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



Table IV. 



X= -375 '400 -450 -500 -600 -700 '800 -900 1*000 



Winter d, (mean of 7 series) 31 88 190 294 328 259 172 111 91 



Spring d, (mean of 9 series) 18 57 139 218 281 274 188 121 94 



The average noon deflections for winter and spring, given 

 in the previous table, require further correction : — first, for the 

 overlapping portion of the (weak) second spectrum, which is 

 considered from provisional experiments to have here an in- 

 tensity one thirtieth that of the first ; secondly, for the selec- 

 tive absorption by silver surfaces ; thirdly, for the selective 

 absorption by one surface of speculum-metal; fourthly, for the 

 diminution of heat in the diffraction-spectrum with increase of 

 the angle of diffraction, which is here provisionally taken as 

 proportional to secant r. The selective absorption by the 

 material of the bolometer is here treated as negligible. 



These corrections are expressed as factors by which the 

 uncorrected deflections are to be successively multiplied, 

 except in the case of the first correction, which is to be sub- 

 tracted. (The second and third corrections have been deter- 

 mined here by special researches on metallic absorption, which 

 will form the subject of a separate memoir.) 



The researches here on the selective absorption of lampblack, 

 it should be added, are incomplete, and the value given may 

 be yet subject to a further correction due to this error. 



Table V. 



\= -375 -400 -450 -500 -600 -700 -800 -900 1-000 



Correction 

 I. (subtr.) -^ Xd- 40 sV Xd. 45 J 5 xd-so 



II. (factor) 3005 2-067 1-606 1448 1-301 1-227 M92 1-166 1-145 



III. „ 2-000 1-923 1-802 1-695 1-550 1-460 1-408 1-389 1-370 



IV. „ 1-034 1-039 1051 1-064 1-096 1-138 1-193 1266 1-366 



We have been measuring thus far " heat,"" by which we 

 mean the solar energy as interpreted by certain agents (that 

 is, silver, lampblack, &c.) in our apparatus. In the degree in 

 which we have above eliminated the selective absorption 

 peculiar to each of these agents are we entitled to speak of 

 the resultant values as proportional to the solar energy itself. 

 We do not suppose ourselves to have accomplished so untried 

 and difficult a task with exactness, but regard these curves as 

 useful as a first approximation to the absolute-energy curve. 



These corrections being applied, the final values of noon 

 deflections at Allegheny become: — 



