as fundamental, yet it appears to be the simplest and
best of any that have occurred to me.


PROPOSITION I.

_All Impulses are propagated in a homogeneous elastic Medium with an
equable Velocity._

Every experiment relative to sound coincides with the observation
already quoted from NEWTON, that all undulations are propagated
through the air with equal velocity; and this is further confirmed
by calculations. (LAGRANGE. Misc. Taur. Vol. I. p. 91. Also, much
more concisely, in my Syllabus of a course of Lectures on Natural and
Experimental Philosophy, about to be published. Article 289.) If the
impulse be so great as materially to disturb the density of the medium,
it will be no longer homogeneous; but, as far as concerns our senses,
the quantity of motion may be considered as infinitely small. It is
surprising that EULER, although aware of the matter of fact, should
still have maintained, that the more frequent undulations are more
rapidly propagated. (Theor. mus. and Conject. phys.) It is possible,
that the actual velocity of the particles of the luminiferous ether may
bear a much less proportion to the velocity of the undulations than in
sound; for light may be excited by the motion of a body moving at the
rate of only one mile in the time that light moves a hundred millions.

_Scholium_ 1. It has been demonstrated, that in different mediums the
velocity varies in the subduplicate ratio of the force directly, and of
the density inversely. (Misc. Taur. Vol. I. p. 91. YOUNG’s Syllabus.
Art. 294.)

_Scholium_ 2. It is obvious, from the phenomena of elastic bodies
and of sounds, that the undulations may cross each other without
interruption. But there is no necessity that the various colours of
white light should intermix their undulations; for, supposing the
vibrations of the retina to continue but a five hundredth of a second
after their excitement, a million undulations of each of a million
colours may arrive in distinct succession within this interval of time,
and produce the same sensible effect, as if all the colours arrived
precisely at the same instant.


PROPOSITION II.

_An Undulation conceived to originate from the Vibration of a single
Particle, must expand through a homogeneous Medium in a spherical Form,
but with different quantities of Motion in different Parts._

For, since every impulse, considered as positive or negative, is
propagated with a constant velocity, each part of the undulation
must in equal times have past through equal distances from the
vibrating point. And, supposing the vibrating particle, in the course
of its motion, to proceed forwards to a small distance in a given
direction, the principal strength of the undulation will naturally
be straight before it; behind it, the motion will be equal, in a
contrary direction; and, at right angles to the line of vibration, the
undulation will be evanescent.

Now, in order that such an undulation may continue its progress
to any considerable distance, there must be in each part of it, a
tendency to preserve its own motion in a right line from the centre;
for, if the excess of force at any part were communicated to the
neighbouring particles, there can be no reason why it should not
very soon be equalised throughout, or, in other words, become wholly
extinct, since the motions in contrary directions would naturally
destroy each other. The origin of sound from the vibration of a
chord is evidently of this nature; on the contrary, in a circular
wave of water, every part is at the same instant either elevated or
depressed. It may be difficult to show mathematically, the mode in
which this inequality of force is preserved; but the inference from
the matter of fact, appears to be unavoidable; and, while the science
of hydrodynamics is so imperfect that we cannot even solve the simple
problem of the time required to empty a vessel by a given aperture,
it cannot be expected that we should be able to account perfectly for
so complicated a series of phenomena, as those of elastic fluids.
The theory of HUYGENS indeed explains the circumstance in a manner
tolerably satisfactory: he supposes every particle of the medium to
propagate a distinct undulation in all directions; and that the general
effect is only perceptible where a portion of each undulation conspires
in direction at the same instant; and it is easy to show that such
a general undulation would in all cases proceed rectilinearly, with
proportionate force; but, upon this supposition, it seems to follow,
that a greater quantity of force must be lost by the divergence of the
partial undulations, than appears to be consistent with the propagation
of the effect to any considerable distance. Yet it is obvious, that
some such limitation of the motion must naturally be expected to take
place; for, if the intensity of the motion of any particular part,
instead of continuing to be propagated straight forwards, were supposed
to affect the intensity of a neighbouring part of the undulation,
an impulse must then have travelled from an internal to an external
circle in an oblique direction, in the same time as in the direction
of the radius, and consequently with a greater velocity; against
the first proposition. In the case of water, the velocity is by no
means so rigidly limited as in that of an elastic medium. Yet it is
not necessary to suppose, nor is it indeed probable, that there is
absolutely not the least lateral communication of the force of the
undulation, but that, in highly elastic mediums, this communication is
almost insensible. In the air, if a chord be perfectly insulated, so
as to propagate exactly such vibrations as have been described, they
will in fact be much less forcible than if the chord be placed in the
neighbourhood of a sounding board, and probably in some measure because
of this lateral communication of motions of an opposite tendency.
And the different intensity of different parts of the same circular
undulation may be observed, by holding a common tuning fork at arm’s
length, while sounding, and turning it, from a plane directed to the
ear, into a position perpendicular to that plane.


PROPOSITION III.

