attached to the mirror by a universal joint. The heavy frame was mounted
on a brick pier, and the whole surrounded by a wooden case to protect it
from the sun.



Adjustment of the Fixed Mirror.


The adjustment was effected as follows: A theodolite was placed at about
100 feet in front of the mirror, and the latter was moved about by the
screws till the observer at the theodolite saw the image of his telescope
reflected in the center of the mirror. Then the telescope attached to the
mirror was pointed (without moving the mirror itself) at a mark on a piece
of card-board attached to the theodolite. Thus the line of collimation of
the telescope was placed at right angles to the surface of the mirror. The
theodolite was then moved to 1,000 feet, and, if found necessary, the
adjustment was repeated. Then the mirror was moved by the screws till its
telescope pointed at the hole in the shutter of the building. The
adjustment was completed by moving the mirror, by signals, till the
observer, looking through the hole in the shutter, through a good
spy-glass, saw the image of the spy-glass reflected centrally in the
mirror.

The whole operation was completed in a little over an hour.

Notwithstanding the wooden case about the pier, the mirror would change
its position between morning and evening; so that the last adjustment had
to be repeated before every series of experiments.