YOL. XC.] PHILOSOPHICAL TRANSACTIONS. 749 



nating from them ; and that such heat-making rays are subject to the laws of re- 

 flection and refraction. The similarity between light and heat in these points is 

 so great, that it did not appear necessary to notice some small difference between 

 them, relating to the refraction of rays to a certain focus, which will be mentioned 

 hereafter. But the next 3 articles of this paper will require, that while we show 

 the similarity between light and heat, we should at the same time point out some 

 striking and substantial differences, 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. 



Art. 4. Different Ref Tangibility of the Rays of Heat. — We might have con- 

 cluded this article in the first part of this paper, as a corollary of the former 3 ; 

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

 the laws of reflection and refraction, as has been shown, 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 shown, we find that 2° 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 7° ; 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 occasion heat be the same with those which occasion the colours, which is a 

 case that our foregoing experiments have not ascertained, the arguments for their 

 different refrangibility rests on the same foundation, namely, their being dispersed 

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

 gibility of the rays of heat follows of course. So far then a great resemblance a ain 

 takes place. 



I must now point out a very material difference, which is, that the rays of heat 

 are of a much more extensive refrangibility than those of light. In order to make 

 this appear, I shall delineate a spectrum of light, by assuming a line of a certain 

 length ; and dividing it into 7 parts, according to the dimensions assigned to the 

 7 colours by Sir Isaac Newton, in the 4th figure of the second part of his Optics, 

 I shall represent the illuminating power of which each colour is possessed, by an 

 ordinate drawn to that line. And here, as the absolute length of the ordinates is 

 arbitrary, provided they be proportional to each other, I shall assume the length 

 of that which is to express the maximum, equal to f£ of the whole line. Thus, 

 let Ga, fig. 12, pi. 13, represent the line that contains the arrangement of the co- 

 lours, from the red to the violet. Then, erecting on the confines of the yellow 

 and green the line lr=-§4 of gq, it will represent the power of illumination of the 

 rays in that place. For, by experiment already delivered, we have shown that the 

 maximum of illumination is in the brightest yellow or palest green rays. From 



