AMERICAN JOURNAL OF SCIENCE. 
[THIRD SERIES.] 
Art. XVII.— The Selective pegs of Solar Energy ; 
y S. P. LANGLEY. 
INTRODUCTION. 
In 1800, Sir William Herschel published his remarkable in- 
Wortiiations in the Philosophical Transactions, on obscure 
eat, in which he reached the conclusion that heat 1 is an entity 
distinct from li 
These last opinions, ae cannot be said even yet to be 
universally accepted by physicists. 
Dr. J. W. Draper ek since pointed out that the maximum 
of heat did not necessarily lie in every case below th 
(though it does so in the prismatic spectrum), and that in a 
normal spectrum it would be in the orange. These conclu- 
sions as to the normal spectrum were unverifiable by direct ex- 
periment by any means he possessed. He found it eect 
with the most delicate papel at his command, to get 
Am. Jour. gre —Tuirp Sertss, VoL. XXV, No. 147.—Manrou, 1883. 
