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XXXIV. On the Amount of the Atmospheric Absorption. 

 By S. P. Langley*. 



THE earth is surrounded by an absorbing atmosphere, and 

 we never see the sun or the stars except through it. 

 When we wish to know what the absolute brightness of 

 the sun or of a star is, we must first ask what the degree 

 and kind of this absorption has been, and must add to the 

 directly observed quantities of light the amount that the 

 atmosphere has taken away. Accordingly, every one engaged 

 in such researches must determine in explicit or implicit 

 terms for himself, or take on trust through another, the 

 amount of the absorption, which there is great unanimity 

 in fixing at about 20 per cent, of the whole (at the sea- 

 level). Thus the earliest observations in the last century 

 give the light-absorption as 19 per cent. The very elaborate 

 ones by Seidel of Munich give 21 per cent., those by 

 Pritchard at Oxford 21 per cent.; the most recent, ^ by 

 Mueller, at Potsdam, 17 per cent. ; while the observations by 

 Pouillet on the sun's heat give 18 to 24 per cent. ; and 

 almost all of a great number which could be cited, whether 

 on light or heat, give about 20 per cent. It has indeed been 

 recognized of late years that the " light "-rays are, on the 

 whole, more absorbable than those of " heat/' and that, in 

 particular, blue light is much more so; but the difference 

 between the mean coefficients of " light " and " heat," as 

 found by the usual methods, is so small that we may here 

 continue to speak of this " light "-absorption of 20 per cent. 

 as closely applicable (in common estimation) to heat also. 

 Thus, the very careful series of Ericsson on the sun's heat 

 gives about 21 per cent. In fact, many thousands of obser- 

 vations by scores of observers, during the past and present 

 century, seem to have determined this proportion with all 

 the exactness of which it is capable. 



I have arrived at a result so wholly different that, in the 

 face of such authority, I almost hesitate to announce it ; for 

 I have been forced to the conclusion that all these determi- 

 nations are in error, and not in some small degree, but by 

 a quantity probably at least equal to the total amount in 

 question. 



I hasten to say, that I do not dispute the general accuracy 

 of the numerous skilful investigators of known repute who 

 have made these determinations, but that I dissent from the 

 method in almost universal use for reducing them ; for since 

 it is certain that none of these observers have been outside 



* Communicated by the Author. 



