1885.] on Sunlujht and the Earth's Atmosphere. 269 



inferences ; but I think it is safe at least to say tliat the sky is not 

 self-luminous, and that, since it can only be shining blue at the 

 expense of the sun, all the light this sky sends us has been taken by 

 our atmosphere away from the direct solar beam, which would grow 

 both brighter and bluer if this were restored to it. 



If all tliat has been said so far renders it jDossible that the sun may 

 be blue, you will still have a right to say that "possibilities" and 

 " maybes " are not evidence, and that no chain of mere hypotheses 

 will draw truth out of her well. We are all of one mind hero, and 

 I desire next to call your attention to what I think is evidence. 



Remembering that the case of our supposed dweller in the cave 

 who could not get outside, or that of the inhabitants of the ocean-floor 

 who cannot rise to the surface, is really like our own, over whose 

 heads is a crystalline roof which no man from the beginning of time 

 has ever got outside of, an upper sea to whose surface we have never 

 risen ; we recognise that if we could rise to the surface, leaving the 

 medium whose effect is in dispute wholly beneath us, we should see 

 the sun as it is, and get proof of an incontrovertible kind ; and that, 

 if we cannot entirely do this, we shall get nearest to proof under our 

 real circumstances by going as high as we can in a balloon, or by 

 ascending a very high mountain. The balloon will not do, because 

 we have to use heavy apparatus requiring a solid foundation. The 

 proof to which I ask your kind attention, then, is that derived from 

 the actual ascent of a remarkable mountain by an expedition under- 

 taken for that purpose, which carried a whole j)hysical laboratory up 

 to a point where nearly cme-half the whole atmosphere lay below us. 

 I wish to describe the difference we found in the sun's energy at the 

 bottom of the mountain and at the top, and then the means we took 

 to allow for the eflfect of that part of the earth's atmosphere still over 

 our heads even here, so that we may be said to have virtually got 

 outside it altogether. 



Before we begin our ascent, let me explain more clearly what we 

 are going to seek. We need not expect to find that the original sun- 

 light is a pure monochromatic blue by any means, but that though its 

 rays contain red, orange, blue, and all the other spectral colours, the 

 blue, the violet and the allied tints were originally there in disjDro- 

 portionate amounts, so that, though all which make white were present 

 from the first, the refrangible end of the spectrum had such an excess 

 of colour that the dominant effect was that of a bluish sun. In the 

 same way, when I say briefly that our atmosphere has absorbed this 

 excess of blue and let the white reach us, I mean, more strictly 

 speaking, that this atmosphere has absorbed all the colours, but, 

 selectively, taking out more orange than red, more green than orange, 

 more blue than green ; so that its action is wholly a taking out — an 

 action like that which you now see going on with this sieve, sifting a 

 mixture of blue and white beads, and holding back the blue while 

 letting the white fall down. 



This experiment only rudely typifies the action of the atmosphere, 