_A Portion of a spherical Undulation, admitted through an Aperture into
a quiescent Medium, will proceed to be further propagated rectilinearly
in concentric Superficies, terminated laterally by weak and irregular
Portions of newly diverging Undulations._

At the instant of admission, the circumference of each of the
undulations may be supposed to generate a partial undulation, filling
up the nascent angle between the radii and the surface terminating the
medium; but no sensible addition will be made to its strength by a
divergence of motion from any other parts of the undulation, for want
of a coincidence in time, as has already been explained with respect
to the various force of a spherical undulation. If indeed the aperture
bear but a small proportion to the breadth of an undulation, the newly
generated undulation may nearly absorb the whole force of the portion
admitted; and this is the case considered by NEWTON in the Principia.
But no experiment can be made under these circumstances with light, on
account of the minuteness of its undulations, and the interference of
inflection; and yet some faint radiations do actually diverge beyond
any probable limits of inflection, rendering the margin of the aperture
distinctly visible in all directions; these are attributed by NEWTON to
some unknown cause, distinct from inflection; (Optics, Third Book, Obs.
5.) and they fully answer the description of this proposition.

Let the concentric lines in Fig. 1. (Plate I.) represent the
contemporaneous situation of similar parts of a number of successive
undulations diverging from the point A; they will also represent the
successive situations of each individual undulation: let the force
of each undulation be represented by the breadth of the line, and
let the cone of light ABC be admitted through the aperture BC; then
the principal undulations will proceed in a rectilinear direction
towards GH, and the faint radiations on each side will diverge from
B and C as centres, without receiving any additional force from any
intermediate point D of the undulation, on account of the inequality of
the lines DE and DF. But, if we allow some little lateral divergence
from the extremities of the undulations, it must diminish their
force, without adding materially to that of the dissipated light;
and their termination, instead of the right line BG, will assume the
form CH; since the loss of force must be more considerable near to C
than at greater distances. This line corresponds with the boundary
of the shadow in NEWTON’s first observation, Fig. 1; and it is much
more probable that such a dissipation of light was the cause of the
increase of the shadow in that observation, than that it was owing to
the action of the inflecting atmosphere, which must have extended a
thirtieth of an inch each way in order to produce it; especially when
it is considered that the shadow was not diminished by surrounding
the hair with a denser medium than air, which must in all probability
have weakened and contracted its inflecting atmosphere. In other
circumstances, the lateral divergence might appear to increase, instead
of diminishing, the breadth of the beam.

As the subject of this proposition has always been esteemed the most
difficult part of the undulatory system, it will be proper to examine
here the objections which NEWTON has grounded upon it.

“To me, the fundamental supposition itself seems impossible; namely,
that the waves or vibrations of any fluid can, like the rays of
light, be propagated in straight lines, without a continual and very
extravagant spreading and bending every way into the quiescent medium,
where they are terminated by it. I mistake, if there be not both
experiment and demonstration to the contrary.” (Phil. Trans. VII. 5089,
Abr. I. 146. Nov. 1672.)

“Motus omnis per fluidum propagatus divergit a recto tramite in spatia
immota.”

“Quoniam medium ibi,” in the middle of an undulation admitted,
“densius est, quam in spatiis hinc inde, dilatabit sese tam versus
spatia utrinque sita, quam versus pulsuum rariora intervalla; eoque
pacto—pulsus eadem _fere_ celeritate sese in medii partes quiescentes
hinc inde relaxare debent;—ideoque spatium totum occupabunt.—Hoc
experimur in sonis.” (Princip. Lib. II. Prop. 42.)

“Are not all hypotheses erroneous, in which light is supposed to
consist in pression or motion, propagated through a fluid medium?—If
it consisted in pression or motion, propagated either in an instant,
or in time, it would bend into the shadow. For pression or motion
cannot be propagated in a fluid in right lines beyond an obstacle which
stops part of the motion, but will bend and spread every way into the
quiescent medium which lies beyond the obstacle.—The waves on the
surface of stagnating water, passing by the sides of a broad obstacle
which stops part of them, bend afterwards, and dilate themselves
gradually into the quiet water behind the obstacle. The waves, pulses,
or vibrations of the air, wherein sounds consist, bend manifestly,
though not so much as the waves of water. For a bell or a cannon may be
heard beyond a hill, which intercepts the sight of the sounding body;
and sounds are propagated as readily through crooked pipes as straight
ones. But light is never known to follow crooked passages, nor to bend
into the shadow. For the fixed stars, by the interposition of any of
the planets, cease to be seen. And so do the parts of the sun, by the
interposition of the moon, Mercury, or Venus. The rays which pass very
near to the edges of any body, are bent a little by the action of the
body;—but this bending is not towards but from the shadow, and is
performed only in the passage of the ray by the body, and at a very
small distance from it. So soon as the ray is past the body, it goes
right on.” (Optics, Qu. 28.)

