Intelligence and Miscellaneous Articles. 155 



NOTE ON THE PHOTOGKAPHIC SPECTRUM OF COMET b 1881. 

 BY WILLIAM HTJGGINS, D.C.L., LL.D., F.R.S. 



On the evening of June 24 I directed the reflector furnished 

 with the spectroscopic and photographic arrangements described 

 in my paper "On the Photographic Spectra of Stars"* to the 

 head of the comet, so that the nucleus should be upon one half of 

 the slit. After one hour's exposure the open half of the slit was 

 closed, the shutter withdrawn from the other half, and the instru- 

 ment 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 lines 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 Eraunhofer 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 own. 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, not- 

 withstanding 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 luminosity can be traced from it to a little 

 beyond the second line, 3870. There can be no doubt, therefore, 

 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 Gr 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 De war's measure 4220. 



* Phil. Trans. 1880, p. 669. 



