Prof. Forbes on the Refraction and Polarization of Heat. 367 



of the retardations of the ordinary and extraordinary rays 

 within the crystal, and X the length of an undulation. The 

 sum of the two is always = F^. 



69. Now the quantity o — e may always be known by re- 

 ferring to the retardation, which produces the corresponding 

 tint in Newton's rings, and which is equal to twice the distance 

 between the plates in that experiment. For example, with 

 the thin mica film mentioned in (56\ which polarized light 

 circularly, the tint produced (between crossed polarizing and 

 analysing plates) corresponded (by Newton's table) to an in- 

 terval of about five millionths of an inch between the surfaces 

 of glass, or to a retardation, [o— e\ of '00001 inch. The 

 film, marked No. 2, which gave plum-red of the first order (65), 

 gives a retardation of -00002. The film No. 1 {Q5), gives 

 •00004. inch. From these data, then, having the value of E^ 

 (68), it is clear that we may calculate the value of A, or the 

 length of an undulation of heat*. 



70. In our present case we have always made i = 45° ; 



whence E« = F^ sin ^^ (^^ ' ^"^ ^^ ^^"^^^ ^' = F^-E^. 

 But in an experiment we must not use the direct indication of 

 the multiplier, when the polarizing and analysing planes are 

 parallel, for the total quantity or F^ ; for a large proportion of 

 the heat is not completely polarized, and in order to compare 

 the values of E"^ and F^, we must determine the value of each 

 directly, that is, not only how much is depolarized, but how 

 much is polarized by the mica plates. This 1 did by ascer- 

 taining alternately with the quantities of depolarization, the 

 total intensity of the polarized part of the heat, which reached 

 the pile. This was effected by rendering the polarizing and 

 analysing plates parallel and perpendicular to one another ; 

 whilst the principal section of the interposed mica remained 

 parallel to one or other, so as to exercise no depolarizing 

 influence. 



71. To illustrate this mode of investigation, I shall give as 

 an example the very last series of experiments made on this 

 subject, which, whilst it points out the mode of operating, will 

 exhibit the constancy and considerable magnitude of these 

 effects, amidst the complicated changes of condition to which 

 the heat is subjected. The columns marked " corrected," 

 have a small correction applied for the gradual alteration in 

 the quantity of heat reaching the pile, which corrections are 



* Of course this is only true on the supposition that rays of heat and 

 light are equally retarded. This is not demonstrated, but it is probable 

 that they are nearly so, since that part of the heat which accompanies the 

 spectrum is so, and the dispersion in the case of double refraction is incon- 

 siderable. 



