Researches in Stellar Photography. 



195 



the whole of these instruments for me, and to whom I am indebted 

 for some valuable suggestions ; and finally to Mr. Landriset, of 

 Geneva, my assistant since last January, whose care and perseverance, 

 especially with reference to the determinations by Pettenkofer's 

 method, have added not a little to the success of the inquiry. 



The drawing of the instrument which accompanies this paper 

 (Plate 2) needs no explanation beyond the few notes referring to 

 letters at the foot of the plate. 



6t Researches in Stellar Photography. 1. In its relation 

 to the Photometry of the Stars; 2. Its applicability to 

 Astronomical Measurements of great Precision." By C. 

 Pritchard, SaviKan Professor of Astronomy in Oxford. 

 Received May 20. Read May 27, 1886. 



Several attempts have already been made to connect the photo- 

 graphic images of stars with their photometric magnitudes, and con- 

 sequently with their relative brightness ; but hitherto, so far as I 

 know, 'this relation has been sought by comparing the impressions 

 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 diameter? 

 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 appli- 

 cation 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. 



Several plates of the Pleiades were taken with varying times of 

 exposure, and on these were impressed images of portions of the 

 group, extending to stars of approximately the tenth magnitude. The 



