The Screen.
[Illustration: Fig. 157. Section through Shutter.]
[Illustration: Fig. 158. General View of Screen.]
The screen, or shutter, of the heliograph was mounted on a separate
tripod, so as to prevent shaking the mirror when it was operated. It was
made something like a window shutter. We cut out two slats, each 2-1/2
inches wide and 6 inches long. They were made of hardwood 3/8 inch thick.
The upper and lower edges were tapered down to a thickness of 3/16 inch.
Light nails were driven into the slats at the ends, and the nail heads
were then filed off so that the projecting ends formed trunnions for the
slats to turn on. The slats were linked to a connecting rod with double
point tacks. A small double point tack was driven into the upper edge of
each slat about 1/2 inch from the right hand end. Then through each of
these tacks we hooked a second double point tack and drove it into the
rod. The tacks on the rod were placed just 2 inches apart. A substantial
frame was then made of 3/4-inch stuff 1-1/2 inches wide. The frame was
square, with an opening that measured 6 inches each way, into which the
slats were fitted. Before nailing the frame together we drilled holes in
the side pieces for the trunnions of the slats to turn in.
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