NATURE 



269 



THURSDAY, JANUARY 22, 1880 



ON THE PHOTOGRAPHIC SPECTRA OF 

 STARS 1 



THE author presented, in December, 1876, a preli- 

 minary note on the subject of this paper, together 

 ■with a diagram of the spectrum of Vega compared with 

 that of the sun. 



The author refers to a paper by Dr. William Allen 

 Miller and himself in 1864, in which they describe an 

 early attempt to photograph the spectra of stars. 



Other investigations prevented the author from re- 

 suming this line of research until 1875, when a more 

 perfect driving clock, by Grubb, enabled him to take up 

 this work with greater prospect of success. 



The author describes the special apparatus and the 

 methods of working which have been employed. 



In consequence of the very limited amount of light 

 received from the stars, it was of great importance not to 

 spread out the spectrum to a greater extent than was 

 necessary for a sufficient separation of the principal lines 

 of the spectrum. The spectrum apparatus finally adopted 

 consists of one prism of Iceland spar and lenses of 

 quartz. The length of the spectrum taken with this appa- 

 ratus is about half an inch, from G to O in the ultra-violet. 

 The definition is so good that in photographs of the solar 

 spectrum at least seven lines can be counted between H 

 andK. 



Though there is considerable loss of light in the 

 employment of a slit, still, for the great advantage which 

 it affords in obtaining spectra of comparison, a narrow 

 slit one-three-hundred-and-fiftieth (5J0) °f an mc h ' n 

 width was always employed. 



This slit is provided with two shutters. By means of 

 these through one half of the slit a solar or other spectrum 

 may be taken on the same plate for comparison, and for 

 the determination of the lines in position in the spectrum. 

 This apparatus was adapted to a Cassegrain reflector with 

 a metallic speculum of 18 inches aperture. The small 

 mirror was removed and the slit of the spectrum appa- 

 ratus p'aced at the principal focus of the mirror. A 

 simple but perfectly successful method was adopted by 

 which the image of a star could be brought exactly upon 

 the slit, and retained there during the whole time of 

 exposure, sometimes for more than one hour, by a system 

 of continuous supervision, and instant control by hand 

 when necessary. 



Various photographic methods were tried, but the great 

 sensitiveness which may be given to gelatine plates, 

 together with the special advantages under long exposures 

 of dry plates led finally to the exclusive adoption of this 

 method. 



The photographs were examined and the lines measured 

 by means of a micrometer attached to a microscope of 

 low power. These measures were reduced to wave- 

 lengths by the help of solar and terrestrial spectra, use 

 being made of M. Cornu's map of the ultra-violet part of 

 the spectrum, and of M. Mascart's determination of the 

 wave-lengths of the lines of cadmium. 



Photographs have been obtained of the stars Sirius, 



■ Abstract of paper by W. Huggins. D.C.L., LI..D.. F.R.S., lead before 

 the Royal Society, December i8, 1879. with additions by the author, 9 



Vol. xxi.— No. 534 



f : 



■= 



