204 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



ment gave me a result between 1.53 and 1.54. Without dwelling more than 

 necessary on this difference (our great object being gained when we have com- 

 pared heat and light under similar circumstances), I will mention the two causes 

 which I believe to produce it. (1.) It is undeniable that the transition from total to 

 partial reflection takes place much more gi-adually than is due to the mere hetero- 

 geneity of the rays; (this the experiment with light makes very obvious). The angle 

 of incidence throughout the range of experiment (from ob z=:14:.o to ab=. 16.5) 

 within the prism varies from 42° 22' to 36° 38'. The intensity at any point is 

 made up of totally and of partially reflected light. Consequently throughout this 

 range of incidence, the partially reflected light must be more intensely reflected 

 as the incidence is greater, and it is easy to see that the effect of this variation in 

 the intensity of the partially reflected rays will have the effect of shifting all the 

 curves towards the right hand in Plate XII. (2.) We have before remarked, that 

 owing to the dimensions of the source of light or heat, the rays do not form a re- 

 fracted beam of uniform intensity. The central rays are usually brightest. Now, 

 it may be shewn that in consequence of the varying angle of incidence the central 

 rays travel across the front of the pile, and consequently there would be a maxi- 

 mum effect produced at one point from this cause alone. 



77. I believe that the former cause is the only one whose effects are sensible, 

 or at least considerable ; and having reason to think that its action is similar upon 

 different kinds of heat, and also of light, we shall probably be very near the truth 

 if we substitute for the indices of refraction above found, others .04 or .05 lower. 

 But relative results are in this case by much the most important. 



78. The results which we have obtained apply, it must be recollected, only 

 to the pi^edominant kind of heat in any source, and that we have as yet got no in- 

 formation respecting the composition of a ray and the amount of dispersion. 



79. It is very easy to see that were the mathematical conditions of the expe- 

 riment (art. 55) fulfilled, we should be led to an exact analysis of heat, more per- 

 fect far than we have any prospect of obtaining in the case of light, considering the 

 difficulty of applying the photometer to coloured light. Were the curve in Plate 

 XIII. Fig. 3, solely representative of the progress of reflection due to the heteroge- 

 neity of the rays, the increment of intensity between any diagonal E and another 

 C, or D / would denote the proportion of the entire heat incident, which lies be- 

 tween the limits of refrangibility assigned by the diagonal, and found by the table 

 in art. 72. Thus an entire ray would be decomposed into parcels of kno^\Ti pro- 

 portions, between given intervals of refrangibility. The case is considerably dif- 

 ferent. Though the points of contrary flexure agree remarkably weU, as we have 

 seen, the curves are in some cases much more flattened than in others, where the 

 source of heat is the same ; owing probably to the greater parallelism of the rays 

 at one time than at another, depending on the distance of the source of heat from 

 the lens. 



