Hat 39, 1S85.1
��lighUieag, but n'hieli really proBscs with nearly a ton to each square foot, so that the weight of all the buildtnga in this great city, for inalajice, Is less than that of the air above them.
I Lope shortly to describe the method of proof that It, too, haa been acting like an optical sieve, holding back the blue; but it ma; uaturaUy be asked, Can our senses have so entirely deceived us that they glee no hint of this tmth, if It be one ? Is the appeal wholly to recondite scientific methods, mid are Uiere DO 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 on orange tnedluni, and keeps the blue light back in the upper sky.
It I hold this piece of glais before my eyes. It seems colorless and transparent; but it is proved not to be so by looking through It edgewUe. when the light, by traversing a greater extent, brings out Its true color, which is yellow. Ever; one knows thia in every-day eiperlence. 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, Hie rosy lint on the sriows of Mont Blanc, forerunning the dawn, or the warm glow of the sun as he seta in Egyptian skies, ahow 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 Gne 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 Tyndnll is In no way responsible for my own inferences ; but I think It is safe, at least, to aay that the sky is not sel (-luminous, and that, since it can only be shining blue at the expense of the sun, all the light this sky sends us haa been taken by our atmos- phere away from the direct aolar beam, which would grow both brighter and bluer if this were restored to it.
If all that has been aaid ao far rendera It possible that the aun may be blue, you will still hare a right to say that 'possibilities' and ' maybes* are not evi- dence, and diat 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.
Bemembe ring that the case of our supposed dweller in the cave who could not gel outside, or that of the inhabitants of rb^ 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 ouisldeof, — an upper sea to whose surface we have never riaeu, — we recogni»e thai 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- cumstancea 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 atmoapbere lay below us. I wish to describe the difference we found in the aun'a enei^y at the bottom of the mountain, and at the top, and then the means we took to allow lor the effect of that part of the earth's atmosphere still over □ur beads even here, so that we may be said to hare 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 lo find that the original sunlight is a pure mono- chromatic blue by any means, but that though its raya contain red, orange, blue, and all the other spectral colors, the blue, the violet, and the allied tints were originally there in disproportionate amoiuila; BO 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 aay briefly that our atmosphere has absorbed this ex- cess of blue and let the while reach us, I mean, more strictly speaking, that this atmosphere has absorbed ull the colors, but selectively taking out more orange than red, more green than orange, mure blue Iban green; ao that Its action is wholly a taking-uul, — an action like that which you now see going on with this sieve, sifting a mixture of blue and white beads, and holding bsck the blue, while letting the white fall down.
This experiment only rudely typllies the action of the atmosphere, which Is discriminating and selec- tive in an amazing degree ; and, as there are really an infinite number of aliadea 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' ot '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 ahow that white light is com- pounded of blue, red, and other colors, where, by
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