Now the proposition quoted from the Principia does not directly
contradict this proposition; for it does not assert that such a motion
must diverge equally in all directions; neither can it with truth be
maintained, that the parts of an elastic medium communicating any
motion, must propagate that motion equally in all directions. (Phil.
Trans. for 1800. p. 109—112.) All that can be inferred by reasoning is,
that the marginal parts of the undulation must be somewhat weakened,
and that there must be a faint divergence in every direction; but
whether either of these effects might be of sufficient magnitude
to be sensible, could not have been inferred from argument, if the
affirmative had not been rendered probable by experiment.

As to the analogy with other fluids, the most natural inference from
it is this: “The waves of the air, wherein sounds consist, bend
manifestly, though not so much as the waves of water;” water being an
inelastic, and air a moderately elastic medium; but ether being most
highly elastic, its waves bend very far less than those of the air, and
therefore almost imperceptibly. Sounds are propagated through crooked
passages, because their sides are capable of reflecting sound, just as
light would be propagated through a bent tube, if perfectly polished
within.

The light of a star is by far too weak to produce, by its faint
divergence, any visible illumination of the margin of a planet
eclipsing it; and the interception of the sun’s light by the moon,
is as foreign to the question, as the statement of inflection is
inaccurate.

To the argument adduced by HUYGENS, in favour of the rectilinear
propagation of undulations, NEWTON has made no reply; perhaps because
of his own misconception of the nature of the motions of elastic
mediums, as dependent on a peculiar law of vibration, which has been
corrected by later mathematicians. (Phil. Trans. for 1800, p. 116.)
On the whole, it is presumed, that this proposition may be safely
admitted, as perfectly consistent with analogy and with experiment.


PROPOSITION IV.

_When an Undulation arrives at a Surface which is the Limit of Mediums
of different Densities, a partial Reflection takes place, proportionate
in Force to the Difference of the Densities._

This may be illustrated, if not demonstrated, by the analogy of
elastic bodies of different sizes. “If a smaller elastic body strikes
against a larger one, it is well known that the smaller is reflected
more or less powerfully, according to the difference of their
magnitudes: thus, there is always a reflection when the rays of light
pass from a rarer to a denser stratum of ether; and frequently an echo
when a sound strikes against a cloud. A greater body striking a smaller
one, propels it, without losing all its motion: thus, the particles
of a denser stratum of ether, do not impart the whole of their motion
to a rarer, but, in their effort to proceed, they are recalled by the
attraction of the refracting substance with equal force; and thus a
reflection is always secondarily produced, when the rays of light pass
from a denser to a rarer stratum.” (Phil. Trans. for 1800. p. 127.) But
it is not absolutely necessary to suppose an attraction in the latter
case, since the effort to proceed would be propagated backwards without
it, and the undulation would be reversed, a rarefaction returning in
place of a condensation; and this will perhaps be found most consistent
with the phenomena.


PROPOSITION V.

_When an Undulation is transmitted through a Surface terminating
different Mediums, it proceeds in such a Direction, that the Sines of
the Angles of Incidence and Refraction are in the constant Ratio of the
Velocity of Propagation in the two Mediums._

(BARROW, Lect. Opt. II. p. 4. HUYGENS, _de la Lum._ cap. 3. EULER,
_Conj. Phys._ Phil. Trans. for 1800, p. 128. YOUNG’s Syllabus. Art.
382.)

_Corollary_ 1. The same demonstrations prove the equality of the angles
of reflection and incidence.

_Corollary_ 2. It appears from experiments on the refraction of
condensed air, that the ratio of the difference of the sines varies
simply as the density. Hence it follows, by Schol. I. Prop. I. that
the excess of the density of the ethereal medium is in the duplicate
ratio of the density of the air; each particle cooperating with its
neighbours in attracting a greater portion of it.


PROPOSITION VI.

_When an Undulation falls on the Surface of a rarer Medium, so
obliquely that it cannot be regularly refracted, it is totally
reflected, at an Angle equal to that of its Incidence._ (Phil. Trans.
for 1800, p. 128.)

_Corollary._ This phenomenon tends to prove the gradual increase
and diminution of density at the surface terminating two mediums, as
supposed in HYPOTHESIS IV; although HUYGENS has attempted to explain it
somewhat differently.


PROPOSITION VII.

_If equidistant Undulations be supposed to pass through a Medium,
of which the Parts are susceptible of permanent Vibrations somewhat
slower than the Undulations, their Velocity will be somewhat lessened
by this vibratory Tendency; and, in the same Medium, the more, as the
Undulations are more frequent._

For, as often as the state of the undulation requires a change in the
actual motion of the particle which transmits it, that change will
be retarded by the propensity of the particle to continue its motion
somewhat longer: and this retardation will be more frequent, and more
considerable, as the difference between the periods of the undulation
and of the natural vibration is greater.

