108 Prof. Forbes's Researches on Heat. 



tion. In the case of heat, we must proceed with more cau- 



tion, the value of being wholly unknown ; we only af5- 



sume (as we are entitled to do) that this quantity increases 

 uniformly with the thickness of the plate, which it necessarily 

 must, since the retardation is as the thickness, and A is inde- 

 pendent of it. By a very simple process, the true value was 

 easily selected. 



Five depolarizing mica plates, of different thicknesses, of 

 exactly the same quality, and each as uniform as possible, 

 were provided. They were cut to the same size, and of such 

 a form that each could at once be placed with its neutral axis 

 (aline in the plane passing through the two axes of double re- 

 fraction) vertical, or inclined 45° at pleasure. Their thickness 

 was next to be determined. The examination of the colours 

 shown by polarized light was the most obvious method, but not 

 susceptible of the exactness which was required. It was, how- 

 ever, used as a check. 



The following were the results of actual measurement, 

 made by means of a pair of callipers constructed for such 

 purposes by Troughton. These results are the mean of ten 

 measures each, which were rendered difficult by the elastic 

 and fissile nature of the substance. 



Thickness in parts 

 of an inch. 



No. 1 -0026 



No. 2 -0044. 



No. 3 -0074 



No. 4 '0060 



No. 5 ; • • • '^^^'^ 



With these mica plates in succession, employed for depo- 



larizing, I proceeded to determine the ratio -p^- (p. 107) for 



the most part exactly in the way described and illustrated by 

 an example in art. 71, First Series, which I found preferable 

 to any other. This laborious investigation I performed for 

 heat from three sources ; (1.), an Argand lamp with glass 

 chimney; (2.), incandescent platinum; and, (3.), brass heated 

 (not to visible redness) by an alcohol flame. The thickness 

 of the plates No. 3. and No. 4. being very nearly the same 

 (and giving, as they ought to do, almost exactly the same 

 measure of depolarization), I preferred using the united thick- 

 ness of Nos. 2. and 3. as an interpolation between Nos. 3. 

 and 5. The swings of the needle, or dynamical effects, vol. xii. 

 p. 547, were always observed, and are alone given. The po- 

 larizing and analysing plates were the same marked I and K, 

 before fully described (vol. xii. p. 550), and a plate is said to 



