J. W. Draper—Distribution of Heat in the Spectrum. 165 
wave-lengths—the length at the line a is 7604, and that at H? 
3933, and these lines are not very far from the less and more 
tefrangible ends of the visible spectrum respectively. The mid- 
dle point of this spectrum is at 57 68, which may therefore be 
called its optical center. This is a little beyond the sodium 
line D, which is 5892. Now if by suitable means we reunite 
all the rays between 7604 and 5768 into one focus, and all the 
tays between 5768 and 3933 into another focus, are we not in a 
Position to determine the true distribution of the heat? Should 
the heat at these two foci be sensibly the same, must not the 
conclusion at present held be abandoned ? a0 
In these investigations the rays of the sun be used, it is 
necessary to restrict the examination to the visible spectrum, 
excluding the invisible red and invisible violet radiations. O 
these the earth’s atmosphere exerts not only a very powerful 
train 
‘pectrum, and that the indications they are giving are reliable. 
This variable absorptive action of the atmosphere depends 
partly on changes in the amount of water vapor, and partly on 
the altitude of the sun. At midday and at midsummer it is at 
‘minimum. The disturbance is not merely a thermochrose, 
for both ends of the spectrum are attacked. It is a matter of 
‘ays. But if the day be clear and the sun’s altitude sufficient, 
the visible spectrum may be considered as unaffected. 
+t should be borne in’ mind that the envelopes of the sun 
himself exert an absorptive action, which is powerfully felt in 
the ultra-violet region, as is indice the numerous fixe 
lines crowded together in that region. e force of this remark 
will be appreciated on examining the plate above referred to, 
m the Philosophical Magazine for May, 1848. 
. It seems then that all the conditions necessary for the solu- 
tion of this problem will be closely approached if we make use 
°t prisms constituted of any substance which is completely color- 
the eye, and confine our measures to the visible spectrum, 
collecting all the radiations between the fixed line A and the 
center of that spectrum just beyond p into one focus, and all 
* 
