Third Series,— Refrangibility of Heat, 183 



be reached when total reflection abruptly commences, which 

 ought to be indicated by a saltm in the movement of the gal- 

 vanometer connected with the pile. This critical angle will 

 be soonest attained for rays of greatest refrangibility, and the 

 calculation of the refractive index of the prism is reduced to 

 a simply mathematical problem. 



The following is the problem to be solved, viz. : A ray of 

 light G D (fig. 2.) falls upon the surface A C of a prisma 'which 

 has the angles at A and B equal ; it falls upon the surface 

 A B a/ the critical angle of total reflection; required the index 

 of refraction (fx,) of the prism, the angle of incidence (a) being 

 given. 



Fig. 2. 



An investigation of little difficulty gives the following result. 



I had a rock-salt prism constructed, so that the incidence 

 on the first surface might be nearly vertical at the critical 

 angle of total reflection, so as to avoid as much as possible 

 any error arising from imperfections of the surface, or want 

 of absolute equality of the angles at A and B; and likewise, 

 that within the limits of the experiment, the loss of heat by 

 reflection at the two surfaces might be nearly unaltered, as it 

 is believed to be almost constant at incidences tolerably nearly 

 perpendicular*. This prism, constructed for me by Mr. John 

 Adie, had two angles of 40° and one of 100°; and so accu- 

 rately was it made, that (satisfying myself with a careful mea- 

 surement by the common goniometer, extreme nicety being 

 unimportant) the angles appeared to be true to those quantities 

 within a iew minutes of a degree. 



By a reference to Plate IV., fig. 1, it will now be under- 

 stood that the required arrangement is of this kind. The 

 heat diverging from the source S, is converted into an ap- 



* See Melloni on the Reflection oiWedAyAnmles de Oiimie, Dec. 1835. 



