520 KBPORT — 1881. 



2, On the Photographic Spectrum of Comet ' b ' 1881. 5y William 

 HuGGiNs, D.G.L., LL.D., F.B.S. 

 [Plate XIV.] 



In the years 1866 and 1868 I applied the spectroscope to the light of comets,, 

 and in the latter year I showed that the three bright bands in the visible part of 

 the spectrum agree with the similar bright bands which are seen when an induction 

 spark is taken in olefiant gas.^ The same bands are also seen in the flames of many 

 compounds of carbon. I was inclined at that time to consider that these bands 

 were due to the vapour of carbon. Subsequent investigations which have been 

 made on the spectra of the compounds of carbon, appear to make it probable that 

 these bands are the spectrum of a compound of carbon with hydrogen. The obser- 

 vations (1868) showed the presence of carbon, probably in combination with 

 hydrogen, in the cometary matter. 



Since that time luitil the present year, no comet has appeared sufficiently bright 

 to allow of the observations on its spectrum being extended to the ultra-violet 

 region. The apparatus with which I had successfully photographed the spectra of 

 stars was especially suited to this purpose." It consists essentially of a spectroscope, 

 furnished with a prism of Iceland spar and lenses of quartz, placed so that the slit 

 shall be in the principal focus of a mirror 18 inches in diameter, equatorially 

 mounted, and driven by an electrically controlled clock. 



On the evening of June 24th (1881), I directed this instrument, armed with a 

 very sensitive gelatine plate, to the head of Comet ' b,' so that the nucleus .should be 

 upon one half of the slit. After an exposure of one hour, the open half of the slit 

 was clo,sed, the shutter withdrawn fi-om the other half, and the instrument was then 

 directed to Arcturus for fifteen minutes. 



After development the plate presented a verj' distinct spectrum of the comet, 

 together with that of the star, for comparison. 



The spectrum of the comet conisists of two spectra supei-posed upon each other : 

 a continuous spectrum, which extends from about F to a little distance beyond H. 

 In this continuous spectrum can be seen the Fraunhofer lines G, h, H, K, and many 

 others. This spectrum is therefore due to reflected solar light. 



The second spectrum consists of two sets of bright lines, and a suspicion of the 

 presence of a third set. These lines are obviously to be referred to original light 

 from the comet. 



The strongest set consists of two bright lines in the commencement of the 

 ultra-violet region. Measures, made by the aid of the compari.son star-.spectrum, 

 ^ive for these 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, 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 the 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, though 

 fainter, impression of a group of lines between G and h. There can be little doubt 

 .that this group is the one for the least refrangible limit of which the wave-length 

 4220 is given by Professors Liveing and Dewar. 



An increase of brightness in the continuous spectrum is also seen between h 

 .and H, which may be due to other bright lines, but the photograph is not strong 

 enough to admit of any certain conclusion on this point. 



On 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 bright lines in the ultra-violet, and the continuous .spectrum. 



These photographs confirm the results of my earlier observations on comets, — 

 that part of their light is reflected sunlight, and part is original light; and, further, 

 that carbon is present in the cometary matter. The new bright groups in the 



1 PJiil. Trans. 1868, p. 556. = PHI. Trans. 1880, p. 669. 



