182 S. P. Langley on the Selective Absorption of Solar Energy. 

 suppose it to terminate at Fraunhofer's A. We then find : — 



Luminous and ultra-violet energy (within") AO , Q 



the smooth curve) j- U'obtS 



Infra-red energy 0*632 



1-000 



The ratio of the invisible (infra-red) to the whole, then, is 

 0*632; and there is reason to believe this value rather too small 

 than too large. If, however, we deduct the space occupied by 

 the gaps in the lower spectrum, the ratio becomes 0'562. The 

 infra-red energy at sea-level may be roughly taken, as thus 

 defined, at three fifths of the whole. At the same time the ratio 

 of luminous to obscure energy without our atmosphere is, we 

 repeat, far greater than within it. 



We conclude (among other consequences of our observations) 

 that since the heat in the shorter wave-lengths (corresponding 

 in a general sense to high solar temperature) was thus 

 relatively greater before absorption, that we are obliged to 

 increase our usual estimates, not only of the amount of heat 

 the sun sends us, but (and very greatly) of the effective tem- 

 perature of the solar surface. 



The relatively small amount of energy corresponding to 

 great wave-lengths in the infra-red is due not so much to 

 absorption as to the fact that there is no considerable solar 

 energy existing there at all. The relatively great amount of 

 energy in the luminous part of the spectrum exists there, not 

 on account of a feeble absorption, but in spite of a strong 

 absorption ; and the original solar energy here was even much 

 more considerable than what we see. 



It is probable, however, that the solar spectrum before 

 absorption, though originally weak below the red,y<?£ extended 

 very much further into the infra-red than our charts indicate. 

 We may even regard it as probable that some agent of the 

 atmosphere acts as an almost complete barrier to the entrance 

 or departure of ra;ys below the point charted. 



It should be understood that these researches have here a 

 practical bearing of great consequence. The temperature of 

 this planet and with it the existence, not only of the human 

 race but of all organized life on the globe, appears, in the light 

 of the conclusions reached by the Mount Whitney expedition, 

 to depend far less on the direct solar heat than on the hitherto 

 too little regarded quality of selective absoption in our atmo- 

 sphere which we are now studying. 



The discussion of these and other points is reserved for a 

 subsequent memoir. Among these will be the fuller con- 

 sideration of the place of the principal absorption of water 



