REPORT ON RADIANT HEAT. 
291 
That the heat is found to accompany the rays of light in the 
most constant and inseparable manner through various refrac¬ 
tions, as in the instance of the four lenses in the eye-piece of a 
telescope after reflection, is also remarked by Sir W. Herschel 
{Phil. Trans. 1800, Exp. 11.) 
II. Solar rays subjected to analysis by the prism. 
1.) The different heating powers belonging to different parts 
of the spectrum, were probably first observed by the Abbe 
Rochon. {Phil. Mag. June 1815 ; and Biot, Traite de Phys. 
iv. 600.) He found the maximum in the yellow-orange rays : 
the prism was of flint glass : his thermometer was filled with 
spirits, probably therefore tinged red : this may account for 
his result. 
I tried some experiments with the bulb of the thermometer 
painted red, which appeared to agree with his result. {Annals 
of Phil. li. 201.) 
Prof. Leslie applied his “ photometer” to these experiments. 
{Inquiry, p. 454.) 
Dr. Hutton observed the different heating powers, and that 
they are not proportional to the illuminating. {Piss, on Light 
and Heat, p. 38.) 
Landriani found the maximum in the yellow rays, as also 
did Senebier. (Volta, Lettere, Sfc. 136.) 
Berard {Mem. d'Arcueil, iii.; Ann. de Chimie, Ixxxv. 309,) 
repeated the experiment with a heliostat. He found the 
maximum in the red, but some heat beyond. He repeated the 
experiment in both the spectra formed by Iceland spar. 
2.) Sir W. Herschel {Phil. Trans. 1800, Part II.) first ob¬ 
served the maximum of heat beyond the red end of the visible 
spectrum, and considered the effect as due to essential!}/ in¬ 
visible rays of a separate kind from those of light. 
Yet he found them subject to the same laws of refraction, 
and their dispersion corrected by another prism : they w^ere con¬ 
centrated by a lens {Ibid. p. 317), and by reflexion (pp. 298, 
302). 
Leslie objects to the conclusion of invisible rays, and tries 
to account for it as owing to an optical cause. ( Inquiry, 
Note, p. 559; see also Nicholson's Journal, 4to, iv. 344 and 
416). 
Sir H. Englefield {Nicholson s Journal, iii. 125,) found heat 
beyond the visible red ; it does not appear whether it was there 
at a maximum : the rays were such as to be concentrated by a 
lens, and he compared the effects on a black and a white bulb. 
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