1 84 Prof. Forbes's Researches on Heat. \ 



proximately parallel beam by the lens L. It then passes 

 through a diaphragm T, placed on one or other side of the 

 prism (it does not much matter which, as the beam which 

 arrives at the pile is always much wider than the second dia- 

 phragm t, placed there to admit only the central rays arriving 

 parallel to the line a c). The use of this diaphragm is, that 

 a narrow enough pencil of rays may be employed, to be in- 

 dependent of the variable breadth under which the surface 

 of the prism is presented to the incident beam. The usual 

 dimension of this diaphragm was one inch in breadth and one 

 and a quarter in height, but in some instances its breadth was 

 reduced to three-eighths of an inch. 



The pile p has its funnel-shaped orifice closed by a screen 

 widi a vertical slit, an inch wide, in the direction of its axis. 

 But there is a peculiarity in the arrangement of the pile very 

 essential to the success of these experiments, where the pile 

 itself is moveable, which I must not omit to mention. Its 

 exposure to currents of air would render the observations, 

 when the pile cannot be entirely inclosed by a box or screen, 

 very capricious in its action. I therefore adapted to the end, 

 bearing the conical reflector, an adjustable wooden tube r, 

 containing a rock-salt lens, which still further increased its 

 sensibility, and totally protected it from aerial currents. 



The abruptness of the effect of transition from partial to 

 total reflection is far from being so complete as might be 

 wished ; and this is easier accounted for than remedied. It 

 arises mainly from the magnitude of the source of heat, the 

 consequent want of parallelism of the refracted rays, the scat- 

 tering of these rays in consequence of the imperfect polish of 

 the surfaces, the unequal intensity of the rays in different 

 parts of the section of the cylinder, and lastly, owing to the 

 want of homogeneity of the rays of heat from any source, 

 which the method would serve to measure, were the other 

 imperfections removed, just as in the course of the total re- 

 flection of light, prismatic colours are successively presented. 



My first rude attempts showed all this very clearly. As the 

 diagonal ab of the lozenge (fig. 1) shortened, total reflection 

 obviously succeeded to partial, and the change was not only 

 very great, but near one point very rapid. The point where 

 the most rapid increase took place, is obviously that where 

 the greater proportion of the incident rays underwent total 

 reflection, and might therefore be taken as a mean represent- 

 ation of the quality of the heat. Still the change w^as too 

 gradual to enable one by mere inspection to determine this 

 point with accuracy, and I speedily resolved to take the sure 

 but laborious method of ascertaining at a number of points 



