186 



Prof. Forbes's Researches on Heat. 



to total reflection, the specific quality of the heat changes, gave 

 countenance to the view that the gradation is in a great mea- 

 sure owing to the want of homogeneity of the heat, and that 

 the figure of the curve becomes (as we have said) a real test 

 of the composition of a ray. 



At the inferior limit of the curve, or when partial reflection 

 takes place, all kinds of heat are equally reflected (in the case 

 of light, the light is white), just as at the superior limit, or 

 after total reflection is complete, the beam has exactly the 

 same relative composition as before. In the intermediate 

 stages the composition is perpetually varying. The first rays 

 totally reflected (and combining with the scattered and partially 

 reflected rays) are the more refrangible, or those more easily 

 transmitted by glass. At a certain point a maximum propor- 

 tion of these enter into the reflected beam. As the angle of 

 incidence becomes greater, more and more of the less re- 

 frangible rays enter into the composition of the reflected heat, 

 which at last possesses the same qualities as at first. This is 

 well illustrated by the following early experiment which I made 

 on the proportion of the reflected rays transmitted by a plate 

 of glass '06 inch thick, at different stages of reflection (7th 

 February 1838). 



In the following experiments on the law of the transition 

 from partial to total reflection, the arrangement was that 

 shown in Plate IV. fig. J, and described (with the adjust- 

 ments) in pages 182 — 1 84-; the centre of the pile p was 1 3 inches 

 from the prism P, and the distance of the source of heat S 

 from P was 12 inches; that a diaphragm T, whose aperture 

 was 1 inch by 1|, was placed in the path of the ray usually 

 between P and L near P ; the aperture of the pile was con- 

 tracted to a breadth of one inch, whose centre was exactly in 

 the line ad; and only that part of the prism was employed 

 which was free from flaws capable of producing total reflection. 



The diagonal of the lozenge frame was varied from 14*5 

 inches up to 16*5 or 17*0, about eight observations of the in- 



