148 LIGHT SCIENCE FOR LEISURE HOURS. 



which afforded doubtful evidence respecting the quality 

 of the light we receive from comets, and thus allowed 

 astronomers to form vague guesses respecting the struc- 

 ture of these mysterious wanderers. But beyond the 

 unsatisfactory indications of this instrument, astrono- 

 mers had no means whatever of ascertaining the phy- 

 sical nature of comets. 



At present, however, an instrument of incomparably 

 higher powers is applicable to the inquiry. The spec- 

 troscope has the power of revealing, not only the 

 general character of any substance which is a source 

 of light, but even of exhibiting, in many instances, 

 the elementary constitution of such a substance. The 

 indications of this wonderful instrument of analysis 

 are not affected by the distance or dimensions of the 

 object under examination. So long as the object is 

 luminous the spectroscope will tell us with the utmost 

 certainty whether the light is inherent or reflected ; 

 and if the light is inherent that is, if the object is 

 self-luminous the spectroscope will tell us with the 

 utmost certainty what terrestrial elements (if any) 

 exist in the constitution of the object. It is with the 

 revelations of the spectroscope respecting Brorsen's 

 comet that I now propose to deal. I must make a 

 few preliminary remarks, however, respecting the vari- 

 ous peculiarities of structure which have been presented 

 by comets. 



I assume that my readers are familiar with the 

 general appearance presented by comets at least by 

 those which are visible to the naked eye. It may be 



