1 80 S. P. Langley on the Selective Absorption of Solar Energy, 



infra-red; and these ordinates not only indicate its character, 

 but give its amount. In contradiction to the statement of 

 many investigators and of present opinion on the point, we 

 find that (according to these measures) the absorption grows 

 on the whole less and less as we go down below the red to a 

 point near wave-length 2^*8. By this it is not meant to deny 

 the existence of regions of very great local absorption in the 

 lower spectrum. These same observations do in fact point out 

 new regions of such local absorption. But, excepting these, 

 they warrant us in saying that, broadly speaking, the absorp- 

 tion through the whole spectrum, visible and invisible, appears 

 to follow one simple law, and to decrease where the wave- 

 length increases; so that not only is the ultra-violet more 

 absorbed than blue, blue than yellow, and yellow than red, 

 but that red is more absorbed than the infra-red, and each 

 degree of infra-red is more so than the next one below it. 



3rd. By the use of the ordinary logarithmic formula, here 

 employed in its legitimate application to homogeneous waves, 

 we can pass from the curve inside to that outside the atmo- 

 sphere ; in other words, we can virtually transport our obser- 

 ving-station to a point wholly above the air, and determine 

 the distribution of the sun's heat before this unequal absorbent 

 action of our atmosphere has affected it. We need only 

 embody the results for selective absorption given by our 

 tables in a simple graphic construction (like that here shown 

 in connexion with the preliminary investigation) to see that 

 the point of maximum heat outside our atmosphere lies near 

 wave-lengths 0*50 to 0*55 — or, as we are entitled to say, that 

 the hottest portion of the spectrum outside the atmosphere 

 will be found rather in the green than, as here, near the 

 yellow. 



It is probable, from our measurements, that the sun would 

 appear of a decidedly bluish tint to the naked eye placed without 

 our air. 



This atmosphere, which we are so accustomed to regard as 

 colourless, has then, in fact, played a part analogous to that 

 of a yellowish or reddish glass, whose impure colour is not a 

 monochromatic yellow or red, but a compound of all spectral 

 tints in unaccustomed proportions. Had we in all our lives 

 had no light but from an electric light, seen through such a 

 reddish glass shade, we should probably have believed this 

 reddishness to be the " natural " or proper colour of the naked 

 carbons, and moreover that it represented " the sum of all 

 radiations." It would apparently answer, in an individual 

 brought up in ignorance of any other light, to our common 

 notion of whiteness ; so that even though it really possessed 



