THE TWO COMETS OF THE YEAR 1868. 171 



of light. A tail could be traced to a distance of about 

 one degree from the nucleus. 



Dr. Huggins quickly subjected the new arrival to 

 spectroscopic analysis. The result, at first sight, seemed 

 to differ little from that which had been noticed in the 

 case of Brorsen's comet. Indeed the astronomers at 

 the Paris observatory and the Padre Secchi at Rome 

 were led to pronounce the spectra of the two comets to 

 be absolutely identical. The more powerful spectro- 

 scopic appliances employed by Dr. Huggins, however, 

 exhibited important differences. 



The spectrum consisted of three bands of light sepa- 

 rated by dark intervals. Of these bands two were 

 greenish blue, the other greenish yellow. The two 

 former were tongue-shaped, the last was narrowed off at 

 both extremities. 



From what I have said above respecting the nature 

 of spectroscopic analysis, it will be understood that the 

 distribution of the comet's light along the length of 

 the spectrum is the most important circumstance to be 

 attended to in endeavouring to form an estimate of the 

 substance of the comet. But as we see that there are, 

 in this instance, peculiarities affecting the breadth of the 

 spectrum, it will be well briefly to consider their mean- 

 ing. The matter is, in reality, simple enough, but 

 requires a little attention. 



The breadth of the spectrum corresponds to the 

 breadth of the object which is the source of light. If 

 that object is uniformly bright the spectrum is also 

 uniformly bright across its breadth, whatever variations 



