1889.] On the Spectrum of the Great Nebula in Orion. 57 



•approximate X 3998 

 3988-0 



2nd S rou P 1 „ 3975-0 



3959-0 

 3896-0 

 3887-0 

 3878-0 

 3870-0 

 3859-0 



3rd S rou P < „ 3854-0 



3848-0 

 3842-0 

 3832-0 

 3825-0 



Chemical Significance of the Lines. 



Until I can obtain more photographs taken on different parts of the 

 nebula, I wish to be understood to speak on this point with much 

 hesitation, and provisionally only. We know certainly that two of 

 the lines are produced by hydrogen. The fineness of these lines 

 points to a high temperature and condition of great tenuity of the 

 hydrogen from which the light was emitted. This condition of the 

 hydrogen may give us a clue as to the probable interpretation of the 

 other lines. These may come from substances of very low vapour- 

 density, and under molecular conditions which are consistent with a 

 high temperature. It is in accordance with this view that the recent 

 measures of Dr. Copeland, since confirmed by Mr. Taylor (loc. cit.), 

 show with great probability that the line known as D 3 , which has 

 been supposed to indicate some substance of low vapour- density, 

 which shows itself only at the hottest region of the sun, is present in 

 the nebular spectrum. The great simplicity of the three pairs of 

 lines seen in the photograph of 1889 suggests a substance of a similar 

 chemical nature. 



If hydrogen can exist at half its usual vapour-density, with a mole- 

 cule of one atom only, we might possibly expect to find it in some of 

 these bodies, but at present we do not know what its spectrum would 

 be in such a condition. It may be possibly that it is in molecular 

 states of our elements other than those we are acquainted with that 

 we may have to look for an interpretation of some of the lines of 

 these bodies. 



[With respect to the groups of lines which cross the star spectra, 

 any statements must also be provisional only. 



These lines are distinct and fairly strong in the star spectra, 

 and extend, some farther than others, into the adjoining nebular 



