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Experiments on the Refrangibility of the invisible Rays of the Sun. 

 By William Herschel, LL.D. F.R.S. Read April 24, 1800. 

 [Phil. Trans. 1800, p. 284.] 



In a paper read to the Society at a former meeting, the Doctor an- 

 nounced some observations which seemed to indicate that there are 

 two sorts of rays proceeding from the sun ; the one the calorific rays, 

 which are luminous and refrangible into a variegated spectrum ; and 

 the other the invisible rays, which produce no illumination, but 

 create a sensible degree of heat, and appear to have a greater range 

 of refrangibility than the colorific rays. To the latter he assigns 

 the name of radiant heat. Having lately had some favourable oppor- 

 tunities to prosecute this investigation, he here delivers an account 

 of the series of experiments he made on the subject, which seem to 

 him to confirm the above conjecture. The mode of conducting these 

 experiments was simply this : 



On a horizontal tablet covered with white paper, and divided into 

 squares, for the conveniency of measurement, a part of the extreme 

 colour of a prismatic spectrum was suffered to fall, the remainder of 

 the coloured rays passing by the edge of the tablet, so as not to in- 

 terfere with the experiment. 



Three thermometers were placed on the tablet, at different distances 

 from the termination of coloured rays. The general results of the ten 

 experiments here described were as follows : 



From the four first it appears, that there actually are rays coming 

 from the sun which are less refrangible than any of those which 

 affect the sight ; that they are invested with a high power of heating 

 bodies, but with none of illuminating objects, which probably is the 

 reason why they have hitherto escaped unnoticed. 



The fifth and sixth experiments showed that the power of heating 

 is extended, though in a feeble degree, to the utmost limits of the 

 most refrangible or visible purple rays, but not beyond them ; and 

 that it is gradually increased as the coloured rays grow less refran- 

 gible. And from the four last experiments we gather that the maxi- 

 mum of the heating power resides among the invisible rays without 

 the prismatic spectrum, and is probably about half an inch beyond 

 the last visible one, or from the confines of the red ray. These like- 

 wise show that the sun's invisible rays, in their less refrangible state, 

 still exert a heating power, considerably beyond this maximum, fully 

 equal to that of the red- coloured light ; and that consequently, if we 

 may infer the quantity of the efficient from the effect produced, these 

 invisible rays of the sun probably far exceed the visible ones in 

 number. 



The inferences deduced from these results are, that the range of 

 refrangibility of radiant heat, or colorific rays, when dispersed by a 

 prism, begins at the purple-coloured light, where they are most re- 

 fracted, and have the least efficacy ; and that their refrangibility 

 lessens and their power increases as they approach the confines of 

 the red-coloured light, but that these confines are not the limits of 



