HENRY, PAUL P. AND PROSPER M.
The Moon at 167 hours, from the Paris Observatory, March 27, 1890, albumen print mounted, 15½ x 11½ inches (395 x 290 mm), captioned above and below in ink.
Astronomers at the Paris Observatory, the brothers Henry inherited in 1872 a project begun twenty years earlierthe mapping of the heavens by means of painstaking observation, calculation, and notation. From 1884 they began to employ photography in their task, and developed an extraordinarily precise mechanism for tracking stars across the night sky during exposures as long as one hour. They were thus able to photograph stars so distant and faint that they had hitherto been invisible to the eye.