A Section of the Constellation Cygnus (August 13, 1885)

Astronomers at the Paris Observatory, the brothers Paul and Prosper Henry inherited in 1872 a project begun twenty years earlier--the mapping of the heavens by means of painstaking observation, calculation, and notation. In a dozen years they charted nearly fifty thousand stars. When, in 1884, their survey approached the Milky Way, the Henry brothers found that the cluster of stars proved far too dense and complex to chart by eye, and they constructed a photographic telescope to produce an exact, objective record of the sky.
That photography might serve astronomy was evident from the very beginning. Daguerre's standard-bearer François Arago, director of the Paris Observatory, declared in July 1839 that the daguerreotype would eventually accomplish with ease the most delicate and difficult astronomical tasks, such as mapping the surface of the moon. Indeed, before the Henry brothers' first use of the medium, other photographers had successfully charted lunar geology, solar and lunar eclipses, the transit of Venus, sunspots, the surface of Mars, the rings of Saturn, and the relative position of the brightest stars.
No one, however, had yet recorded stars so distant and faint that they were not visible to the eye. This the Henry brothers achieved in 1885 by constructing a still more powerful photographic telescope, with an extraordinarily precise mechanism for tracking the stars across the night sky during exposures as long as one hour. The resulting photographs, each seemingly infinite expanse showing but a three-degree section of the firmament, remain among the most sublime conceptions of scientific photography.