PRELUDE TO THE EPOCH OF YOUNG AND FRESNEL.
BY _Physical_ Optics we mean, as has already been stated, the
theories which explain optical phenomena on mechanical principles.
No such explanation could be given till true mechanical principles
had been obtained; and, accordingly, we must date the commencement
of the essays towards physical optics from Descartes, the founder of
the modern mechanical philosophy. His hypothesis concerning light
is, that it consists of small particles emitted by the luminous
body. He compares these particles to balls, and endeavors to
explain, by means of this comparison, the laws of reflection and
refraction.[62\9] In order to account for the production of colors
by refraction, he ascribes to these balls an alternating rotatory
motion.[63\9] This form of the _emission theory_, was, like most of
the physical speculations of its author, hasty and gratuitous; but
was extensively accepted, like the rest of the Cartesian doctrines,
in consequence of the love which men have for sweeping and simple
dogmas, and deductive reasonings from them. In a short time,
however, the rival optical _theory of undulations_ made its
appearance. Hooke in his _Micrographia_ (1664) propounds it, upon
occasion of his observations, already noticed, (chap. **vii.,) on the
colors of thin plates. He there asserts[64\9] light to consist in a
"quick, short, vibrating motion," and that it is propagated in a
homogeneous medium, in such a way that "every pulse or vibration of
the luminous body will generate a sphere, which will continually
increase and grow bigger, just after the same manner (though
indefinitely swifter) as the waves or rings on the surface of water
do swell into bigger and bigger circles about a point in it."[65\9]
He applies this to the explanation of refraction, {86} by supposing
that the rays in a denser medium move more easily, and hence that
the pulses become oblique; a far less satisfactory and consistent
hypothesis than that of Huyghens, of which we shall next have to
speak. But Hooke has the merit of having also combined with his
theory, though somewhat obscurely, the _Principle of Interferences_,
in the application which he makes of it to the colors of thin
plates. Thus[66\9] he supposes the light to be reflected at the
first surface of such plates; and he adds, "after two refractions
and one reflection (from the second surface) there is propagated a
kind of fainter ray," which comes behind the other reflected pulse;
"so that hereby (the surfaces AB and EF being so near together that
the eye cannot discriminate them from one), this compound or
duplicated pulse does produce on the retina the sensation of a
yellow." The reason for the production of this particular color, in
the case of which he here speaks, depends on his views concerning
the kind of pulses appropriate to each color; and, for the same
reason, when the thickness is different, he finds that the result
will be a red or a green. This is a very remarkable anticipation of
the explanation ultimately given of these colors; and we may observe
that if Hooke could have measured the thickness of his thin plates,
he could hardly have avoided making considerable progress in the
doctrine of interferences.
[Note 62\9: _Diopt._ c. ii. 4.]
[Note 63\9: _Meteor._ c. viii. 6.]
[Note 64\9: _Micrographia_, p. 56.]
[Note 65\9: _Micrographia_, p. 57.]
[Note 66\9: _Micrographia_, p. 66.]
But the person who is generally, and with justice, looked upon as
the great author of the undulatory theory, at the period now under
notice, is Huyghens, whose _Traité de la Lumière_, containing a
developement of his theory, was written in 1678, though not
published till 1690. In this work he maintained, as Hooke had done,
that light consists in undulations, and expands itself spherically,
nearly in the same manner as sound does; and he referred to the
observations of Römer on Jupiter's satellites, both to prove that
this difference takes place successively, and to show its exceeding
swiftness. In order to trace the effect of an undulation, Huyghens
considers that every point of a wave diffuses its motion in all
directions; and hence he draws the conclusion, so long looked upon
as the turning-point of the combat between the rival theories, that
the light will not be _diffused_ beyond the rectilinear space, when
it passes through an aperture; "for," says he,[67\9] "although the
_partial_ waves, produced by the particles comprised in the
aperture, do diffuse themselves beyond the rectilinear space, these
waves do not _concur_ anywhere except in front of the {87}
aperture." He rightly considers this observation as of the most
essential value. "This," he says, "was not known by those who began
to consider the waves of light, among whom are Mr. Hooke in his
_Micrography_, and Father Pardies; who, in a treatise of which he
showed me a part, and which he did not live to finish, had
undertaken to prove, by these waves, the effects of reflection and
refraction. But the principal foundation, which consists in the
remark I have just made, was wanting in his demonstrations."
[Note 67\9: _Tracts on Optics_, p. 209.]
