188 SIR DAVID GILL, ON THE SIDEREAL UNIVERSE. 



viewed at a like distance would present the appearance of a spiral 

 nebula. But there is one point that I omitted to bring before you. 

 It was this. If there is dust scattered through space there is no 

 doubt that it would absorb the blue rays of light more than the red 

 or other less refrangible rays. Those of you who remember the 

 great volcanic explosion at Krakatoa may remember that the dust 

 of that explosion was carried right round the world, and gave rise 

 to glorious red sunsets, due to the absorption of the blue light by 

 the dust. Now suppose two stars having similar spectra (that is to 

 say, originally of the same colour) — if dust pervades space, and if the 

 two stars are at very different distances, then the blue light of the 

 more distant of the two stars would be more absorbed than that of 

 the nearer star. 



Now ordinary photographic plates are more sensitive to blue than 

 to red light — therefore the difference between the photographic and 

 the visual magnitude of a star becomes a means of estimating its 

 distance. The process is difficult to explain in a few words, and the 

 results are not A^ery accurate. But on the assumption that cosmical 

 dust is uniformly distributed in space, Professor Kapteyn has 

 estimated that the Andromeda nebula is distant 10,000 light-years. 

 But if, as I think most probable, when you get to that part of 

 space where there are fewer stars, then I think it probable there 

 will also be less cosmical dust, and if that is so Kapteyn's estimate of 

 the distance of the Andromeda nebula will require to be largely 

 increased — and that is one of the reasons why I think that some 

 spiral nebulae are so distant as to be beyond our so-called universe, 

 and may be considered universes in themselves. 



There are other reasons for this view which it would take me too 

 long to explain now, but I quite agree with the Astronomer Eoyal 

 and Mr. Maunder that the latter part of my lecture is, and was 

 intended to be, speculative — and that the view I expressed with 

 regard to the spiral nebulae, though it seems to me probable, is 

 not to be as yet accepted as a proved scientific fact. 



As to Mr. Tuckwell's question — about the light of nebulae — I 

 suppose the question refers to those nebulae which do not give a 

 continuous spectrum — whose spectra are not continuous like stellar 

 spectra. 



These purely gaseous nebulae (such as 37 H. iv Draconis) give 

 a spectrum of bright lines — coincident with those emitted by 



