THIRD SERIES— REFRANGIBILITY OF HEAT. 193 



the pile was at d. The other parts maintaining the same positions, it is evident 

 that the pile must he moved into the position d', if the source of heat be now one 



yielding rays of gi*eater refrangibUity. Although the radius of the circulai- arc 

 was (if I understand the account rightly) eleven inches, but little deviation of 

 position was requu-ed for heat from different som-ces ; and M. Melloni admits 

 that, whilst his experiment indimtes the difference of refi-angibiMty, it is inade- 

 quate to measure it. 



49. There are many reasons why such a form of apparatus must be rejected 

 for accurate observations. I ^viU mention only the impossibility of obtaining a 

 beam of heat which shall jireserve the same breadth at different distances from 

 its source (of course supposing the rays rendered as pai'aUel as possible by re- 

 fraction through a rock-salt lens), arising, 1. from the angular magnitude of the 

 source ; 2. from the scattered reflection and refraction at the surfaces of the lens 

 and prism ; 3. from the want of homogeneity of the ray. On aU these accounts, 

 the beam must have acquired a very sensible breadth at the distance of the pUe, 

 and consequently the effect of heat must be perceptible, and even nearly uni- 

 form, through a certain space. I may also add from experience, that the diffi- 

 culty of varying the an-angement of an experiment, so as to get a maximum heat- 

 ing effect at the pile, is so considerable, that no delicate result can be deduced 

 fi'om the merely tentative procedm-e. Finally, the smallness of the variation of 

 refrangibUity, seems to require some more critical method of ascertaining its mea- 

 sure. On all these grounds, it seemed to me desh-able to discover a method in 

 some degree less open to objection. 



50. The phenomenon of total reflection, successfully employed by Dr Wol- 

 L ASTON in the measm'ement of refractive indices in the case of Ught,* presents the 

 advantage of being (theoretically 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 Ught to 

 many times the intensity of the smaller effect. It seemed reasonable to expect, 

 that an apparatus constructed on the principle of determining the critical angle 



» Phil. Trans. 1802. 

 VOL. XIV. PART I. B b 



