174 oo. W. Draper—Dstribution of Heat in the Spectrum. 
TasieE IIll.—Distribution of heat by bisulphide of carbon. 
Seri 
sI. Series I. 
(1) Heat of the whole visible spectrum, 100 100 
(2) . more refrangible region,_--.-.--- 52 49 
(3) = less “3 Dt iin ee 48 51 
The sulphide employed was devoid of any yellowish tinge; it 
was quite clear. Series I. is the mean of eight experiments, 
Series IT. of ten. 
Taste [V.— Distribution of heat by quartz. 
: Series I. Series Il. 
(1) Heat of the whole visible spectrum, ------_--- 100 100 
(2 ye more refrangible region,_------- 49 53 
£3) * less . sf Ge SEN NE 51 47 
Series I. represents twenty-seven experiments, Series IL. 
twelve. In the former two quartz prisms were used to increas€ 
the dispersion ; in the latter only one was employed. 
Perhaps it may not be unnecessary for me to say that I have 
repeated these experiments many hundred times during a perio 
of several months, including the winter and the summer, vary- 
ing the conditions as to the hour of the day, arrangement of the 
paratus, &c., as much as I could, and present the foregoing 
tables as fair examples of the results. Apprehending that the 
heliostat mirror, which was of speculum metal, might exert 
some disturbing influence on account of its faint reddish tinge, I 
replaced it with one of glass silyered on the front face, but 
could not detect any substantial difference in the results. k 
The important fact clearly brought into view by these exper 
ments is, that if the visible spectrum be divided into two equal 
portions, the ray having a wave-length of 5768 being con 
sidered as the optical center of such a spectrum, these portions 
will present heating powers so nearly equal that we may impute 
the differences to errors of experimentation. Assuming this as 
true, it necessarily follows that in the spectrum any two series 
of undulations will have the same heating power, no matter 
what their wave-lengths may be. 
But this conclusion leads unavoidably to a most important 
modification of the views now universally held as regards the 
transmutation of force. ae 
From this point of view the conception that there exists 1? 
an incident ray various principles disappears altogether. We 
have to consider an incident ray as consisting solely of etherial 
