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.  These holes were just 1-3/4 inches apart.  After the slats had been set in place, the frame was fastened together and then nailed to a baseboard, which was fastened by a bolt to the tripod.  The shutter was operated by a key something like a telegraph key.  It was made of a narrow stick of wood hinged at one end to the lower strip of the shutter frame, and a spool sawed in two was fastened to the other end to serve as a handle for the key.  A string connected the key with the connecting rod.  The slats were kept closed by a spring, which was fastened at one end to the connecting rod and at the other to the top of the frame.  At first we used a rubber band for this purpose, but it soon wore out, so we then made a spiral spring out of stiff spring brass wire by wrapping it around a pencil.  When the key was pressed down the slats would be turned open, as shown in Fig. 159; but as soon as the key was released the spring would pull them back again.

Focusing the Instrument.

[Illustration:  Fig. 159.  The Heliograph in Operation.]

We were now ready to commence operations with our instruments.  The heliograph was set up on the ledge at the top of the cliff.  First the disk was turned down, uncovering the point of the sighting rod.  Then Bill sighted through the unsilvered spot in the mirror and shifted the rod up and down until the tip end came squarely in line with the door of our straw hut, where Jack was seated, notebook in hand, to take down our message.  Reddy stood by him with his wigwag flag to answer back.  When the instrument was properly sighted the shutter was set up directly in front of it and the sighting disk turned up to cover the point of the sighting rod.  Then came the rather troublesome task of focusing the mirror.  The mirror reflected a square panel of light, in the center of which there was a small shadow spot made by the unsilvered peep hole.  The object was to get this shadow to fall on the center of the sighting disk.  We knew that then the mirror would reflect the sunlight squarely on the straw hut.  We found it quite easy to direct this shadow spot to the disk by holding a sheet of paper in front of the mirror six or eight inches away, and following up the spot on the paper until it reached the disk.