194 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



of total reflection of heat from difl'erent sources within a prism, would afford much 

 more definite information as to the refrangibility of heat than any other method. 

 After much consideration, an apparatus of the following kind was adopted. 



51. It is fundamentally composed of a jointed frame, resembling a box ex- 

 actly square, ten inches in the side, without top or bottom, and having hinges at 

 every angle, so that it may be formed into a lozenge of any degree of obliquity. 

 This is seen in Plate XIII, Fig. 1, and marked AB. By an arrangement present- 

 ly to be described, the rays of heat are made to pass parallel to the edge a c of 

 one of the sides of the box, and to faU upon the prism P, whence, after under- 

 going reflection (total or partial) at the posterior surface of the prism, they pro- 

 ceed parallel to the line a d, and fall upon the sentient extremity of the pile at p. 

 Now, in order that this course may be taken by the reflected rays, it is necessary 

 that, supposing the prism to be an isosceles one, the posterior reflecting surface 

 a' V Fig. 2, should form equal angles with the incident and reflected rays c e and 

 fd. It was to effect this that the arrangement of the jointed lozenge was adopt- 

 ed. The prism P (Fig. 1) rests on a column 0, moveable round the line of junc- 

 tion of the sides C and D of the lozenge. The column has connected with it a 

 tail-piece of brass a E passing through the diagonal of the frame, and preserved 

 constantly in that position by a slit parallel to its length, through which passes a 

 clamping screw h, serving at once to maintain this constancy of direction, to se- 

 cure the form of the moveable lozenge, and by means of an index pointing to a 

 graduated scale of inches reckoned from a, along a E, to determine the length of 

 the diagonal a 5 at any moment, and consequently the angles of the lozenge. 



52. A little consideration of this mechanical an-angement, will shew how it 

 is adapted to the end in view. The rays from a source of heat S, rendered pa- 

 rallel by the lens of rock-salt L, fall upon the prism P, and, after undergoing two 

 refractions and one reflection, they fall upon the sentient surface of the pile p. 

 This will always take place so long as the posterior surface of the prism forms 

 equal angles with the lines a c, a d, which will be secured by making it truly per- 

 pendicular to the tail-piece a E, by which it is guided, and which of com-se always 

 bisects the angle cad. Now, it is evident that, whilst the angle cad remains 

 small, the reflection wiU continue partial, but that as the diagonal a h is shortened, 

 a point win be reached when total reflection abruptly commences, which ought 

 to be indicated by a saltus in the movement of the galvanometer connected with 

 the pile. This critical angle will be soonest attained for rays of greatest refran- 

 gibility, and the calculation of the refractive index of the prism is reduced to a 

 simply mathematical problem. 



53. Before going farther, we shall proceed to solve this problem, viz. : A ray oj 

 light GD (fig. next i^age) falls upon the surface AC of a prism, which has the angles 

 at A and B equal ; it falls wpon the surface AB at the critical angle of total reflection ; 

 required the index of refraction (/*) of the prism the angle of incidence («) being given. 