_Corollary._ It was long an established opinion, that heat consists
in vibrations of the particles of bodies, and is capable of being
transmitted by undulations through an apparent vacuum. (NEWT. Opt. Qu.
18.) This opinion has been of late very much abandoned. Count RUMFORD,
Professor PICTET, and Mr. DAVY, are almost the only authors who have
appeared to favour it; but it seems to have been rejected without any
good grounds, and will probably very soon recover its popularity.

Let us suppose that these vibrations are less frequent than those of
light; all bodies therefore are liable to permanent vibrations slower
than those of light; and indeed almost all are liable to luminous
vibrations, either when in a state of ignition, or in the circumstances
of solar phosphori; but much less easily, and in a much less degree,
than to the vibrations of heat. It will follow from these suppositions,
that the more frequent luminous undulations will be more retarded than
the less frequent; and consequently, that blue light will be more
refrangible than red, and radiant heat least of all; a consequence
which coincides exactly with the highly interesting experiments of
Dr. HERSCHEL. (Phil. Trans. for 1800. p. 284.) It may also be easily
conceived, that the actual existence of a state of slower vibration
may tend still more to retard the more frequent undulations, and that
the refractive power of solid bodies may be sensibly increased by an
increase of temperature, as it actually appears to have been in EULER’s
experiments. (Acad. de Berlin. 1762. p. 328.)

_Scholium_. If, notwithstanding, this proposition should appear to be
insufficiently demonstrated, it must be allowed to be at least equally
explanatory of the phenomena with any thing that can be advanced on
the other side, from the doctrine of projectiles; since a supposed
accelerating force must act in some other proportion than that of the
bulk of the particles; and, if we call this an elective attraction,
it is only veiling under a chemical term, our incapacity of assigning
a mechanical cause. Mr. SHORT, when he found by observation the
equality of the velocity of light of all colours, felt the objection
so forcibly, that he immediately drew an inference from it in favour
of the undulatory system. It is assumed in the proposition, that when
light is dispersed by refraction, the corpuscles of the refracting
substance are in a state of actual alternate motion, and contribute to
its transmission; but it must be confessed, that we cannot at present
form a very decided and accurate conception of the forces concerned in
maintaining these corpuscular vibrations.


PROPOSITION VIII.

_When two Undulations, from different Origins, coincide either
perfectly or very nearly in Direction, their joint effect is a
Combination of the Motions belonging to each._

Since every particle of the medium is affected by each undulation,
wherever the directions coincide, the undulations can proceed no
otherwise than by uniting their motions, so that the joint motion
may be the sum or difference of the separate motions, accordingly as
similar or dissimilar parts of the undulations are coincident.

I have, on a former occasion, insisted at large on the application of
this principle to harmonics; (Phil. Trans. for 1800. p. 130.) and it
will appear to be of still more extensive utility in explaining the
phenomena of colours. The undulations which are now to be compared
are those of equal frequency. When the two series coincide exactly in
point of time, it is obvious that the united velocity of the particular
motions must be greatest, and, in effect at least, double the separate
velocities; and also, that it must be smallest, and if the undulations
are of equal strength, totally destroyed, when the time of the greatest
direct motion belonging to one undulation coincides with that of the
greatest retrograde motion of the other. In intermediate states, the
joint undulation will be of intermediate strength; but by what laws
this intermediate strength must vary, cannot be determined without
further data. It is well known that a similar cause produces in sound,
that effect which is called a beat; two series of undulations of nearly
equal magnitude cooperating and destroying each other alternately, as
they coincide more or less perfectly in the times of performing their
respective motions.

COROLLARY I. _Of the Colours of striated Surfaces._

BOYLE appears to have been the first that observed the colours of
scratches on polished surfaces. NEWTON has not noticed them. MAZEAS and
Mr. BROUGHAM have made some experiments on the subject, yet without
deriving any satisfactory conclusion. But all the varieties of these
colours are very easily deduced from this proposition.

Let there be in a given plane two reflecting points very near each
other, and let the plane be so situated that the reflected image of
a luminous object seen in it may appear to coincide with the points;
then it is obvious that the length of the incident and reflected ray,
taken together, is equal with respect to both points, considering them
as capable of reflecting in all directions. Let one of the points
be now depressed below the given plane; then the whole path of the
light reflected from it, will be lengthened by a line which is to the
depression of the point as twice the cosine of incidence to the radius.
Fig. 2.

If, therefore, equal undulations of given dimensions be reflected
from two points, situated near enough to appear to the eye but as one,
wherever this line is equal to half the breadth of a whole undulation,
the reflection from the depressed point will so interfere with the
reflection from the fixed point, that the progressive motion of the one
will coincide with the retrograde motion of the other, and they will
both be destroyed; but, when this line is equal to the whole breadth
of an undulation, the effect will be doubled; and when to a breadth
and a half, again destroyed; and thus for a considerable number of
alternations; and, if the reflected undulations be of different kinds,
they will be variously affected, according to their proportions to the
various length of the line which is the difference between the lengths
of their two paths, and which may be denominated the interval of
retardation.

