Photographic Standard of Light.

Experiments are being conducted at the Observatory with a view to using the light of the stars as a standard of light in photography. It is the practice among manufacturers of photographic supplies to sensitize plates with lamplight of different degrees of brilliancy, and so the numbers of different makes of plates have no comparative significance. By this method, an unvarying standard of measurement will be obtained. The apparatus at the Observatory is very simple, consisting merely of a contrivance for allowing the light to shine through a small hole to the plate. A secondary and more convenient standard will be formed by using a lens in connection with the other apparatus. This will give a light one hundred times as bright as that in the simpler instrument. By means of this same device, the relative brightness of the different stars and planets will also be obtained. The visular comparative brilliancy has already been well ascertained by astronomers, but their comparative brightness for photographic work is not accurately known.