Third Series. — Depolarization of Heat. 107 



double refraction ; or, if the latter be assumed or known, we 

 may find the length of a wave. Considering the latter ele- 

 ment as the more important, and not being then in possession 

 of any more direct mode of determining it numerically, I 

 proposed to assume the retardation due to double refraction 

 as the same for heat as in the case of light, (considering heat 

 as but less refrangible light), and to determine the length of 

 a wave in the way which I fully explained in the First Series, 

 art. 68-75*. 



Two circumstances require notice by way of precaution. 

 The first is, that, for the very reason that we have periodical 

 colours in the case of light, there are different thicknesses of 

 mica and different measures of retardation, which, for the 

 same length of a wave, will give the same measure of depo- 

 larization; these dubious cases (which the formula of depo- 

 larization completely expresses) must be distinguished. The 

 second remark is, that all our sources of heat furnishing he- 

 terogeneous rays, each has its own period of maximum and 

 minimum intensity, just as in the case of solar light, and 

 since our means of numerical estimation embraces the sum 

 of all the effects of heterogeneous rays, we cannot expect re- 

 sults which shall rigorously satisfy a formula, in which homo- 

 geneity (or constancy of A, the length of a wave), is assumed, 

 but consider the approximate result as representing the mean 

 or predominating character of the heat employed. 



Recalling, then, Fresnel's formula, quoted in art. 70 of the 

 First Seriesf , we have 



p5- = sm^ 180^ 



'° f-r } 



where F* is the intensity of the whole incident polarized ray ; 

 E^ the intensity of that portion which, after transmission 

 through the depolarizing plate, is capable of being analysed 

 in a perpendicular plane. These two quantities being deter- 

 mined from observation, the first side of this equation, or 

 their ratio, becomes known. On the second side we have 

 two quantities, either of which may be assumed, and the 

 other becomes known, viz o—e the retardation of the one 

 doubly refracted ray upon the other within the crystal, and 

 A the length of a wave. Now, it is obvious from the form of 



o—e 

 the expression, that an infinite number of values of will 



A 



satisfy the equation; in light there can be little ambiguity 

 arising from this cause, because the phaenomena of periodic 

 colours at once afford the means of selecting the true solu- 



• Lond. & Edinb. Phil. Mag., vol. vi. p. 366. f liid., p. 367. 