In order that the effect may be the more perceptible, a number of pairs
of points must be united into two parallel lines; and, if several such
pairs of lines be placed near each other, they will facilitate the
observation. If one of the lines be made to revolve round the other as
an axis, the depression below the given plane will be as the sine of
the inclination; and, while the eye and luminous object remain fixed,
the difference of the length of the paths will vary as this sine.

The best subjects for the experiment are Mr. COVENTRY’s exquisite
micrometers; such of them as consist of parallel lines drawn on
glass, at the distance of one five hundredth of an inch, are the most
convenient. Each of these lines appears under a microscope to consist
of two or more finer lines, exactly parallel, and at the distance of
somewhat more than a twentieth, of that of the adjacent lines. I placed
one of these so as to reflect the sun’s light at an angle of 45°, and
fixed it in such a manner, that while it revolved round one of the
lines as an axis, I could measure its angular motion; and I found, that
the brightest red colour occurred at the inclinations 10-1/4°, 20-1/4°,
32°, and 45°; of which the sines are as the numbers 1, 2, 3, and 4.
At all other angles also, when the sun’s light was reflected from the
surface, the colour vanished with the inclination, and was equal at
equal inclinations on either side.

This experiment affords a very strong confirmation of the theory.
It is impossible to deduce any explanation of it from any hypothesis
hitherto advanced; and I believe it would be difficult to invent any
other that would account for it. There is a striking analogy between
this separation of colours, and the production of a musical note by
successive echoes from equidistant iron palisades; which I have found
to correspond pretty accurately with the known velocity of sound, and
the distances of the surfaces.

It is not improbable that the colours of the integuments of some
insects, and of some other natural bodies, exhibiting in different
lights the most beautiful versatility, may be found to be of this
description, and not to be derived from thin plates. In some cases, a
single scratch or furrow may produce similar effects, by the reflection
of its opposite edges.

COROLLARY II. _Of the Colours of thin Plates_.

When a beam of light falls on two parallel refracting surfaces, the
partial reflections coincide perfectly in direction; and, in this case,
the interval of retardation, taken between the surfaces, is to their
distance as twice the cosine of the angle of refraction to the radius.
For, in Fig. 3, drawing AB and CD perpendicular to the rays, the times
of passing through BC and AD will be equal, and DE will be half the
interval of retardation; but DE is to CE as the sine of DCE to the
radius. Hence, that DE may be constant, or that the same colour may be
reflected, the thickness CE must vary as the secant of the angle of
refraction CED: which agrees exactly with NEWTON’s experiments; for the
correction is perfectly inconsiderable.

Let the medium between the surfaces be rarer than the surrounding
mediums; then the impulse reflected at the second surface, meeting a
subsequent undulation at the first, will render the particles of the
rarer medium capable of wholly stopping the motion of the denser, and
destroying the reflection, (PROP. IV.) while they themselves will
be more strongly propelled than if they had been at rest; and the
transmitted light will be increased. So that the colours by reflection
will be destroyed, and those by transmission rendered more vivid, when
the double thicknesses, or intervals of retardation, are any multiples
of the whole breadths of the undulations; and, at intermediate
thicknesses the effects will be reversed; according to the NEWTONIAN
observations.

If the same proportions be found to hold good with respect to thin
plates of a denser medium, which is indeed not improbable, it will be
necessary to adopt the corrected demonstration of PROP. IV. but, at
any rate, if a thin plate be interposed between a rarer and a denser
medium, the colours by reflection and transmission may be expected to
change places.

From NEWTON’s measures of the thicknesses reflecting the different
colours, the breadth and duration of their respective undulations
may be very accurately determined; although it is not improbable,
that when the glasses approach very near, the atmosphere of ether may
produce some little irregularity. The whole visible spectrum appears
to be comprised within the ratio of three to five, or a major sixth in
music; and the undulations of red, yellow, and blue, to be related in
magnitude as the numbers 8, 7, and 6; so that the interval from red
to blue is a fourth. The absolute frequency expressed in numbers is
too great to be distinctly conceived, but it may be better imagined by
a comparison with sound. If a chord sounding the tenor ̅/c, could be
continually bisected 40 times, and should then vibrate, it would afford
a yellow green light: this being denoted by 41/c, the extreme red would
be 40/a, and the blue 41/d. The absolute length and frequency of each
vibration is expressed in the table; supposing light to travel in 8-1/8
minutes 500,000,000000 feet.

