SIR DAVID GILL, ON THE SIDEREAL UNIVERSE. 



189 



incandescent hydrogen, helium and nitrogen — and another line, 

 which, so far, has not been identified with any terrestrial spectrum 

 and which, for want of a better name, we call " nebulum." As to 

 the blank spaces seen in the photograph of portions of the Milky 

 Way. My belief is that they are quite possibly due to absorbing 

 clouds of matter. We have nebulous clouds which shine ; I can 

 conceive that there are others which do not shine but which absorb 

 light. But here also we are not yet certain — the matter is not yet 

 proved. 



I thank you once more for the kind reception you have given to 

 my remarks. 



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FIG. 1. 



