The mirror is mounted on a
metal plate, which is shaped to a peculiar curve. A clamp and slow
motion provide for rapid and for fine adjustment. The eye piece is set
at an angle, and contains a half silvered mirror, the upper portion
being transparent. This allows direct vision along the axis of the eye
piece, and at the same time vision in another direction, after two
reflections, one in the eye piece and the other at the adjustable
mirror. Fig. 2 is an outline plan of the instrument when closed. In
the first form of the instrument only one mirror was provided, but by
the double reflection in the improved pattern, any accidental twisting
of the rod or handle produces no displacement of the images, since the
inclination of one mirror neutralizes the equal and opposite
inclination of the other. No cross line is required with the new
arrangement, since it is only necessary that the two images should
coincide.
[Illustration: FIG. 2.--OUTLINE OF INSTRUMENT SHOWING THE PATH OF THE
DIRECT AND OF THE REFLECTED RAY.
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