184 STELLAR PHOTOGRAPHT. 



centre of the object-glass. The portion of the light falling upon this will form a 

 companion to each star. The ratio of the true brightness of the star to that of its 

 companion is a constant quantity, and is readily determined from the absorption of 

 the prism and the ratio of its area to that of the uncovered portion of the lens. 

 If the stars and companions are then measured upon any empirical scale, this ratio 

 of light serves to reduce the results to absolute measures. 



To remedy the variation arising from the color of the stai's, an attempt was 

 made to render the light monochromatic. One method proposed for this purpose 

 consisted in using as an object-glass an uncorrected lens. The focus of this varies 

 very greatly with the color of the light. The rays out of focus would be spread 

 over so large a circle that they would have but little effect upon the image. By 

 covering the centre of the lens by a diaphragm, or by a prism of small angle, the 

 light at the centre of the image of the star would be nearly monochromatic. By 

 taking a series of plates with various foci, the relative intensity of the light of each 

 different color could be determined. The most important of these methods, and the 

 results attained by them, were described and exhibited to the Royal Astronomical 

 Society at their meeting on June 8, 1883. (Observatory, VII. 199 ; Astron. Register, 

 XXI. 149.) 



Photographic Apparatus. 



A second series of experiments was undertaken in March, 1885, with results 

 that seemed to justify their repetition on a larger scale. An application was accord- 

 ingly made to the Board of Directors of the Bache Fund, and an appropriation 

 was granted by means of which the following work was accomplished. A Yoigt- 

 lander lens having an aperture of 8 inches and a focal length of about 45 inches 

 was obtained, and intrusted to Messrs. Alvan Clark and Sons for correction and 

 mounting. Errors which are quite inappreciable in ordinary photographic work may 

 be large enough to ruin a stellar photograph. This was found to be the case with the 

 lens mentioned above. It was also desirable that its focal length should be 114.6 cm., 

 so that the scale of the photographs should be as nearly as possible 2 cm. to 1°, 

 which is the scale of the maps of the Durchmusterung. These difficulties were 

 finally overcome, but it was necessary to regrind two of the glass surfaces. The 

 lens was next mounted equatorially, as is shown in Plate I. The brass tube carry- 

 ing the lens is screwed into the end of a steel tube which is mounted in trunnions 

 at the ends of a large fork. This fork forms the prolongation of the polar axis, and 

 possesses some important advantages over the usual equatorial mounting. It is much 



