Theory of New Method.



[Illustration: FIG. 1.]

Let S, Fig. 1, be a slit, through which light passes, falling on R, a
mirror free to rotate about an axis at right angles to the plane of the
paper; L, a lens of great focal length, upon which the light falls which
is reflected from R. Let M be a plane mirror whose surface is
perpendicular to the line R, M, passing through the centers of R, L, and
M, respectively. If L be so placed that an image of S is formed on the
surface of M, then, this image acting as the object, its image will be
formed at S, and will coincide, point for point, with S.

If, now, R be turned about the axis, so long as the light falls upon the
lens, an image of the slit will still be formed on the surface of the
mirror, though on a different part, and as long as the returning light
falls on the lens an image of this image will be formed at S,
notwithstanding the change of position of the first image at M. This
result, namely, the production of a stationary image of an image in
motion, is absolutely necessary in this method of experiment. It was first
accomplished by Foucault, and in a manner differing apparently but little
from the foregoing.

[Illustration: FIG. 2.]

In his experiments L, Fig. 2, served simply to form the image of S at M,
and M, the returning mirror, was spherical, the center coinciding with the