same results as
the other experiments. The colors were produced at certain definite
and measurable angles, and the theory of interference of undulations
explained them perfectly, while, as Young affirmed with confidence, no
other hypothesis hitherto advanced would explain them at all. Here are
his words:

"Let there be in a given plane two reflecting points very near each
other, and let the plane be so situated that the reflected image of a
luminous object seen in it may appear to coincide with the points; then
it is obvious that the length of the incident and reflected ray, taken
together, is equal with respect to both points, considering them as
capable of reflecting in all directions. Let one of the points be
now depressed below the given plane; then the whole path of the
light reflected from it will be lengthened by a line which is to the
depression of the point as twice the cosine of incidence to the radius.

"If, therefore, equal undulations of given dimensions be reflected
from two points, situated near enough to appear to the eye but as one,
whenever this line is equal to half the breadth of a whole undulation
the reflection from the depressed point will so interfere with the
reflection from the fixed point that the progressive motion of the one
will coincide with the retrograde motion of the other, and they will
both be destroyed; but when this line is equal to the whole breadth of
an undulation, the effect will be doubled, and when to a breadth and
a half, again destroyed; and thus for a considerable number of
alternations, and if the reflected undulations be of a different kind,
they will be variously affected, according to their proportions to the
various length of the line which is the difference between the lengths
of their two paths, and which may be denominated the interval of a
retardation.

"In order that the effect may be the more perceptible, a number of pairs
of points must be united into two parallel lines; and if several such
pairs of lines be placed near each other, they will facilitate the
observation. If one of the lines be made to revolve round the other as
an axis, the depression below the given plane will be as the sine of the
inclination; and while the eye and the luminous object remain fixed the
difference of the length of the paths will vary as this sine.

"The best subjects for the experiment are Mr. Coventry's exquisite
micrometers; such of them as consist of parallel lines drawn on