1886.] 



Researches in Stellar Photography. 



449 



friction becomes very sensibly larger, and does partly, if not mainly, 

 depend upon the permanent rotation to-and-fro of the molecules 

 about their axes. The above-mentioned limit can be widened by 

 allowing the wire to rest after suspension with oscillations at 

 intervals, by annealing, and by repeated heating and cooling. 



VII. "Researches in Stellar Photography. 1. In its Relation to 

 the Photometry of the Stars; 2. Its Applicability to Astro- 

 nomical Measurements of Great Precision." By the Rev. 

 C. Pritchard, D.D., F.R.S., Saviiian Professor of Astro- 

 nomy in Oxford. Received May 20, 1886. 



(Abstract.) 



I. The objects are, first to enquire, by means of accurate measure- 

 ment, whether there does not exist a definite relation between the area 

 of the disk of a star image impressed on a photographic film and the 

 " photometric magnitude " of that star as determined by instrumental 

 means. 



For this purpose," several plates of portions of the Pleiades were 

 taken by varying exposures in the focus of the De la Rue reflector of 

 13 inches aperture, in the Oxford University Observatory. The 

 diameters of the star disks on these plates were then carefully 

 measured, both with the macro-micrometer and with a double image 

 micrometer, in the same establishment. The result is that the 

 relation sought for is expressed by — 



D— D'=£{log M'-logM}, 



where D, D' are the measured diameters of two star disks on the same 

 plate, and M, M' their corresponding " magnitudes" as recorded in the 

 " Uranometria Nova Oxonienses." 



The mean difference between the observed and computed magni- 

 tudes as derived from the foregoing formula, applied to 28 stars 

 all impressed on each of the four plates and ranging from magnitude 

 3 to magnitude 9*5, is 016 magnitude. A few stars (3) here 

 stand out, in all the plates, as was to be expected, arising from the 

 peculiar actinic action of their spectra. Similar anomalies, as is well 

 known, exist in the application of the photometer. 



II. In the next section of the enquiry the effect of alteration of the 

 time of exposure on the areas of the star images is referred to. The 

 enquiry is not fully completed, but as far as it extends, it indicates 

 that for stars not very faint, the areas of the disks of the same star on 

 the same plate vary as the square root of the time of exposure. 



VOL. XL. 2 H 



