_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]