If the mirror swings through a horizontal arc, the reflected imagewill move, in virtue of a simple geometrical principle, through an arc of twice as many degrees. The scale can be placed far from the mirror, so that the ray of lightwill represent a weightless index of very great length, and minute deflections of the needlewill be shown distinctly upon the scale.
In the cut, Fig. 182, the ray of light from the lamp passes through the aperture, m m, and is made parallel by the lens, L. At s is the mirror attached to the needle and moving with it. A scale placed at t receives the reflection from the mirror. The cut, Fig. 183, shows one form of the instrument set up for use.
Definitions of galvanometer, reflecting
