Astronomers at the Paris Observatory, the brothers Paul and Prosper Henry inherited in 1872 a project begun twenty years earlier—mapping 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.

Although photography’s potential value to astronomy had been imagined from the very outset, no one had yet recorded stars so distant and faint that they were not visible to the eye. The Henry brothers achieved this feat 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 that lasted, in the case of this image, two hours. 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.