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.
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