1885.] Professor S. P. Laivjleij on Sunlhjht, dc. 265 



WEEKLY EVENING MEETING, 



Friday, April 17, 1885. 



Sir William Bowman, Bart. LL.D. F.E.S. Honorary Secretary 

 and Vice-President, in the Chair. 



Professor S. P. Langley. 



Sunlight and the Earth's Atmosphere. 



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 splendours 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 thiuk of it as one, it is because it seems so transparent 

 and colourless. 



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 trans- 

 parent as it appears, and that this seeming colourlessness is a sort of 

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

 seen the true colour 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 coloured 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. 



1 will first ask you, then, to consider that this seeming colourless- 

 ness 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 

 "coloured" 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 colourless 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 colour in the glass so 

 long as he cannot 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 



