SIR DxVVID GILL, ON THE SIDEREAL UNIVERSE. 



187 



showing us the different ways in which astronomical knowledge 

 is being pursued, and it always seems to me that this makes a 

 lecture much more valuable and interesting than a mere statement of 

 results. 



The difficulty in sidereal astronomy is to bring together the lines 

 of thought so to obtain from them as correct an idea as possible of 

 the stellar universe. Sir David's idea is that many of the stars are 

 comparatively near the sun, but as we go further out we come 

 to the Milky Way. All these, together, constitute what he calls our 

 universe, and it is, he thinks, similar to one of the spiral nebulae we 

 see. These spiral nebula? and the nebula of Andromeda are, in his 

 view, much more distant, and may be said to constitute separate 

 universes. One reason for this is that the spectroscopic observa- 

 tions of the Andromeda nebula made at Mount Wilson suggest that 

 it is made up of a collection of bodies like our sun — but so far away 

 that we cannot separate them in our largest telescopes. We cannot 

 say that this is definitely proved, but must rather regard it as a 

 speculation for the present. 



Mr. Martin Rouse said : W^hen a photographic plate is exposed 

 beyond a certain number of hours, does it not cease to give any 

 more stars, and does not the number of stars gradually imprinted 

 on the plate gradually diminish with the time of the exposure ? 

 Does the lecturer think this indicates that we are approaching the 

 limit of the universe "? 



Sir David Gill said : That has been investigated, and we find 

 that there is a very rapid decrease in the number of stars and that 

 they are much more sparsely scattered in space, the farther we go. 



The Rev. John Tuckwell asked if Sir David would just say a 

 word as to the nature of the light emanating from the nebulae 1 



Professor Orchard asked what was the explanation of those 

 extraordinary black spaces shown in some of the slides ; do they 

 indicate that the universe is limited and beyond this there is outer 

 darkness 1 



Sir David Gill in reply said : I entirely agree with Mr. 

 Maunder and the Astronomer Royal that what I have said to 

 you in the latter part of my address is speculative. We have no 

 absolute proof as yet that the stars immediately surrounding the 

 sun if viewed say from the great nebula in Andromeda would 

 constitute a nebulous-looking cluster, and that the Milky Way if 



