1869.] On some Becent Spectroscoine Researches. 223 



the middle part of each band, which was due to the Hght of this 

 part of the comet, to be traced to a greater distance than the 

 marginal parts which were produced by the fainter portions of the 

 comet. 



The same evening the writer put this apparent identity of the 

 comet's spectrum with that of carbon to the test of a direct com- 

 parison of the two spectra. For this purpose a jar of olefiant gas 

 was prepared, and the comparison was made in the manner shown 

 in the woodcut. 



The glass gas-holder a, containing olefiant gas, was connected 

 by a flexible tube with a glass tube, h, into which platinum wires 

 were soldered. This tube was so fixed that the spark between the 

 wires was suitably reflected by the small mirror c into the spectro- 

 scope attached to the telescope, so that the spectrum of carbon 

 appeared directly below the spectrum of the comet. 



Very careful comparisons on that evening and similar direct 

 comparisons on a subsequent night showed that in every particular 

 the spectrum of the comet was similar to that of the spectrum of 

 carbon as obtained by the decomposition of olefiant gas. The lines 

 of hydrogen which are prominent in the spectrum of the gas were 

 not present in the comet's spectrum. 



Considering that the apparent identity of the spectra rests upon 

 the positions of three bands and also upon the special characters of 

 the distribution of the light in them, the conclusion may, perhaps, 

 be considered well founded, that the source of a part of the cometary 

 light is really due to carbon. 



The difliculty of accepting what appears to be the obvious 

 teaching of these observations arises from the very high tempera- 

 ture necessary to raise carbon to a state of vapour ; for it appears to 

 be alone when carbon is in the condition of luminous vapour that 

 the characteristic spectrum of the bright bands appears. A degree of 

 heat sufficient perhaps even for this purpose has been experienced by 

 a very few comets. A temperature less excessive might be indeed 

 sufficient, if we were free to suppose that comets consist of some 

 compound of carbon which is decomposed by the sun's heat. 



It is right to state that phosphorescent and fluorescent bodies 

 give spectra containing bright bands. The former phenomenon 

 appears to be restricted to solid and highly-reflective bodies, and the 

 phosphorescence emitted by them is not seen so long as they are 

 exposed to light. The phenomenon of fluorescence, when a nearly 

 transparent liquid becomes an object of some brightness by absorb- 

 ing the nearly invisible rays of the spectrum, and then dispersing 

 them in a degraded and more luminous form, is less inconsistent 

 with the small reflective power of cometary matter. 



Tlie violent commotions and internal changes of comets when 

 near the sun seem, however, to connect the light they emit rather 

 Avith a condition of great heat. 



