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S. P. Langley—The Actinic Balance. 187 
Art. XXII.—The Actinic Balance; by S. P. LANGLEY. 
THE writer has been, during some time, making experi- 
ments on the device and construction of an instrument more 
delicate and more prompt than the thermopile; an advance 
which his recent researches into the distribution of radiant 
energy in the spectrum have proved to be. indispensable. 
These researches have involved expenses for special apparatus 
which have been in part met by a grant from the American 
principal results obtained was made to that society in the early 
part of December, and will appear with illustrations of the 
apparatus in a forthcoming volume of their Proceedings, to 
which the reader who desires fuller details is referred. 
The following independent description of the newly devised 
apparatus is rendered necessary here, as an introduction to a 
future account of researches in the true distribution of radiant 
with the minimum of error which the vicious method of the 
prism admits. Even the use of the prism, however, deman 
most delicate means of measurement. Tyndall, employing 
every instrumental aid science commanded in his experiments 
on the electric light, was obliged to operate on a spectrum only 
an inch and a half in length, and it is from this that the well- 
nown heat curves of our text-books are derived. When we 
form a much longer spectrum, we must either make the face 
of our thermopile larger, or expect to find the radiation so 
weakened that we cannot measure it. 
Its results, has been almost completely neglected. It concerns 
Am. Jour. ape ay Series, Vou. XXI, No. 123.—Marozn, 1881. 