  +---------------+--------------+-----------+-------------------------+
  |               | Length of an | Number of |                         |
  |   Colours.    |  Undulation  |Undulations|  Number of Undulations  |
  |               |in parts of an|in an Inch.|      in a second.       |
  |               | Inch, in Air.|           |                         |
  +---------------+--------------+-----------+-------------------------+
  |  Extreme -    |   .0000266   |   37640   |463 millions of millions |
  |Red   -   -    |   .0000256   |   39180   |482                      |
  |  Intermediate |   .0000246   |   40720   |501                      |
  |Orange    -    |   .0000240   |   41610   |512                      |
  |  Intermediate |   .0000235   |   42510   |523                      |
  |Yellow    -    |   .0000227   |   44000   |542                      |
  |  Intermediate |   .0000219   |   45600   |561  (= 2^{48} nearly)   |
  |Green -   -    |   .0000211   |   47460   |584                      |
  |  Intermediate |   .0000203   |   49320   |607                      |
  |Blue  -   -    |   .0000196   |   51110   |629                      |
  |  Intermediate |   .0000189   |   52910   |652                      |
  |Indigo    -    |   .0000185   |   54070   |665                      |
  |  Intermediate |   .0000181   |   55240   |680                      |
  |Violet    -    |   .0000174   |   57490   |707                      |
  |  Extreme -    |   .0000167   |   59750   |735                      |
  +---------------+--------------+-----------+-------------------------+

_Scholium_. It was not till I had satisfied myself respecting all
these phenomena, that I found in HOOKE’s Micrographia, a passage which
might have led me earlier to a similar conclusion. “It is most evident
that the reflection from the under or further side of the body, is the
principal cause of the production of these colours....Let the ray fall
obliquely on the thin plate, part therefore is reflected back by the
first superficies,...part refracted to the second surface,...whence it
is reflected and refracted again.... So that, after two refractions and
one reflection, there is propagated a kind of fainter ray,...and, by
reason of the time spent in passing and repassing,...this fainter pulse
comes behind the former reflected pulse; so that hereby, (the surfaces
being so near together that the eye cannot discriminate them from one,)
this confused or duplicated pulse, whose strongest part precedes, and
whose weakest follows, does produce on the retina,...the sensation of
a yellow.... If these surfaces are further removed asunder, the weaker
pulse may become coincident with the reflection of the second, or next
following pulse, from the first surface, and lagg behind that also,
and be coincident with the third, fourth, fifth, sixth, seventh, or
eighth; so that, if there be a thin transparent body, that from the
greatest thinness requisite to produce colours, does by degrees grow
to the greatest thickness,...the colours shall be so often repeated,
as the weaker pulse does lose paces with its primary or first pulse,
and is coincident with a subsequent pulse.... And this, as it is
coincident, or follows from the first hypothesis I took of colours, so
upon experiment have I found it in multitudes of instances that seem
to prove it.” (P. 65—67.) This was printed about seven years before
any of NEWTON’s experiments were made. We are informed by Newton, that
Hooke was afterwards disposed to adopt his “suggestion” of the nature
of colours; and yet it does not appear that HOOKE ever applied that
improvement to his explanation of these phenomena, or inquired into
the necessary consequence of a change of obliquity, upon his original
supposition, otherwise he could not but have discovered a striking
coincidence with the measures laid down by Newton from experiment. All
former attempts to explain the colours of thin plates, have either
proceeded on suppositions which, like NEWTON’s, would lead us to expect
the greatest irregularities in the direction of the refracted rays;
or, like Mr. MICHELL’s, would require such effects from the change of
the angle of incidence, as are contrary to the effects observed; or
they are equally deficient with respect to both these circumstances,
and are inconsistent with the most moderate attention to the principal
phenomena.

COROLLARY III. _Of the Colours of thick Plates._

When a beam of light passes through a refracting surface, especially
if imperfectly polished, a portion of it is irregularly scattered, and
makes the surface visible in all directions, but most conspicuously in
directions not far distant from that of the light itself: and, if a
reflecting surface be placed parallel to the refracting surface, this
scattered light, as well as the principal beam, will be reflected,
and there will also be a new dissipation of light, at the return
of the beam through the refracting surface. These two portions of
scattered light will coincide in direction; and, if the surfaces be of
such a form as to collect the similar effects, will exhibit rings of
colours. The interval of retardation is here, the difference between
the paths of the principal beam and of the scattered light between the
two surfaces; of course, wherever the inclination of the scattered
light is equal to that of the beam, although in different planes, the
interval will vanish, and all the undulations will conspire. At other
inclinations, the interval will be the difference of the secants from
the secant of the inclination or angle of refraction of the principal
beam. From these causes, all the colours of concave mirrors observed
by NEWTON and others are necessary consequences: and it appears that
their production, though somewhat similar, is by no means, as NEWTON
imagined, identical with the production of those of thin plates.

COROLLARY IV. _Of Blackness._

In the three preceding corollaries, we have considered the refracting
and reflecting substances as limited by a mathematical surface; but
this is perhaps never physically true. The ethereal atmospheres may
extend on each side the surface as far as the breadth of one or more
undulations; and, if they be supposed to vary equally in density at
every part, the partial reflections from each of the infinite number
of surfaces, where the density changes, will very much interfere with
each other, and destroy a considerable portion of the reflected light,
so that the substance may become positively black; and this effect may
take place in a greater or less degree, as the density of the ethereal
atmosphere varies more or less equably; and, in some cases, particular
undulations being more affected than others, a tinge of colour may be
produced. Accordingly, M. BOUGUER has observed a considerable loss of
light, and in some instances a tinge of colour, in total reflections at
the surface of a rarer medium.

