438 Dr. Herschel's Experiments on the solar, and 



the same time point out some striking and substantial diffe- 

 rences, which will occur in our experiments on the rays which 

 occasion them, and on which hereafter we may proceed to 

 argue, when the question reserved for the conclusion of this 

 Paper, whether light and heat be occasioned by the same or by 

 different rays, comes to be discussed. 



Article iv. — Different Refrangibility of the Rays of Heat. 



We might have included this article*in the first part of this 

 Paper, as a corollary of the former three ; since rays that have 

 been separated by the prism, and have still remained subject to 

 the laws of reflection and refraction, as has been shewn, could 

 not be otherwise than of different refrangibility ; but we have 

 something to say on this subject, which will be found much more 

 circumstantial and conclusive than what might have been drawn 

 as a consequence from our former experiments. However, to 

 begin with what has already been shewn, we find that two 

 degrees of heat were obtained from that part of the spectrum 

 which contains the violet rays, while the full red colour, on the 

 opposite side, gave no less than seven degrees ;* and these facts 

 ascertain the different refrangibility of the rays which occasion 

 heat, as clearly as that of light is ascertained by the dispersion 

 and variety of the colours. For, whether the rays which occa- 

 sion heat be the same with those which occasion the colours, 

 which is a case that our foregoing experiments have not ascer- 

 tained, the arguments for their diffe^bnt refrangibility rests on 

 the same foundation, namely, their being dispersed by the 

 prism ; and that of the rays of light being admitted, the different 



* See 2d and 4th experiments, pages 258 and 259. 



