10 LIGHT SCIENCE FOR LEISURE HOURS. 



receive 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 a hypothetical case, for a 

 portion of the light of every comet yet examined does 

 in reality give a rainbow-coloured 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. 

 Accordingly, we find that our spectroscopists always 

 accompany the record of such an observation with an 

 expression of doubt as to the real nature of the object 

 which is the source of light. 



Secondly, when an electric spark flashes through any 

 vapour, its light gives a spectrum which indicates the 

 nature, not only of the vapour through which the spark 

 has passed, but of the substances between which the 

 spark has travelled. Thus, if we cause an electric flash 

 to pass between iron points through common air, we 

 see in the spectrum the numerous bright lines which 

 form the spectrum of iron, and in addition we see the 

 bright lines belonging to the gases which form our 

 atmosphere. 



Both the considerations above discussed are of the 

 utmost importance in studying the subject of the auroral 

 light as analysed by the spectroscope, because there are 

 many difficulties in forming a general opinion as to the 

 nature of the auroral light, while there are circum- 