COROLLARY V. _Of Colours by Inflection._

Whatever may be the cause of the inflection of light passing through
a small aperture, the light nearest its centre must be the least
diverted, and the nearest to its sides the most: another portion of
light falling very obliquely on the margin of the aperture, will be
copiously reflected in various directions; some of which will either
perfectly or very nearly coincide in direction with the unreflected
light, and, having taken a circuitous route, will so interfere with it,
as to cause an appearance of colours. The length of the two tracks will
differ the less, as the direction of the reflected light has been less
changed by its reflection, that is, in the light passing nearest to the
margin; so that the blues will appear in the light nearest the shadow.
The effect will be increased and modified, when the reflected light
falls within the influence of the opposite edge, so as to interfere
with the light simply inflected by that also.

But, in order to examine the consequences more minutely, it will be
convenient to suppose the inflection caused by an ethereal atmosphere,
of a density varying as a given power of the distance from a centre,
as in the eighth proposition of the last Bakerian Lecture. (Phil.
Trans. for 1801, p. 83.) Putting r = 3, and x = 1/2, I have constructed
a diagram, (Fig. 4.) which shows, by the two pairs of curves, the
relative position of the reflected and unreflected portions of any
one undulation at two successive times, and also, by shaded lines
drawn across, the parts where the intervals of retardation are in
arithmetical progression, and where similar colours will be exhibited
at different distances from the inflecting substance. The result fully
agrees with the observations of NEWTON’s third book, and with those
of later writers. But I do not consider it as quite certain, until
further experiments have been made on the inflecting power of different
substances, that Dr. HOOKE’s explanation of inflection, by the tendency
of light to diverge, may not have some pretensions to truth. I am sorry
to be obliged to recall here the assent which, at first sight, I was
induced to give to a supposed improvement of a late author. (Phil.
Trans. for 1800, p. 128.)

_Scholium_. In the construction of the diagram, it becomes necessary
to find the time spent by each ray in its passage. Since the velocity
was denoted by x^{–1/r}, on the supposition of a projectile, it
will be as x^{1/r} on the contrary supposition, (Phil. Trans. for
1801, p. 27. _Schol_. 2. Prop. I.) and the fluxion of the distance
described being ẋ/sqrt(1–yy), that of the time will be x^{(–1/r)ẋ}/sqrt(1–yy) or
rs/(1–r)· ẏ/(yy·sqrt(1–yy)), of which the fluent is r/(1–r)·s/y·sqrt(1–yy).
Therefore, with the radius x^{1–(1/r)}, describe a circle concentric with
the surfaces of the inflecting atmosphere, then the angle described
by the ray during its passage through the atmosphere, will always be
to the angle subtended by the line cut off by this circle from the
incident ray produced, in the ratio of r to r–1; and the time spent in
this passage, will be in the same ratio to the time that would have
been spent in describing this intercepted portion with the initial
velocity. For y, being equal to sx^{(1/r)–1} is the sine of the
inclination of the incident ray to the radius, where it meets this
circle; therefore, by the proposition quoted, the angle described is in
a given ratio to the angle at the centre, which is the difference of
the inclinations. Making x^{1–(1/r)} or s/y radius, the sine, instead
of y, becomes s, and the cosine sqrt((ss/yy)–ss) or s/y sqrt(1–yy),
and, when y = ss, sqrt(1–ss); therefore the line intercepted is to the
difference of the fluents as r to r–1. (See also YOUNG’s Syllabus, Art.
372.)


PROPOSITION IX.

_Radiant Light consists in Undulations of the luminiferous Ether._

This proposition is the general conclusion from all the preceding;
and it is conceived that they conspire to prove it in as satisfactory
a manner as can possibly be expected from the nature of the subject.
It is clearly granted by NEWTON, that there are undulations, yet he
denies that they constitute light; but it is shown in the three first
Corollaries of the last Proposition, that all cases of the increase or
diminution of light are referable to an increase or diminution of such
undulations, and that all the affections to which the undulations would
be liable, are distinctly visible in the phenomena of light; it may
therefore be very logically inferred, that the undulations are light.

A few detached remarks will serve to obviate some objections which may
be raised against this theory.

1. NEWTON has advanced the singular refraction of the Iceland
crystal, as an argument that the particles of light must be projected
corpuscles; since he thinks it probable that the different sides of
these particles must be differently attracted by the crystal, and
since HUYGENS has confessed his inability to account in a satisfactory
manner for all the phenomena. But, contrarily to what might have been
expected from NEWTON’s usual accuracy and candour, he has laid down a
new law for the refraction, without giving a reason for rejecting that
of HUYGENS, which Mr. HAUY has found to be more accurate than NEWTON’s;
and, without attempting to deduce from his own system any explanation
of the more universal and striking effects of doubling spars, he has
omitted to observe that HUYGENS’s most elegant and ingenious theory
perfectly accords with these general effects, in all particulars, and
of course derives from them additional pretensions to truth: this he
omits, in order to point out a difficulty, for which only a verbal
solution can be found in his own theory, and which will probably long
remain unexplained by any other.

