Very sensitive gelatine films, such as are used in photography, when exposed during several hours to the sky in taking stellar photographs, become more or less darkened during development. The darkening is chiefly due to atmospheric glare caused by starlight; and the nebulous circles seen round the bright stars are caused by the glare and by diffraction effects produced by the objectives, or mirrors, of the instruments employed in photographing.

I have made some experiments to enable us to judge to what extent the glare and diffraction affect the finished photographs, a summary of which experiments may be given here. They were made by exposing simultaneously plates in the 20-inch reflector, the 5-inch lens camera, and to the sky in a blackened box, measuring 7-inches square by 12-inches in height, with the open end exposed to the zenith, the exposures respectively being made during precisely equal intervals of time. The plates were selected so as to be equal in sensitiveness, and the development was performed in a similar manner in each coincident trial.

The plates exposed in the box were 6-inches square, and equal areas on each of them were (1) left uncovered; (2) covered with black paper; (3) covered with different thicknesses of polished plate glass. The plates when developed showed the comparative effects of the unobstructed full sky glare as well as the effects of the application of complete and partial covering with plates of glass or with sensitometer figured scales.