May 20, 1885.] 



SCIENCE 



443 



lightness, but which really presses with nearly a ton 

 to each square foot, so that the weight of all the 

 buildings in this great city, for instance, is less than 

 that of the air above them. 



I hope shortly to describe the method of proof that 

 it, too, has been acting like an optical sieve, holding 

 back the blue; but it may naturally be asked, Can 

 our senses have so entirely deceived us that they 

 give no hint of this truth, if it be one ? Is the appeal 

 wholly to recondite scientific methods, and are there 

 no indications, at least, which we may gather for 

 ourselves ? I think there are, even to our unaided 

 eyes, indications that the seemingly transparent air 

 really acts as an orange medium, and keeps the blue 

 light back in the upper sky. 



If I hold this piece of glass before my eyes, it seems 

 colorless and transparent; but it is proved not to be 

 so by looking through it edgewise, when the light, 

 by traversing a greater extent, brings out its true 

 color, which is yellow. Every one knows this in 

 every-day experience. We shall not get the color of 

 the ocean by looking at it in a wineglass, but by 

 gazing through a great depth of it ; and so it is with 

 the air. If we look directly up, we look through 

 where it is thinnest; but if we look horizontally 

 through it towards the horizon, through great thick- 

 nesses, as at sunrise or sunset, is it not true that 

 this air, where we see its real color most plainly, 

 makes the sun look very plainly yellow or orange ? 

 We not only see here, in humid English skies, the 

 ' orange sunset waning slow,' but most of us, in these 

 days of travel,- can perfectly testify that the clearest 

 heavens the earth affords, the rosy tint on the snows 

 of Mont Blanc, forerunning the dawn, or the warm 

 glow of the sun as he sets in Egyptian skies, show 

 this most clearly, — show that the atmosphere holds 

 back the blue rays by preference, and lets the orange 

 through. 



If next we ask, What has become of the blue that 

 it has stopped? does not that very blue of the mid- 

 day sky relate the rest of the story, — that blue which 

 Professor Tyndall has told us is due to the presence 

 of innumerable fine particles in the air, which act se- 

 lectively on the solar waves, diffusing the blue light 

 towards us ? I hope it will be understood that Pro- 

 fessor Tyndall is in no way responsible for my own 

 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 ' maybes ' are not evi- 

 dence, 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 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 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 incon- 

 trovertible kind ; and that, if we cannot 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 purpose, which car- 

 ried 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 mountain, 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 ex- 

 pect to find that the original sunlight is a pure mono- 

 chromatic 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 disproportionate 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 ex- 

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

 strictly speaking, 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-oiti, — 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, which is discriminating and selec- 

 tive in an amazing degree ; and, as there are really an 

 infinite number of shades of color in the spectrum, 

 it would take forever to describe the action in de- 

 tail. It is merely for brevity, then, that we now 

 unite the more refrangible colors under the general 

 word ' blue,' and the others under the corresponding 

 terms ' orange ' or ' red.' 



All that I have the honor to lay before you is less 

 an announcement of absolute novelty than an appeal 

 to your already acquired knowledge, and to your rea- 

 son as superior to the delusions of sense. I have, 

 then, no novel experiment to offer, but to ask you to 

 look at some familiar ones in a new light. We are 

 most of us familiar, for instance, with that devised 

 by Sir Isaac Newton to show that white light is com- 

 pounded of blue, red, and other colors, where, by 



