1885. J on Sunlight and the Earth's Atmosphere. 267 



orange, and you see that the original light was not whito, but intensely 

 blue. If we couLl 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 

 colours which make up white, but blue in superabun lance ; so that, 

 though the white is, so to say, latent there, the dominant effect is 

 blue. The glass coloured veil does not put anything m, 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 beghi to see that it is at any rate conceivable 

 tliat we may have been hitherto under a delusion about the true 

 colour of the sun, though of course this is not proving that wi; 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 oj^tics, because we should find that white 

 light is not the sum of the sun's radiations, but only of those dregs of 

 them which have filtered down to us ; in meteorology because it is 

 snggeste 1 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 selected portions come 

 to us, and so altering the conditions on which this heat by which we 

 live, depends ; in otlier ways, innumerable, because, as we know, the 

 sun's heat and light are facts of such central imj^ortance, that they 

 affect almost every part of scientific knowledge. 



It may be asked what suggested the idea that the sun may be blue 

 rather than any other colour. 



My ow^n attention was first directed this way many years ago 

 when measuring the heat and light from different parts of the sun's 

 disk. It is known that the sun has an atmosj^here 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 connection with the absorbing vapours of sodium, 

 iron, and so forth, which produce them ; but my own attention was 

 ^particularly given to the regions of absorption, or to the colour it 

 caused, and I found that the sun's body must be deeply bluish, and 

 that it would shed blue liglit except for this apparently colourless 

 solar at UK sphere, 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 eflect 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 

 islands would rise above that of the torrid zone, and this directed my 



