Third Series. — Refrangibility of Heat. 181 



heat be now one yielding rays of greater refrangibility. Al- 

 though the radius of the circular arc was (if I understand the 



p 



Fig. 1. K * 



account rightly) eleven inches, but little deviation of position 

 was required for heat from different sources ; and M. Melloni 

 admits that, whilst his experiment indicates the difference of 

 refrangibility, it is inadequate to measure it. 



There are many reasons why such a form of apparatus 

 must be rejected for accurate observations. I will mention 

 only the impossibility of obtaining a beam of heat which shall 

 preserve the same breadth at different distances from its source 

 (of course, supposing the rays rendered as parallel as possible 

 by refraction through a rock-salt lens), arising, 1. from the 

 angular magnitude of the source; 2. from the scattered re- 

 flection and refraction at the surfaces of the lens and prism; 

 3. from the want of homogeneity of the ray. On all these ac- 

 counts, the beam must have acquired a very sensible breadth 

 at the distance of the pile, and consequently the effect of heat 

 must be perceptible, and even nearly uniform, through a cer- 

 tain space. I may also add from experience, that the diffi- 

 culty of varying the arrangement of an experiment, so as to 

 get a maximum heating effect at the pile, is so considerable, 

 that no delicate result can be deduced from the merely ten- 

 tative procedure. Finally, the smallness of the variation of 

 refrangibility seems to require some more critical method of 

 ascertaining its measure. On all these grounds, it seemed to 

 me desirable to discover a method in some degree less open 

 to objection. 



The phsenomenon of total reflection, successfully employed 

 by Dr. Wollaston in the measurement of refractive indices in 

 the case of light*, presents the advantage of being (theoretic- 

 ally at least) abrupt in its action, the transition from partial 

 to total reflection being (with the necessary exception arising 

 from the want of homogeneity) an instantaneous change, 

 amounting in the case of light to many times the intensity of 



* Phil. Trans. 1802. 



