SUNLIGHT AND THE EARTH'S ATMOSPHERE. 647 



inferences ; but I think it is safe at least to say that 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 atmos- 

 phere away from the direct solar beam, which would grow both brighter 

 and bluer if this were restored to it. 



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

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

 " may-bes " are not evidence, and that no chain of mere hypotheses 

 will draw Truth out of her well. We are all of one mind here, 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 can not 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 

 recognize 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 can 

 not entirely do this, we shall get nearest to proof under our real cir- 

 cumstances 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 undertaken for that pur- 

 pose, which carried a whole physical laboratory up to a point where 

 nearly one 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 mount- 

 ain and at the top, and then the means we took to allow for the effect 

 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 colors, the 

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

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

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

 of color 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 speak- 

 ing, that this atmosphere has absorbed all the colors, 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. 