2. Mr. MICHELL has made some experiments, which appear to show that
the rays of light have an actual momentum, by means of which a motion
is produced when they fall on a thin plate of copper delicately
suspended. (PRIESTLEY’s Optics.) But, taking for granted the exact
perpendicularity of the plate, and the absence of any ascending current
of air, yet since, in every such experiment, a greater quantity of
heat must be communicated to the air at the surface on which the
light falls than at the opposite surface, the excess of expansion
must necessarily produce an excess of pressure on the first surface,
and a very perceptible recession of the plate in the direction of
the light. Mr. BENNET has repeated the experiment, with a much more
sensible apparatus, and also in the absence of air; and very justly
infers from its total failure, an argument in favour of the undulatory
system of light. (Phil. Trans. for 1792, p. 87.) For, granting the
utmost imaginable subtility of the corpuscles of light, their effects
might naturally be expected to bear some proportion to the effects of
the much less rapid motions of the electrical fluid, which are so very
easily perceptible, even in their weakest states.

3. There are some phenomena of the light of solar phosphori, which at
first sight might seem to favour the corpuscular system; for instance,
its remaining many months as if in a latent state, and its subsequent
re-emission by the action of heat. But, on further consideration, there
is no difficulty in supposing the particles of the phosphori which
have been made to vibrate by the action of light, to have this action
abruptly suspended by the intervention of cold, whether as contracting
the bulk of the substance or otherwise; and again, after the restraint
is removed, to proceed in their motion, as a spring would do which had
been held fast for a time in an intermediate stage of its vibration;
nor is it impossible that heat itself may, in some circumstances,
become in a similar manner latent. (NICHOLSON’s Journal. Vol. II. p.
399.) But the affections of heat may perhaps hereafter be rendered
more intelligible to us; at present, it seems highly probable that
light differs from heat only in the frequency of its undulations or
vibrations; those undulations which are within certain limits, with
respect to frequency, being capable of affecting the optic nerve, and
constituting light; and those which are slower, and probably stronger,
constituting heat only; that light and heat occur to us, each in
two predicaments, the vibratory or permanent, and the undulatory or
transient state; vibratory light being the minute motion of ignited
bodies, or of solar phosphori, and undulatory or radiant light the
motion of the ethereal medium excited by these vibrations; vibratory
heat being a motion to which all material substances are liable, and
which is more or less permanent; and undulatory heat that motion of the
same ethereal medium, which has been shown by Mr. KING, (Morsels of
Criticism. 1786. p. 99.) and M. PICTET, (_Essais de Physique_. 1790,)
to be as capable of reflection as light, and by Dr. HERSCHEL to be
capable of separate refraction. (Phil. Trans. for 1800. p. 284.) How
much more readily heat is communicated by the free access of colder
substances, than either by radiation or by transmission through a
quiescent medium, has been shown by the valuable experiments of Count
RUMFORD. It is easy to conceive that some substances, permeable to
light, may be unfit for the transmission of heat, in the same manner as
particular substances may transmit some kinds of light, while they are
opaque with respect to others.

On the whole it appears, that the few optical phenomena which admit
of explanation by the corpuscular system, are equally consistent
with this theory; that many others, which have long been known, but
never understood, become by these means perfectly intelligible; and
that several new facts are found to be thus only reducible to a
perfect analogy with other facts, and to the simple principles of the
undulatory system. It is presumed, that henceforth the second and third
books of NEWTON’s Optics will be considered as more fully understood
than the first has hitherto been; but, if it should appear to impartial
judges, that additional evidence is wanting for the establishment of
the theory, it will be easy to enter more minutely into the details
of various experiments, and to show the insuperable difficulties
attending the NEWTONIAN doctrines, which, without necessity, it would
be tedious and invidious to enumerate. The merits of their author in
natural philosophy, are great beyond all contest or comparison; his
optical discovery of the composition of white light, would alone have
immortalised his name; and the very arguments which tend to overthrow
his system, give the strongest proofs of the admirable accuracy of his
experiments.

Sufficient and decisive as these arguments appear, it cannot
be superfluous to seek for further confirmation; which may with
considerable confidence be expected, from an experiment very
ingeniously suggested by Professor ROBISON, on the refraction of the
light returning to us from the opposite margins of Saturn’s ring; for,
on the corpuscular theory, the ring must be considerably distorted when
viewed through an achromatic prism: a similar distortion ought also to
be observed in the disc of Jupiter; but, if it be found that an equal
deviation is produced in the whole light reflected from these planets,
there can scarcely be any remaining hope to explain the affections of
light, by a comparison with the motions of projectiles.

[Illustration: Single Plate containing four Figures]



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