414 Mr. David Gill [May 29, 



Here is another interesting cosmical revelation wliicli we owe to 

 photography. 



You all know the beautiful constellation Orion, and many in this 

 theatre have before seen the photograph of the nebula which is now 

 on the screen, taken by Mr. Roberts. 



Here is another photograph of the same object taken with a much 

 longer exposure. You see how over-exposed, in fact, burnt out, the 

 brightest part of the picture is, and yet what a wonderful develop- 

 ment of faint additional nebulous matter is revealed. 



But I do not think that many persons in this room have seen this 

 picture, and probably very few have any idea what it represents. It 

 is from the original negative taken by Professor Pickering, with a 

 small photographic lens of short focus, after six hours' exposure in 

 the clear air of the Andes, 10,000 feet above sea-level. 



The field embraces the three well-known stars in the belt of Orion 

 on the one hand, and /? Orionis (Eigel) on the other. You can hardly 

 recognise these great white patches as stars ; their ill-defined character 

 is simply the result of excessive over-exposure. But mark the wonders 

 which this long exposure with a lens of high intrinsic brilliancy of 

 image has revealed. Here is the great nebula, of course terribly 

 over-exposed, but note its wonderful fainter ramifications. See how 

 the whole area is more or less nebulous, and surrounded as it were 

 with a ring fence of nebulous matter. This nebulosity shows a special 

 concentration about /S Orionis. 



Well, when Professor Pickering got this wonderful picture, know- 

 ing that I was occupied with investigations on the distances of the 

 fixed stars, he wrote to ask whether I had made any observations to 

 determine the distance of (3 Orionis, as it would be of great interest 

 to know from independent evidence whether this very bright star was 

 really near to us or not. It so happens that the observations were 

 made, and their definitive reduction has shown that /S Orionis is really 

 at tbe same distance from us as are the faint comparison stars. 

 /3 Orionis is, therefore, probably part and parcel of an enormous 

 system in an advanced but incomplete state of stellar evolution, and 

 that what we have seen in this wonderful picture is all a part of that 

 system. 



I should explain what I mean by an elementary or by an ad- 

 vanced state of stellar evolution. There is but one theory of 

 celestial evolution which has so far survived the test of time and com- 

 parison with observed facts, viz. the nebular hypothesis of Laplace, 

 Laplace supposed that the sun was originally a huge gaseous or 

 nebulous mass of a diameter far greater than the orbit of Neptune. 

 I say originally^ do not misunderstand me. We have finite minds ; 

 we can imagine a condition of things which might be supposed to 

 occur at any particular instant of time however remote, and at any 

 particular distance of space however great, and we may frame a theory 

 beginning at another time still more remote, and so on. But we can 

 never imagine a theory beginning at an infinite distance of time or at 



