.272 Dr. Herschel's Investigation of the Poivers 



but is also subject to the laws of the dispersion arising from its 

 different refrangibility ; and, as this subject is new, I may be 

 permitted to dwell a few moments upon it. The prism refracts 

 radiant heat, so as to separate that which is less efficacious, from 

 that which is more so. The whole quantity of radiant heat 

 contained in a sun-beam, if this different refrangibility did not 

 exist, must inevitably fall uniformly on a space equal to the area 

 of the prism ; and, if radiant heat were not refrangible at all, it 

 would fall upon an equal space, in the place where the shadow 

 of the prism, when covered, may be seen. But, neither of these 

 events taking place, it is evident that radiant heat is subject to 

 the laws of refraction, and also to those of the different refran- 

 gibility of light. May not this lead us to surmise, that radiant 

 heat consists of particles of light of a certain range of momenta, 

 and which range may extend a little farther, on each side of 

 refrangibility, than that of light? We have shewn, that in a 

 gradual exposure of the thermometer to the rays of the prismatic 

 spectrum, beginning from the violet, we come to the maximum 

 of light, long before we come to that of heat, which lies at the 

 other extreme. By several experiments, which time will not 

 allow me now to report, it appears that the maximum of illumi- 

 nation has little more than half the heat of the full red rays ; 

 and, from other experiments, I likewise conclude, that the full 

 red falls still short of the maximum of heat; which perhaps 

 lies even a little beyond visible refraction. In this case, radiant 

 heat will at least partly, if not chiefly, consist, if I may be per- 

 mitted the expression, of invisible light ; that is to say, of rays 

 coming from the sun, that have such a momentum as to be 

 unfit for vision. And, admitting, as is highly probable, that the 

 organs of sight are only adapted to receive impressions from 



