490 LECTURE LI. 



through the conjugate or corresponding focus ; and if a body at the same 

 temperature be placed in that focus, the radiation will still be the same : but 

 if a substance absolutely cold were placed there, the whole of the heat before 

 reflected by the mirror would be intercepted, that is, almost half of that 

 which was received by the thermometer from the surrounding bodies ; and 

 if a piece of ice be put in the conjugate focus, a delicate thermometer will 

 instantly show its effect in depressing the temperature ; as if the cold were 

 absolutely reflected in the same manner as heat or light. 



Dr. Herschel's experiments have shown that radiant heat consists of 

 various parts, which are differently refrangible, and that in general, in- 

 visible heat is less refrangible than light. This discovery must be allowed 

 to be one of the greatest that has been made since the days of Newton, 

 although the theories of some speculative philosophers might have led to it 

 a few years earlier. Dr. Herschel was occupied in determining the pro- 

 perties of various kinds of coloured glass, which rendered them more or less 

 fit for enabling the eye to view the sun through a telescope ; and for this 

 purpose it was necessary to inquire which of the rays would furnish the 

 greatest quantity of light, without subjecting the eye to the inconvenience 

 of unnecessary heat. He first observed that the heat became more and 

 more considerable as the thermometer approached the extreme red rays 

 in the prismatic spectrum ; and pursuing the experiment, he found not 

 only that the heat continued beyond the visible spectrum, but that it was 

 even more intense when the thermometer was at a little distance without 

 the limits of the spectrum, than in any point within it.* (Plate XXXIX. 

 Fig. 546, 547.) 



Sir Henry Englefieldt has repeated these experiments with many ad- 

 ditional precautions, and Mr. Davy was a witness of their perfect accuracy : 

 the excess of heat beyond the spectrum was even considerable enough to 

 be ascertained by the sense of warmth occasioned by throwing it on the 

 hand. The skin appears, when compared with a thermometer, to have its 

 sensibility more adapted to the perception of radiant heat than to that of 

 heat imparted by contact, perhaps because a much smaller quantity of 

 heat is sufficient to raise the temperature of the thin cuticle very consider- 

 ably, than would be required in order to affect any thermometer in the 

 same degree. 



It was first observed in Germany by Hitter, and soon afterwards hi 

 England by Dr. Wollaston, that the muriate of silver is blackened by 

 invisible rays, which extend beyond the prismatic spectrum, on the violet 

 side. It is therefore probable that these black or invisible rays, the 

 violet, blue, green, perhaps the yellow, and the red rays of light, and the 

 rays of invisible heat, constitute seven different degrees of the same scale, 

 distinguished from each other into this limited number, not by natural 



* Herschel, Ph. Tr. 1800, p. 255, &c. Leslie, in Nich. Jour. iv. 244, called in 

 question this experiment. Landriani (Volta Lettere sull' Aria delle Paludi, 1777, 

 p. 136) andRochon (Recueil des Mem. 1785, p. 348) had placed the point of 

 greatest heat near the yellow. The matter was completely investigated by Seebeck, 

 Abhand. der Akad. Berlin, 1818-19, p. 305, and he found that the difference was 

 due to the substance of the prism : with water the point of greatest heat is in the 

 yellow ray ; with crown glass in the red ; and with flint-glass, beyond the red. 



f Jour, of the Royal Institution, 1802, p. 202. 



