442 



SCIENCE, 



Vol. V., No. 121 



risen above the ocean-floor were an intelligent being, 

 might he not plausibly reason that the dim greenish 

 light of his heaven — which is all he has ever known 

 — was the full splendor of the sun, shining through 

 a medium which all his experience shows is trans- 

 parent ? We ourselves are, in very fact, living at the 

 floor of a great aerial sea, whose billows roll hundreds 

 of miles above our heads. Is it not, at any rate, con- 

 ceivable that we may have been led into a like fallacy 

 from judging only by what we see at the bottom? 

 May we not, that is, have been led into the fallacy of 

 assuming that the intervening medium above us is 

 colorless because the light which comes through it 

 is so? 



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 show that 

 it is the sun which is blue, and the air really acts like 

 an 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 them by the light of reason; and 

 before this audience it is to that interpretation that 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 enfeebled disk of the sun when so 

 viewed. Is this intervening glass colored, or not ? It 

 seems nearly colorless ; but have we any right to con- 

 clude that it is so because it seems so? Are we not 

 taking it for granted that the original light which we 

 see through it is white, and that the glass is color- 

 less because the light seems unaltered ? and is not an 

 appeal to be made here from sense to reason, which, 

 in the educated observer, recalls that white light is 

 made of various colors, and that whether the original 

 light is really white and the glass transparent, or the 

 glass really colored and so making 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 between us and the sun, how can we say that 

 the same result might not follow? To make the 

 meaning of our illustration clearer, observe that this 

 blueness is not a pure spectral blue. It has in it red, 

 yellow, blue, and all the colors which make up white, 

 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, but acts, I repeat, like a sieve 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 

 too? 



I think we already begin to see that it is, at any 

 rate, conceivable that we may have been hitherto un- 



der a delusion about the true color of the sun, though 

 of course this is not proving that we have been so. 

 And it will at any rate, I hope, be evident that here 

 is a question raised which ought to be settled : for the 

 blueness of the sun, if proven, evidently affects our 

 present knowledge in many ways, and will modify 

 our present views in optics, in meteorology, and 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 have 

 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 action of our air, which picks out parts of 

 the solar heat (for instance, that connected with its 

 blue light), and holds them back, letting other se- 

 lected portions come to us, and so altering the con- 

 ditions 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 such central im- 

 portance, that they affect almost every part of scien- 

 tific knowledge. 



It may be asked, What suggested the idea that the 

 sun may be blue rather than any other color ? My 

 own attention was first directed this way many years 

 ago, when measuring the heat and light from differ- 

 ent parts of the sun's disk. It is known that the 

 sun has an atmosphere of its own, which tempers its 

 heat, and by cutting off certain radiations, and not 

 others, produces the spectral lines we are all familiar 

 with. These lines we customarily study in connec- 

 tion with the absorbing vapors of sodium, iron, and 

 so forth, which produce them; but my own attention 

 was particularly given to the regions of absorption, 

 or to the color it caused; and I found that the sun's 

 body must be deeply bluish, and that it would shed 

 blue light, except for this apparently colorless solar 

 atmosphere which really plays the part of a reddish 

 veil, letting a little of the blue appear on the centre 

 of the sun's disk where it is thinnest, and staining 

 the edge red, so that to delicate tests the centre of 

 the sun is a pale aqua-marine, and its edge a garnet. 

 The effect I found to be so important, that, if this 

 all but invisible solar atmosphere were diminished by 

 but a third part, the temperature of the British Is- 

 lands would rise above that of the torrid zone; and 

 this directed my attention to the great practical im- 

 portance of studying the action of our own terres- 

 trial atmosphere on the sun, and the antecedent 

 probability that our own air was also and independ- 

 ently making the really blue sun into an apparently 

 white one. We actually know, then, beyond con- 

 jecture, by a comparison of the sun's atmosphere 

 where it is thickest, and where it is thinnest, that an 

 apparently colorless atmosphere can have such an 

 effect; and analogous observations which I have car- 

 ried on for many years, but do not now detail, show 

 that the atmosphere of our own planet, this seemingly 

 clear air in which we exist like creatures at the bot- 

 tom of the sea, does do so. We look up through our 

 own air as through something so limpid in its purity, 

 that it appears scarcely matter at all; and we are apt 

 to forget the enormous mass of what seems of such 



