on a line with the trunnions and just half-way between them.  This done, we took two sticks of 3/8 inch wood, 1 inch wide and 3-1/2 inches long.  In the upper end of each stick a slot was cut 1/2 inch deep and 1/4 inch wide.  Into these slots the trunnions of the mirror were placed, and then the nuts were screwed tightly on, clamping the sticks against the sides of the mirror.  The sticks were now connected by nailing a 1/2-inch strip at the bottom, and braced by a couple of corner pieces.  This formed a swiveled frame for the mirror, which was clamped to the base of the instrument by means of a bolt 1-1/2 inches long.  The bolt passed through the bottom board of the frame, squarely under the peep hole of the mirror and through the baseboard of the instrument near one end.  The baseboard was 2 inches wide, 10 inches long and 3/4 inch thick.

The Sight Rod.

[Illustration:  Fig. 155.  The Sight Rod.]

[Illustration:  Fig. 156.  Nut Set in Baseboard.]

At the end opposite to where the mirror frame was swiveled we mounted a sight rod, which was merely a round stick of wood 1/2 inch in diameter and about 8 inches long.  We cut the stick from one of the rounds of an old broken chair.  The upper end of the rod was whittled to a point and one side was flattened as shown in Fig. 155.  Out of a piece of heavy white cardboard we cut a round disk about 1/4 inch in diameter, with a shank 1 inch long sticking out at one side.  This was fastened with a single tack to the flattened end of the rod in such a position that the point lay exactly against the center of the disk.  The disk could then be turned up or down, to cover or uncover the point of the rod, as desired.  The rod was fitted snugly into a hole in the baseboard, and could be raised or lowered to any extent desired, but we had to provide some sort of an arrangement for making it stay where it was put.  A small hole was drilled from the edge of the baseboard through to the hole in which the rod was fitted.  A square socket was chiseled out around the small hole to receive a nut.  The nut was firmly wedged in and held in place by driving in nails along the edges.  A bolt or machine screw was threaded through the nut, so that its inner end pressed against the sighting rod.  By tightening this screw the rod could be secured at any height desired.

The instrument was mounted on a tripod similar to the one used for our surveying instrument.  To this it was attached by means of a bolt, which passed through the center of the baseboard and the tripod head.

The Screen.

[Illustration:  Fig. 157.  Section through Shutter.]

[Illustration:  Fig. 158.  General View of Screen.]