J. W. Draper—Distribution of Heat in the Spectrum. 167 
(4.) If waves of light falling upon an absolutely black sur- 
face, and becoming extinct thereby, are transmuted into heat, if 
the warming of surfaces by incident light be nothing more than 
the conversion of motion into heat—an illustration of the modern 
doctrine of the correlation of forces—heat itself being only “a 
duce the same amount. For though an undulation of the latter 
Description of the Apparatus employed. 
The optical arrangement I have employed for carrying the 
foregoing suggestions into practice is represented by fig. 2, and 
™ a horizontal section by fig. 8. 
lack 
cient to permit the light of the slit to pass. After refraction 
the dispersed rays fall as a spectrum on a concave me 
lel rays. I have sometimes used one of speculum metal, 
Ut more frequently one silvered on its 
