THE TWO COMETS OF THE YEAR 1868. 173 



difference, in character, between the light emitted by 

 the nucleus and that emitted by the coma ? At first 

 sight it seems that no other conclusion could be come 

 to. But a little consideration enabled Dr. Huggins to 

 arrive at a different result. The tongue-shaped bands 

 were not only narrower but very much fainter towards 

 one end. They were also fainter along their outer 

 edges, on account, of course, of the faintness of the 

 coma as compared with the nucleus. Now it was 

 possible that the narrowing down of the bands might 

 be only apparent, and due to the fact that their outer 

 parts, though really existent, became invisible at the 

 fainter end. And there were two modes of attacking 

 the question. First the observer could determine by a 

 careful inspection whether the light at the narrower 

 end of the tongues was so faint that it ought to disap 

 pear at the edges merely by undergoing the same sort 

 of reduction as the brighter light at the broader end of 

 the tongue : this would show that the coma does not 

 differ in constitution from the nucleus. Secondly, if 

 the strip brought under examination were narrowed by 

 any contrivance, it is clear that any difference which 

 might exist in the constitution of the coma and of the 

 nucleus ought to be exhibited in a more marked 

 manner. 



Dr. Huggins applied both methods, and each resulted 

 in showing that the nucleus has the same constitution 



O 



as the coma, excepting only that the exterior part of 

 the coma seems to give a continuous spectrum. In 

 other words, the nucleus and all the coma except its 



