Pinned to it was a little concave mirror, for making triumphant signal flashes from the summit.

It illuminates a smaller area on the concave mirror, leading to a small beam-width on the way to the flat mirror.

He later modified the theory to simply involve a concave mirror and a comparator mirror.

This involved creating a set up which observed how the position of a concave mirror would affect the real object that it reflected.

He often painted with the aid of a concave mirror, and to obtain exactness looked at his subject through a frame crossed with squares of silk thread.