NOTES ON NEBULA. 137 



Nebula in Orion. Further confirmation of this important 

 discovery was forthcoming when the photographs of the 

 spectrum of the Great Nebula were subsequently obtained. 

 On those photographs lines were present which are con- 

 stituted by light of such a nature as to be wholly invisible 

 to the eye, though perceptible on the photographic plate. 

 It is of the greatest interest to discover that these invi- 

 sible rays from the nebula are also indicative of the pre- 

 sence of hydrogen. Thus we obtain a beautiful confirmation 

 of the fact that the nebula is partly composed of glowing 

 hydrogen. 



There are, however, some remaining lines, the character 

 of which has not yet been ascertained. 



It would be a little premature to assert that there must 

 be some substance in the Great Nebula not at present 

 known to us on the earth. This would be, no doubt, one 

 interpretation of the facts. We must, however, admit the 

 possibility of another explanation. It is frequently found 

 that the lines yielded by an incandescent material vary to 

 some extent when the physical conditions of temperature 

 and of pressure are modified. It is, therefore, not impos- 

 sible that the unknown lines in the spectrum of the Great 

 Nebula may be due to some element known to us, but 

 which has not yet been tested under the conditions which 

 would make it yield the particular rays we are speaking of. 



The composition of a nebula as disclosed to us by these 

 researches is very instructive. Here we are looking at 

 an object which seems to lie at the very limits of the 

 visible universe an object so remote that our attempts to 

 fathom its distance are quite unsuccessful; yet in this 

 inconceivably distant part of our system we find at least 



