SUNLIGHT AND THE EARTH'S ATMOSPHERE. 643 

 SUNLIGHT AND THE EARTH'S ATMOSPHERE.* 



By Professor S. P. LANGLEY. 



THERE is, we may remember, a passage in which Plato inquires 

 what would be the thoughts of a man who, having lived from 

 infancy under the roof of a cavern, where the light outside was inferred 

 only by its shadows, was brought for the first time into the full splen- 

 dors of the sun. 



We may have enjoyed the metaphor without thinking that it has 

 any physical application to ourselves, who appear to have no roof over 

 our heads, and to see the sun's face daily ; while the fact is, that if we 

 do not see that we have a roof over our heads in our atmosphere, and 

 do not think of 'it as one, it is because it seems so transparent and 

 colorless. 



Now, I wish to ask your attention to-night to considerations in 

 some degree novel, which appear to me to show that it is not transpar- 

 ent as it appears, and that this seeming colorlessness is a sort of delusion 

 of our senses, owing to which we have never in all our lives seen the 

 true color of the sun, which is in reality blue rather than white, as it 

 looks, so that this air all about and above us is acting like a colored 

 glass roof over our heads, or a sort of optical sieve, holding back the 

 excess of blue in the original sunlight, and letting only the white sift 

 down to us. 



I will first ask you, then, to consider that this seeming colorlessness 

 of the air may be a delusion of our senses, due to habit, which has never 

 given us anything else to compare it with. 



If that cave had been lit by sunshine coming through a reddish 

 glass in its roof, would the perpetual dweller in it ever have had an 

 idea but that the sun was red ? How is he to know that the glass 

 is " colored " if he has never in his life anything to compare it with ? 

 How can he have any idea but that this is the sum of all the sun's 

 radiations (corresponding to our idea of white or colorless light) ; will 

 not the habit of his life confirm him in the idea that the sun is red ; 

 and will he not think that there is no color in the glass so long as he 

 can not go outside to see ? Has this any suggestion for us, who have 

 none of us ever been outside our crystal roof to see ? 



We must all acknowledge, in the abstract, that habit is equally 

 strong in us, whether we dwell in a cave or under the sky ; that what 

 we have thought from infancy will probably appear the sole possible 

 explanation ; and that, if we want to break its chain, we should put 

 ourselves, at least in imagination, under conditions where it no longer 

 binds us. 



* A lecture delivered at the Royal Institution of Great Britain, Friday evening, April 

 17, 1885. 



