196 



Prof. C. Pritchard. 



first use made of these photographic plates was to measure the 

 diameter of those stars whose photometric magnitudes have already- 

 been ascertained by the Wedge photometer, and published in the 

 " Uranometria Nova Oxoniensis," p. 94. 



Before proceeding further, it is necessary to observe that the photo- 

 graphic images of the stars formed on the film possess in general a 

 very sensible size, even when exposed for moderate intervals of time. 

 It is not necessary here to enter upon the probable cause or causes of 

 these considerable images, or of their peculiar nature ; it is sufficient 

 to indicate, as an example, that the image of Alcyone (mag. 3*12) on 

 a plate exposed for (say) ten minutes, is a disk having a diameter of 

 about 0"02 inch, equivalent to a diameter of 30" in the focal plane of 

 the telescope. As to the peculiar form of these images, it was long 

 ago shown by Bond (in 1858)*, that when viewed under considerable 

 magnifying power, they are not simply sharply defined circular disks, 

 but are fringed all round with discrete black dots, separable under 

 the higher powers of a microscope, and extending to some distance 

 beyond the main black circular disk ; nevertheless, when viewed in the 

 comparatively low power of the double-image micrometer, or of the 

 microscope attached to the macro-micrometer, actually used in the 

 measurements, these discrete fringes are not salient, and do not inter- 

 fere sensibly with the exactitude of the repeated measures. 



As a matter of precaution, the diameters of the photographed 

 images were measured, both by means of Airy's double-image micro- 

 meter (Greenwich observations), and by the De La Bue macro- 

 micrometer described in vol. 47, "Mem. Boy. Astron. Soc," and the 

 trustworthiness of these measures was still further confirmed by 

 observations made by means of a stage micrometer applied to an 

 achromatic microscope of very considerable power. The results of 

 these combined measures were then graphically plotted on paper pre- 

 pared with ruled squares, the measured diameters being taken as the 

 abscissae, and the corresponding photometric magnitudes taken as 

 ordinates. As usual, a curve was drawn by hand among the points 

 plotted, and thus graphically connecting magnitudes in general with 

 their corresponding diameters. A mere inspection at once suggested 

 the mathematical connexion between these abscissae and their ordi- 

 nates, indicating a logarithmic curve, of which the equation would 

 be— 



y = Ae~~a> 



where x is the measured diameter, and y is the corresponding magni- 

 tude of the star, here expressed on the scale that 2*05 shall represent 

 the magnitude of Polaris, and 2*512 the light ratio in passing from 

 one magnitude to the next. All this is in accordance with the ordi- 



* " Astr. Nadir.," vol. 48 (1858), col. L 



