lens, and to the distant mirror.



The Heliostat.


The heliostat was one kindly furnished by Dr. Woodward, of the Army
Medical Museum, and was a modification of Foucault's form, designed by
Keith. It was found to be accurate and easy to adjust. The light was
reflected from the heliostat to a plane mirror, M, Fig. 3, so that the
former need not be disturbed after being once adjusted.



The Revolving Mirror.


The revolving mirror was made by Fauth & Co., of Washington. It consists
of a cast-iron frame resting on three leveling screws, one of which was
connected by cords to the table at S, Fig. 3, so that the mirror could be
inclined forward or backward while making the observations.

[Illustration: FIG. 4.]

Two binding screws, S, S, Fig. 4, terminating in hardened steel conical
sockets, hold the revolving part. This consists of a steel axle, X, Y,
Figs. 4 and 5, the pivots being conical and hardened. The axle expands
into a ring at R, which holds the mirror M. The latter was a disc of plane
glass, made by Alvan Clark & Sons, about 1¼ inch in diameter and 0.2 inch