Researches in Stellar Photography. 



199 



plate (A) after seven minutes' exposure, was 12*82 mag., and that the 

 diameter of a star of the magnitude 1*28, would occupy on the plate 

 in question a circular disk 46*36" in diameter, or, linearly expressed, 

 0*024 inch. 



Similarly for plates B and 0, with the exception that the exposures 

 and the altitudes of the stars are different, to say nothing of other 

 possible differences, such as the chemical constitution of the films, the 

 development of the images, and the meteorological circumstances 

 attending the exposures. It is observable that in Plate B, although 

 the exposure was twelve minutes instead of the seven for Plate A, 

 nevertheless the zero star was brighter than for A, owing probably to the 

 lower altitude or to the other causes mentioned above. For Plate C 

 the exposure was still longer, viz., fifteen minutes, nevertheless, as 

 the group was still lower in altitude, the zero star is the brightest of 

 any of the three plates : the instructive character of these phenomena 

 is obvious. 



The results of applying the normal equations A, B, C severally to 

 the twenty -four stars whose diameters were measured, are given in 

 the subjoined Tables I, II, III, where, in the first column, is given the 

 star's designation as originally applied by Bessel. In the second is 

 given the measured diameter of its photographed image on the plate 

 in question. This diameter expressed in seconds of arc corresponds 

 to the angular magnitude under which that image would be seen 

 when placed in the focus of the mirror (10 feet), and viewed from 

 the centre of that mirror. In the third column of each table is given 

 the photographic magnitude, computed from the normal equations 

 A, B, C severally. Column 4 contains the photometric magnitude, as. 

 given in the " Uranometria Nova Oxoniensis," p. 94. The last* 

 column contains the difference between the two magnitudes. 



