S. P. Langley on the Selective Absorption of Solar Energy. 181 



colour, the medium would (previous to investigation) be 

 deemed colourless. In the same way common opinion regards 

 our air as colourless; yet it cannot be so, but must necessarily 

 (according to these observations) be considerably coloured. 



As we have been accustomed to regard it as colourless, 

 however, it is clear that were it removed we should, in seeing 

 the sun's true appearance for the first time, regard the sun 

 itself as coloured. 



Our w f hite light, then, is not the sum of all radiations, but 

 only of a part, even of the visible ones. 



4th. We can, by measuring the area of the curve outside 

 our atmosphere and comparing it with the area of the curve 

 within, obtain by a method never before pursued, which is in 

 close accord with theory, a value for the solar constant. 



Previous observations have found from 1*7 cal., in the 

 time of Pouillet, to 2*5 cal. in that of Violle, with a ten- 

 dency to increase. The value here given from our preliminary 

 investigation is 2*84 cal. The last figure is of little weight ; 

 and the exactness of that in the first decimal place is probably 

 open to doubt. The conclusion which we are entitled to draw r 

 from these investigations in the stage here presented is that 

 the solar constant is in reality greater than has been heretofore 

 supposed, and that it is probable that it is not very greatly 

 inferior to 3 calories. This important point will be discussed 

 fully in connexion with the Mount-Whitney observations, 

 with which the complete graphical constructions elucidatory 

 of our present tables will be given. 



5th. These observations show heat in extreme ultra-violet 

 rays, and the change of temperature (hitherto unobserved) in 

 the Fraunhofer lines. They lend increased probability to the 

 belief that all the energy in any ray can be exhibited as heat, 

 if there be a proper medium to receive this energy. Their 

 evidence, so far as it goes, then, favours the conception of one 

 solar energy, which is interpreted in terms of heat, or of light, 

 or of chemical action, according to the medium by means of 

 w T hich we choose to observe it. 



6th. The ratio of luminous to dark heat has evidently been 

 wholly changed by the selective absorption. The ratio at the 

 sea-level may be found with close approximation by measuring 

 the tw r o areas — (1) above the point where we assume the lumi- 

 nous spectrum to end, and (2) below it. This point each one 

 may define differently ; for the extent of the luminous spectrum 

 depends much upon our precautions for observing it. If we 

 assume it to end near B, then three quarters of the energy 

 must be termed invisible ; if at the actual visual extremity 

 (far below A), then less than half. To fix our ideas, let us 



