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Several attempts have already been made to connect the photographic images of stars with their photometric magnitudes, and consequently with their relative brightness; but hitherto, so far as I know, this relation has been sought by comparing theimpressions made on the eye rather than as resulting from rigorous measures. With a view to the removal of this indefiniteness, unscientific unless it be unavoidable, I have undertaken a series of instrumental measures of the diameters of the photographic images impressed on sensitised films, which has led to the establishment of a remarkable physical relation (mathematically expressed) between the diameters of the stellar images and their photometric magnitudes, as determined by instrumental means : a method which seems to me to be free from systematic error and personal bias. With this end chiefly in view, though accompanied also with the hope of obtaining still further, and perhaps even more valuable application of the photographic method to astronomical observations, I procured a number of gelatine dry plates, each being about 2 inches square. The comparative smallness of these plates was determined or suggested by my desire to obtain pictures of such small parts only of the sky as would fall within the ascertained limits of astronomical accuracy of the telescopic field of view,i. e ., a field possessing no measurable distortion, and consequently restricted to about a square degree. These plates were exposed in the focus of the well-known De La Rue reflecting telescope, of 13 inches aperture, erected in the University Observatory, at Oxford.

proceedings of the royal society of london

Researches in stelllar photography. 1. In its relation to the photometry of the stars; 2. Its applicability to astronomical measurements of great precision