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��rlieii abovu the ocean-floor were an Intelligent being, might he not pUnalbly reaaon that the dim greaiiiah light of big heaven — which Is all he has ever known — wu the full splendor of the sun, shining through a medium which all hie experience shows ia trans- pareDt ? We ounelrea are, in very fact, living at the fioor of a. great aerial sea, whose billows roll hundreds of rolles above our heads. Is it not, at any rate, con- ceivable that we loay have been led Into a like fallacy from judging only by what we see at the bottom? May we not. that is, have beati led into the fallacy of assuralng that the intervening medium above us is colorless because the light which comes Uirough it I8»0?

I freely admit that all men, educated or ignorant, appear to have the evidence of their senses that the air is colorless, and that pure sunlight is white; so that, if I venture to ask you to listen to considerations which have lately been brought forward to fihow that it Is the sun which is blue, and the air really acts tike au orange veil, or like a sieve which picks out the blue and leaves the white, I do so in the confidence that I may appeal to you on other grounds than those I could submit to the primitive man, who has his senses alone to trust to; for the educated intelligence possesses those senses equally, and, in addition, the ability to Interpret tbera by the light of reason; and before this audience It is to that Interpretation tJiat I address myself.

Permit me a material illustration. You see through this glass, which may typify the intervening medium of air or water, a circle of white light, which may represent the enteehled dislt of the sun when so viewed. Is this intervening glass colored, or not ? It seems nearly colorless; but have we any right tu con- clude that it is so becaiue it seems so? Are we not taking Ufor grantti that the original light which we see through it is white, and that the gloss is color- less because the light seems unaltered f and is not an appeal to be made here from sense to rea^ion, which, in the educated observer, recalls that white light Is mode of various colors, and that whether the original light is really while and the glass transparent, or the glass really colored and so mnking the white. Is to be decided only by experiment, by taking away the possibly deceptive medium? I can take away this glass, which was not colorless, but of a deep orange, and you see that the original light was not white, but intensely blue. If we could take the atmosphere away lietween us and the sun, how can we say that the same result might not follow? To make the meaning of oar illustration clearer, observe that this blaeness is not a pure spectral blue. It has la it red, yellow, blue, and all the colors which make up while, but blue In superabundance; so that, though the white is, so to say, latent there, the dominant effect is blue. The glass colored veil does not put any thing in, bat acts, I repeat, like aaieve straining out the blue, and letting through to us the white light which was there In the bluishness ; and so may not our air do so

��der a delusion about the true color of the sun, thoii^ of course this is nut proving that we have been to. And it will at any rale, I hope, he evident th«t here is a question raised which ought to be settled ; for the biueness of the sun, if proven, evidently affect* oar present knowledge in many ways, and will modify our present views in optics, in meteorology, suid In numerous other things, — In optics, because we should find that white light Is not the sum of the sun's radi- ations, but only of those dregs of them which hare filtered down to us; in meteorology, because it is suggested that the temperature of the globe, and the condition of man on it, depend in part on a curious selective actiori of our air, which picks out parts of the solar heat (for instance, thnt connected with Ita blue light}, and holds them back, letting other ae- tected portions come to us, and so altering the con- dltiona on which this heat by which we live depends; in other ways Innumerable, because, as we know,, the sun's heat and light are facts of »uch central ii portance, that they aSect almost every part of scIeii-4 tiflc knowledge.

It may be asked. What su^ested the idea that the sun may be blue rather than any other color ? Uy own attention was first directed this way many jears ago, when measuring the heat and light from differ- ent paris of the sun's disk. It is known that the sun baa an atmosphere of its own, which tempera It* heat, and by cutting oS certain radiations, and not others, produces the spectral lines we are all familiar with. These lines we cnatomarlly study in conneo- tlon with the absorbing vapors of sodium, iron, and i so forth, which produce tbem; but my own attenttoll ■" was particularly given to the regions of absorption, . or to the color it caused; and 1 found that the nm'a body must be deeply bluish, and that it would shed I blue light, except for this apfiarently colorless boIw \ atmosphere which really plays the part of a reddlsb, veil, letting a little of the bine appear on the ceatn 1 of the sun's disk where it Is thinnest, and staining f the edge red, so that to delicate tests the centre of 1 the sun Is a pale aqua-marine, and its edge a garnet. | The effect I found to be so important, that, if ttati-l all but invisible solar atmosphere weredlrainished by 'I but a third pan, the temperature of the Brllisb !»• | lands would rise above that of the torrid i this directed my attention to the great practical Im portance of studying the action of our o trial atmosphere on the sun, and the antecedei probability that our own air was also and independ*! ently making the really blue sun into an apparently J white one. We actually know, then, beyond con- jecture, by a comparison of the sun's atmosphen I where it is thickest, and where it is ihlnnest, that an \ apparently colorless atmosphere eon have such i effect; and analogous otiservations which I have ci ried on for many years, but do not now detail, ahow J that the atmosphere of our own planet, this seemingly 4 clear air in which we exist like creatures at the boU ./ lom of the sea, docs do so. We look up through our I own air as through something so limpid in its parity, J appears scarcely matter at all; and we are ^it.J

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