220 On some Recent Spectroscopic Researches. [April, 



one of the lines of hydrogen. These objects were thus sho-wn to be 

 masses of luminous gas, of which the principal constituents were 

 hydrogen and nitrogen. 



Now it appeared possible that the application to these objects 

 of the more powerful spectroscope, and more exact methods already 

 described, by which the apparent coincidence of the nebular lines 

 with those of nitrogen and hydrogen would be subjected to the test 

 of a spreading out of the spectrum, three or fom- times as great as 

 that previously emjjloyed, might furnish new information on two 

 important points. 



If the coincidence of the hnes was found to be no longer main- 

 tained, then if it should be seen that both lines were, to about the 

 same amount, a httle more or a httle less refrangible than the 

 corresponding terrestrial lines, there would be reason to conclude 

 that the want of coincidence was due to a motion of the nebula 

 towards or from the earth. If a want of coincidence were obsen^ed 

 in one line only, or in both lines, but in different directions, then 

 there might be reason to infer that one or both linos were not 

 really due to the substances nitrogen and hydrogen. 



A careful re-examination of the nebula in Orion on several nights 

 showed that the coincidence of one hue with nitrogen and one hne 

 with hydrogen was perfectly maintained, a result which appeai-s to 

 show that the lines are really emitted by hydrogen and nitrogen ; 

 and further, that this nebula is not receding from us with a velocity 

 so great as 12 miles per second, for such a velocity added to the 

 earth's motion at the time would have produced a sensible shift of 

 the lines. If, however, the nebula were moving in the opposite 

 direction, it might have had a velocity of approach as great as 

 20 miles per second, for half of this amount would have been 

 annulled by the earth's motion from the nebula. 



The method of placing the spark before the object-glass brought 

 into notice a fact of sufficient interest to be recorded. The great 

 loss of brightness which followed from the distance at which the 

 induction spark j^assiug in nitrogen was observed, showed itself 

 alike m the case of hydrogen and nitrogen by the total extinction 

 of all the lines of the spectrum, with the exception of the one line 

 in each spectrum which is found in the nebulae. The obvious 

 inquiry suggests itself whether the other lines of hydrogen and 

 nitrogen were also originally present in the light of the nebuke, 

 and have been quenched on their way to us. As the nebube are 

 objects of sensible size, we cannot attribute the extinction of the 

 lines to the effect of distance alone. If one had reason to believe 

 that the fainter lines of nitrogen and hydrogen have been extingiiished 

 on their way to us, we should have experimental evidence of the 

 absorptive- power on light, which, from theoretical grounds, was 

 ascribed to cosmical space by Cheseaux and the elder Struvo. 