By the help of this view, Huyghens gave a perfectly satisfactory and
correct explanation of the laws of reflection and refraction; and he
also applied the same theory, as we have seen, to the double
refraction of Iceland spar with great sagacity and success. He
conceived that in this crystal, besides the spherical waves, there
might be others of a spheroidal form, the axis of the spheroid being
symmetrically disposed with regard to the faces of the rhombohedron,
for to these faces the optical phenomena are symmetrically related.
He found[68\9] that the position of the refracted ray, determined by
such spheroidal undulations, would give an oblique refraction, which
would coincide in its laws with the refraction observed in Iceland
spar; and, as we have stated, this coincidence was long after fully
confirmed by other observers.
[Note 68\9: _Tracts on Optics_, 237.]
Since Huyghens, at this early period, expounded the undulatory
theory with so much distinctness, and applied it with so much skill,
it may be asked why we do not hold him up as the great Author of the
induction of undulations of light;--the person who marks the epoch
of the theory? To this we reply, that though Huyghens discovered
strong presumptions in favor of the undulatory theory, it was not
_established_ till a later era, when the fringes of shadows, rightly
understood, made the waves visible, and when the hypothesis which
had been assumed to account for double refraction, was found to
contain also an explanation of polarization. It is _then_ that this
theory of light assumes its commanding form; and the persons who
gave it this form, we must make the great names of our narrative;
without, however, denying the genius and merit of Huyghens, who is,
undoubtedly, the leading character in the prelude to the discovery.
The undulatory theory, from this time to our own, was unfortunate in
its career. It was by no means destitute of defenders, but these
were not experimenters; and none of them thought of applying it to
{88} Grimaldi's experiments on fringes, of which we have spoken a
little while ago. And the great authority of the period, Newton,
adopted the opposite hypothesis, that of emission, and gave it a
currency among his followers which kept down the sounder theory for
above a century.
Newton's first disposition appears to have been by no means averse
to the assumption of an ether as the vehicle of luminiferous
undulations. When Hooke brought against his prismatic analysis of
light some objections, founded on his own hypothetical notions,
Newton, in his reply, said,[69\9] "The hypothesis has a much greater
affinity with his own hypothesis than he seems to be aware of; the
vibrations of the ether being as useful and necessary in this as in
his." This was in 1672; and we might produce, from Newton's writing,
passages of the same kind, of a much later date. Indeed it would
seem that, to the last, Newton considered the assumption of an ether
as highly probable, and its vibrations important parts of the
phenomena of light; but he also introduced into his system the
hypothesis of emission, and having followed this hypothesis into
mathematical detail, while he has left all that concerns the ether
in the form of queries and conjectures, the emission theory has
naturally been treated as the leading part of his optical doctrines.
[Note 69\9: _Phil. Trans._ vii. 5087.]
The principal propositions of the _Principia_ which bear upon the
question of optical theory are those of the fourteenth Section of
the first Book,[70\9] in which the law of the sines in refraction is
proved on the hypothesis that the particles of bodies act on light
only at very small distances; and the proposition of the eighth
Section of the second Book;[71\9] in which it is pretended to be
demonstrated that the motion propagated in a fluid must diverge when
it has passed through an aperture. The former proposition shows that
the law of refraction, an optical truth which mainly affected the
choice of a theory, (for about reflection there is no difficulty on
any mechanical hypothesis,) follows from the theory of emission: the
latter proposition was intended to prove the inadmissibility of the
rival hypothesis, that of undulations. As to the former point,--the
hypothetical explanation of refraction, on the assumptions there
made,--the conclusion is quite satisfactory; but the reasoning in
the latter case, (respecting the propagation of undulations,) is
certainly inconclusive and vague; and something better might the
more reasonably have been expected, since Huyghens had at least {89}
endeavored to prove the opposite proposition. But supposing we leave
these properties, the rectilinear course, the reflection, and the
refraction of light, as problems in which neither theory has a
decided advantage, what is the next material point? The colors of
thin plates. Now, how does Newton's theory explain these? By a new
and special supposition;--that of _fits of easy transmission and
reflection_: a supposition which, though it truly expresses these
facts, is not borne out by any other phenomena. But, passing over
this, when we come to the peculiar laws of polarization in Iceland
spar, how does Newton's meet this? Again by a special and new
supposition;--that the rays of light have _sides_. Thus we find no
fresh evidence in favor of the emission hypothesis springing out of
the fresh demands made upon it. It may be urged, in reply, that the
same is true of the undulatory theory; and it must be allowed that,
at the time of which we now speak, its superiority in this respect
was not manifested; though Hooke, as we have seen, had caught a
glimpse of the explanation, which this theory supplies, of the
colors of thin plates.
[Note 70\9: _Principia_, Prop. 94, _et seq._]
[Note 71\9: Ib. Prop. 42.]
