of Heat i7i the Prismatic Spectnan. 449 



liovvever, comparative investigations should also be made with 

 achromatic lenses, since the simple lenses produce coloured 

 streaks, which are probably not without influence on the result. 

 M. Rochon mentions, in his Recueil de Memoires sur la 

 Mecanique et la Physique, p. 652, &c. Paris 1 783, fifteen ex- 

 periments on the difference of heat in the red and in the oranae 

 ot his flint-glass prism ; in twelve of which he found the heat 

 between the yellow and the red (mjatine orange as he calls it) 

 greater than in the red, and less in three. He made use 

 ot an air-thermometer, but the bulb does not seem to have 

 been blackened ; which probably compelled him to use a lens, 

 in order to obtain exact results. He also distinctly says (at 

 p. 351) that he did make use of a lens, which he inclined and 

 directed according to the altitude of the sun. 



Among the few experiments mentioned by Sir H. C. Engle- 

 held, a similar deviation, although in a different place, is're- 

 c^rded : the heat in the first experiment was found less on 

 the limit of the red than in the red; but further below the li- 

 mit, greater again. But he had also concentrated the light by 

 a lens of four inches in diameter. 



As for myself, I found in no instance the heat greater in the 



*'''nF\7''*S '" *^ ''^'' ' ^"' ^^'^" ^ "^''^'' '"^'^^ "^e of a lens. 



Ul M. Berard's experiments I only know as much as is 

 contained in the report which was made on them by the com- 

 mittee ot the Institute, and in Biot's Traite de Physique (vol. iv. 

 p. 602, &c.). I find it no where mentioned of what species of 

 glass was the prism which he used, or whether he used more 

 than one. If the latter were not the case, we must suppose that 

 the pri^i he used belonged to the class mentioned in my 5th 

 position ; for M. Berard found the maximum of heat near the 

 limit of the red, the bulb of the thermometer bein^, however 

 still covered with red light. ' 



After M. Berard, M. Ruhland undertook experiments on 

 the evolution of heat in the prismatic spectrum, as may be 

 seen in his prize-work, crowned by the Academy of Berlin*. 

 He there says that he found the place of the maximum of 

 heat variable: with some prisms of glass (of which, however, 

 he gives no further description), and with one made of borax,' 

 he had found the maximum to be beyond the red, with others 

 in the red, and with several liquid bodies in the yellow. How 

 far M. Ruhland's experiments tend to confirm mine, cannot be 

 decided, since he has omitted to give the detail of liis experi- 

 mentsf. ^ q*. 



* Ueher die polaruche Wirlcung Sfc. i. e. On the polar effect of co- 

 loured heterogeneous light. Berlin 1817, p. 50. 



t During my residence at Munich in 1»14, I communicated to Messrs 

 V0I.6G. No. 332. Dec. 1825. 3 L Ruhland 



