Our heliographing instrument did excellent service sending flashes from the cliff to the island, but we couldn't make it work very well sending messages from the island to the cliff, because we had to face almost due north, and then the sun was nearly always at our backs and couldn't shine squarely on the mirror. This led to our building a double mirrored heliograph the following summer. To begin with, we built an instrument which was the exact duplicate of our first heliograph; then, in addition, to fit in the socket of the sighting rod, we rigged up a second mirror, which was mounted in exactly the same way as the first. The second mirror was called the station mirror, and differed from the other, or sun mirror, in having a small patch of white paper pasted at the center instead of a peep hole. When using this instrument, we set it up so that the station mirror faced the ledge, then by sighting through the hole in the sun mirror at the reflection in the station mirror we could see just what was in focus. The station mirror had to be moved until the patch at its center hid the ledge from view. After that the sun mirror was shifted until the shadow spot fell on the white patch of the station mirror. When once the station mirror was focused, it could be clamped tightly in place by screwing up the trunnion and swivel nuts. But the sun mirror had to be constantly shifted to keep the shadow on the patch. Another way of focusing the mirrors was to stand behind the instrument with the head close to the station mirror, shift the sun mirror until the entire station mirror was reflected in it, with the white patch squarely over the unsilvered spot; then still looking at the sun mirror, the station mirror was shifted until the reflection of the distant station was brought squarely in line with the unsilvered spot on the mirror. The station mirror was now firmly bolted and the sun mirror adjusted until the shadow spot fell on the paper patch.

Fig. 161. The Double Mirror Instrument.

Fig. 162. Top View, showing position of the Two Mirrors and the Screen.