At a later period, Newton certainly seems to have been strongly
disinclined to believe light to consist in undulations merely. "Are
not," he says, in Question twenty-eight of the _Opticks_, "all
hypotheses erroneous, in which light is supposed to consist in
pression or motion propagated through a fluid medium?" The arguments
which most weighed with him to produce this conviction, appear to
have been the one already mentioned,--that, on the undulatory
hypothesis, undulations passing through an aperture would be
diffused; and again,--his conviction, that the properties of light,
developed in various optical phenomena, "depend not upon new
modifications, but upon the original and unchangeable properties of
the rays." (Question twenty-seven.)
But yet, even in this state of his views, he was very far from
abandoning the machinery of vibrations altogether. He is disposed to
use such machinery to produce his "fits of easy transmission." In
his seventeenth Query, he says,[72\9] "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 at the point of incidence? .
. . . and do not these vibrations overtake the rays of light, and by
overtaking them successively, do they not put them into the fits of
easy reflection and easy {90} transmission described above?" Several
of the other queries imply the same persuasion, of the necessity for
the assumption of an ether and its vibrations. And it might have
been asked, whether any good reason could be given for the
hypothesis of an ether as a _part_ of the mechanism of light, which
would not be equally valid in favor of this being the _whole_ of the
mechanism, especially if it could be shown that nothing more was
wanted to produce the results.
[Note 72\9: _Opticks_, p. 322.]
The emission theory was, however, embraced in the most strenuous
manner by the disciples of Newton. That propositions existed in the
_Principia_ which proceeded on this hypothesis, was, with many of
these persons, ground enough for adopting the doctrine; and it had
also the advantage of being more ready of conception, for though the
propagation of a wave is not very difficult to conceive, at least by
a mathematician, the motion of a particle is still easier.
On the other hand, the undulation theory was maintained by no less a
person than Euler; and the war between the two opinions was carried
on with great earnestness. The arguments on one side and on the
other soon became trite and familiar, for no person explained any
new class of facts by either theory. Thus it was urged by Euler
against the system of emission,[73\9]--that the perpetual emanation
of light from the sun must have diminished the mass;--that the
stream of matter thus constantly flowing must affect the motions of
the planets and comets; that the rays must disturb each other;--that
the passage of light through transparent bodies is, on this system,
inconceivable: all such arguments were answered by representations
of the exceeding minuteness and velocity of the matter of light. On
the other hand, there was urged against the theory of waves, the
favorite Newtonian argument, that on this theory the light passing
through an aperture ought to be diffused, as sound is. It is curious
that Euler does not make to this argument the reply which Huyghens
had made before. The fact really was, that he was not aware of the
true ground of the difference of the result in the cases of sound
and light; namely, that any ordinary aperture bears an immense ratio
to the length of an undulation of light, but does not bear a very
great ratio to the length of an undulation of sound. The
demonstrable consequence of this difference is, that light darts
through such an orifice in straight rays, while sound is diffused in
all directions. Euler, not perceiving this difference, rested his
answer mainly upon a circumstance by no means {91} unimportant, that
the partitions usually employed are not impermeable to sound, as
opake bodies are to light. He observes that the sound does not all
come through the aperture; for we hear, though the aperture be
stopped. These were the main original points of attack and defence,
and they continued nearly the same for the whole of the last
century; the same difficulties were over and over again proposed,
and the same solutions given, much in the manner of the disputations
of the schoolmen of the middle ages.
[Note 73\9: Fischer, iv. 449.]
The struggle being thus apparently balanced, the scale was naturally
turned by the general ascendancy of the Newtonian doctrines: and the
emission theory was the one most generally adopted. It was still
more firmly established, in consequence of the turn generally taken
by the scientific activity of the latter half of the eighteenth
century: for while nothing was added to our knowledge of optical
laws, the chemical effects of light were studied to a considerable
extent by various inquirers;[74\9] and the opinions at which these
persons arrived, they found that they could express most readily, in
consistency with the reigning chemical views, by assuming the
materiality of light. It is, however, clear, that no reasonings of
the inevitably vague and doubtful character which belong to these
portions of chemistry, ought to be allowed to interfere with the
steady and regular progress of induction and generalization, founded
on relations of space and number, by which procedure the mechanical
sciences are formed. We reject, therefore, all these chemical
speculations, as belonging to other subjects; and consider the
history of optical theory as a blank, till we arrive at some very
different events, of which we have now to speak. {92}
[Note 74\9: As Scheele, Selle, Lavoisier, De Luc, Richter,
Leonhardi, Gren, Girtanner, Link, Hagen, Voigt, De la Metherie,
Scherer, Dizé, Brugnatelli. See Fischer, vii. p. 20.]