The Single Mirror Instrument.
[Illustration: Fig. 153. Trunnion for Mirror.]
[Illustration: Fig. 154. The Single Mirror Instrument.]
The first thing we did was to procure a small mirror about 4 inches
square, mounted in a wooden frame. Then we got a pair of small square head
bolts about 1/4 of an inch in diameter and 1 inch long, also two strips of
brass 1/2 inch wide and 3 inches long. In the center of each brass strip
we drilled a hole just large enough to admit the shank of one of the
bolts, and then the strips were fastened with screws tight against
opposite edges of the mirror frame, with the heads pressed against the
frame and the shanks sticking out at each side, as shown in Fig. 153.
These projecting shanks served as "trunnions" (that is, pivots) for the
mirror to turn on when it was mounted in place. After the trunnions had
been set in place we made a peep hole in the center of the mirror by
cutting out a piece of the wooden back of the frame and scratching away
the silver from the back of the glass. Only a very small hole was
required, about 1/8 inch in diameter. Great care was taken to have the
unsilvered spot exactly on a line with the trunnions and just half-way
between them.
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