112 Prof. Forbes's Researches on Heat. 



for extreme violet, '000167 inch. Hence for a plate of mica 



•001 inch thick the values of are 



A 



55 

 For extreme red light = '207 



^ 266 



For extreme violet light ... -^ — = "329 

 ° 167 



For heat = '07. 



If we assume the retardation, or o—e, to be the same for 

 all lengths of waves, and for heat as for light, we imme- 

 diately deduce the value of A, or the length of a wave of heat. 



For since for a plate '001 inch thick, = '07, as above, 



A 



o-e = -0000055, we have 



o — e '00055 «^«^^^ . 1 

 A = --— = — - — = -000079 mch, 

 •07 7 



about three times as long as a wave of red light, and four and 

 a half times that of violet. But it is always to be remembered, 

 that this proceeds on the supposition of the retardation being 

 invariable. 



I have taken the trouble to calculate and project in a similar 

 manner my original observations on depolarization given in 

 the First Series of these researches, art. 74, in order that, 

 though probably less accurate, they might form a check upon 

 the results just given. The plates then employed, and marked 

 No. 1 and No. 2, (which are not to be confounded with those 

 so designated in this paper) had thicknesses (deduced from 

 the retardation) of -0072 and •OOSe inch. I have the gratifi- 

 cation to find that the computed results agree almost pre- 

 cisely with those just obtained, although from the accidental 

 thicknesses of the two plates employed the observations with 

 these alone do not enable us to select the appropriate value of 



Q £ 



, there being at least two values which still remain am- 

 biguous ; but when taken in conjunction with the observations 

 of art. 41, the ambiguity is at once removed, and the nu- 

 merical value of A comes out almost exactly as stated above, 

 for incandescent platinum and dark heat, and somewhat 

 smaller for that of the Argand lamp. 



I desire it to be recollected, that, in speaking of these 

 somewhat startling lengths of waves of heat, I am using the 

 language of only one of the two hypotheses which serve to 

 interpret the results of this section ; for, if the variation be in 

 o—e, or the diiFerence of the velocities of the doubly reflected 



