f 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 be also 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 colors. 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
colors 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 thin plates."(2)

By following up this clew with mathematical precision, measuring the
exact thickness of the plate and the space between the different rings
of color, Young was able to show mathematically what must be the length
of pulsation for each of the different colors of the spectrum. He
estimated that the undulations of red light, at the extreme lower end
of the visible spectrum, must number about thirty-seven thousand six
hundred and forty to the inch, and pass any given spot at a rate of four
hundred and sixty-three millions of millions of undulations in a second,
while the extreme violet numbers fifty-nine thousand seven hundred and
fifty undulations to the inch, or seven hundred and thirty-five millions
of millions to the second.

The Colors of Striated Surfaces

Young similarly examined the colors that are produced by scratches on
a smooth surface, in particular testing the light from "Mr. Coventry's
exquisite micrometers," which consist of lines scratched on glass at
measured intervals. These microscopic tests brought the