THE TWO COMETS OF THE YEAR 1868. 155 



Two small comets have been analysed with the spec- 

 troscope, and each presented similar results. The 

 spectrum in each case consisted of thin bright lines on 

 a faint continuous streak of light. And from the fact 

 that the bright lines did not extend across the whole 

 breadth of the faint streak of light, it became evident 

 that they formed the spectrum of the nucleus, the 

 faint continuous spectrum belonging to the coma. 

 Hence it resulted that the nucleus of each of these 

 small comets consisted of self-luminous gas, while the 

 coma either consisted of incandescent solid matter or 

 shone by reflecting the light of the sun. The latter is 

 far the more probable hypothesis. In fact, when we 

 consider the extreme tenuity of the substance of a 

 comet, and therefore the certainty that if composed of 

 solid matter such matter must be dispersed in very 

 minute fragments, we shall recognise the extreme 

 improbability that these fragments should be self- 

 luminous through intensity of heat. If the comets 

 had been brighter, I may remark, there would have 

 been no dubiety respecting this point, since it would 

 have been possible to compare the continuous streak of 

 light with the solar spectrum, and by the resemblance 

 or dissimilarity of the two spectra, to determine whether 

 the coma really shines by reflecting the sun's light or 

 not. 



Brorsen's comet has now been examined with the 

 spectroscope, and with results quite different from 

 .those which attended the analysis of the other two. 

 Dr. Huggins, the physicist, who examined the latter, 

 says of Brorsen's comet : 



