162 J. W. Draper—Distribution of Heat in the Spectrum. 
under unexceptionable circumstances; the beam of electric 
light had practically undergone no atmospheric absorption, 
and the optical refracting train was of rock-salt. 
Sir J. Herschel had shown in 1840 that when the sun’s rays 
are dispersed by a flint-glass prism, the distribution of the heat 
toward the less refrangible region is not continuous, but there 
are three maximum points. These points, as shown by Dr. 
of the spectrum. 
In view of the preceding statement and others that might be 
given, it may, I think, be affirmed that the general opinion held 
at the present day as to the constitution of the spectrum is this, 
that there exists a heat spectrum in the less refrangible regions, 
a light spectrum in the intermediate, and a spectrum producing 
chemical action in the more refrangible regions. An exper 
mental attempt to correct this view, and to introduce a more 
. 
accurate interpretation, will not be without interest, especially 
subject of photometry. In this memoir I shall offer some 
experiments and suggestions respecting the heat of the spec 
trum, and in another, shortly to be published, shall consider the — 
distribution of the so-called chemical rays. Among the numer — 
ous problems of actino-chemistry there are none more 1mp0l” 
tant than these. 
All the experiments hitherto made on the heat of the ee : 
trum have been conducted on the principle of exposing a the? 
mometer in the differently colored spaces. Such was Sir W- 
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