16 LIGHT SCIENCE FOR LEISURE HOURS. 



object whose light illuminates it. Tims we cannot 

 pronounce positively as to the nature of an object 

 merely from the appearance of its spectrum, unless we 

 are quite certain that the object is self-luminous. For 

 example, we observe the solar spectrum to be a rain- 

 bow-colored streak crossed by a multitude of dark lines, 

 and we conclude accordingly that the sun is an incan- 

 descent globe shining through a complex vaporous at- 

 mosphere. We feel no doubt on this point, because 

 we are absolutely certain that the sun is self-luminous. 

 Again, we observe the spectrum of the moon to be 

 exactly similar to the solar spectrum, only, of course, 

 much less brilliant. And here also we feel no doubt 

 in interpreting the result. We know, certainly, that 

 the moon is not self-luminous, and therefore we con- 

 clude with the utmost certainty that the light we re- 

 ceive from her is simply reflected solar light. So far 

 all is clear. But now take the case of an object like 

 a comet, which may or may not be self-luminous. If 

 we find that a comet's spectrum resembles the sun's 

 and this is not altogether an hypothetical case, for a 

 portion of the light of every comet yet examined does 

 in reality give a rainbow-colored streak resembling the 

 solar spectrum we cannot form, in that case, any such 

 positive conclusion. The comet may be a self-luminous 

 body, but, on the other hand, its light may be due 

 merely to the reflection of the solar beams. Accord- 

 ingly, we find that our spectroscopists always accom- 



