2 On the Photographic Spectrum of Comet b 1881. [Nov. 17, 



withdrawn from the other half, and the instrument then directed 

 to Arcturus for fifteen minutes. 



After development, the plate presented a very distinct spectrum of 

 the comet, together with the spectrum of the star, which I have 

 already described in the paper referred to above. 



The spectrum of the comet consists of a pair of bright Hues in the 

 ultra-violet region, and a continuous spectrum which can be traced 

 from about F to some distance beyond H. 



The bright lines, a little distance beyond H, with an approximate 

 wave-length from 3870 to 3890, appear to belong to the spectrum of 

 carbon (in some form, possibly in combination with hydrogen), which 

 I observed in the spectra of the telescopic comets of 1866 and 1868. 



In the continuous spectrum shown in the photograph, the dark 

 lines of Fraunhofer can be seen. 



This photographic evidence supports the results of my previous 

 observations in the visible spectra of some telescopic comets. Part of 

 the light from comets is reflected solar light, and another part is light 

 of their owu. The spectrum of this light shows the presence in the 

 comet of carbon, possibly in combination with hydrogen. 



On the next night, June 25, a second photograph was obtained with 

 an exposure of an hour and a half. This photograph, notwithstanding 

 the longer exposure, is fainter, but shows distinctly the two bright 

 lines and the continuous spectrum, which is too faint to allow the 

 Fraunhofer lines to be seen. 



(Postscript, July 9, 1881.) 



I have since measured the photographs of the comet's spectrum, 

 and I find for the two strong bright lines the wave-lengths 3883 and 

 3870. The less refrangible line is much stronger, and a faint lumi- 

 nosity can be traced from it to a little beyond the second line 3870. 

 There can be, therefore, no doubt that these lines represent the 

 brightest end of the ultra-violet group which appears under certain 

 circumstances in the spectra of the compounds of carbon. Professors 

 Liveing and Dewar have found for the strong line at the beginning of 

 this group the wave-length 3882' 7, and for the second line 3870-5. 



I am also able to see upon the continuous solar spectrum, a 

 distinct impression of the group of lines between G and h, which is 

 usually associated with the group described above. My measures for 

 the less refrangible end of this group give a wave-length of 4230, 

 which agrees as well as can be expected with Professors Liveing and 

 Dewar' s measure 4220. 



In their paper " On the Spectra of the Compounds of Carbon," 

 "Proc. Hoy. Soc," vol. 30, p. 494, Professors Liveing and Dewar 

 show that these two groups indicate the presence of cyanogen, and 



