ATTEMPTS TO DISCOVER THE LAWS OF OTHER PHENOMENA.
THE phenomena which result from optical combinations, even of a
comparatively simple nature, are extremely complex. The theory which
is now known accounts for these results with the most curious
exactness, and points out the laws which pervade the apparent
confusion; but without this key to the appearances, it was scarcely
possible that any rule or order should be detected. The undertaking
was of {79} the same kind as it would have been, to discover all the
inequalities of the moon's motion without the aid of the doctrine of
gravity. We will enumerate some of the phenomena which thus employed
and perplexed the cultivators of optics.
The fringes of shadows were one of the most curious and noted of
such classes of facts. These were first remarked by Grimaldi[50\9]
(1665), and referred by him to a property of light which he called
_Diffraction_. When shadows are made in a dark room, by light
admitted through a very small hole, these appearances are very
conspicuous and beautiful. Hooke, in 1672, communicated similar
observations to the Royal Society, as "a new property of light not
mentioned by any optical writer before;" by which we see that he had
not heard of Grimaldi's experiments. Newton, in his _Opticks_,
treats of the same phenomena, which he ascribes to the _inflexion_
of the rays of light. He asks (Qu. 3), "Are not the rays of light,
in passing by the edges and sides of bodies, bent several times
backward and forward with a motion like that of an eel? And do not
the three fringes of colored light in shadows arise from three such
bendings?" It is remarkable that Newton should not have noticed,
that it is impossible, in this way, to account for the facts, or
even to express their laws; since the light which produces the
fringes must, on this theory, be propagated, even after it leaves
the neighborhood of the opake body, in curves, and not in straight
lines. Accordingly, all who have taken up Newton's notion of
inflexion, have inevitably failed in giving anything like an
intelligible and coherent character to these phenomena. This is, for
example, the case with Mr. (now Lord) Brougham's attempts in the
_Philosophical Transactions_ for 1796. The same may be said of other
experimenters, as Mairan[51\9] and Du Four,[52\9] who attempted to
explain the facts by supposing an atmosphere about the opake body.
Several authors, as Maraldi,[53\9] and Comparetti,[54\9] repeated or
varied these experiments in different ways.
[Note 50\9: _Physico-Mathesis, de Lumine, Coloribus et Iride._
Bologna, 1665.]
[Note 51\9: _Ac. Par._ 1738.]
[Note 52\9: _Mémoires Présentés_, vol. v.]
[Note 53\9: _Ac. Par._ 1723.]
[Note 54\9: _Observationes Opticæ de Luce Inflexâ et Coloribus._
Padua, 1787.]
Newton had noticed certain rings of color produced by a glass
speculum, which he called "colors of thick plates," and which he
attempted to connect with the colors of thin plates. His reasoning
is by no means satisfactory; but it was of use, by pointing out this
as a case in which his "fits" (the small periods, or cycles in the
rays of light, of {80} which we have spoken) continued to occur for
a considerable length of the ray. But other persons, attempting to
repeat his experiments, confounded with them extraneous phenomena of
other kinds; as the Duc de Chaulnes, who spread muslin before his
mirror,[55\9] and Dr. Herschel, who scattered hair-powder before
his.[56\9] The colors produced by the muslin were those belonging to
shadows of _gratings_, afterwards examined more successfully by
Fraunhofer, when in possession of the theory. We may mention here
also the colors which appear on finely-striated surfaces, and on
mother-of-pearl, feathers, and similar substances. These had been
examined by various persons (as Boyle, Mazeas, Lord Brougham), but
could still, at this period, be only looked upon as insulated and
lawless facts.
[Note 55\9: _Ac. Par._ 1755.]
[Note 56\9: _Phil. Trans._ 1807.]