glass,
at a 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 a distance of somewhat more than
a twentieth more than the adjacent lines. I placed one of these so as to
reflect the sun's light at an angle of forty-five degrees, 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; I found that the longest red
color occurred at the inclination 10 1/4 degrees, 20 3/4 degrees, 32
degrees, and 45 degrees; 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 color 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 colors 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 colors 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."(3)

This doctrine of interference of undulations was the absolutely novel
part of Young's theory. The all-compassing genius of Robert Hooke had,
indeed, very nearly apprehended it more than a century before, as Young
himself points out, but no one else bad so much as vaguely conceived
it; and even with the sagacious Hooke it was only a happy guess, never
distinctly outlined in his own mind, and utterly ignored by all
others. Young did not know of Hooke's guess until he himself had fully
formulated the theory, but he hastened then to give his predecessor
all the credit that could possibly be adjudged his due by the most
disinterested observer. To Hooke's contemporary, Huygens, who was the
originator of the general doctrine of undulation as the e