and Polarization of Heal. 163 



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

 tween 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 analyzing 

 plates) corresponded (by Newton's table) to an interval of about 

 five-millionths of an inch between the surfaces of glass, or to a re- 

 tardation, (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 (65), gives .00004 inch. From these 

 data, then, having the value of E 2 (68), it is clear that we may 

 calculate the value of x, or the length of an undulation of heat* 



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



E 2 = F 2 sin V ( ° = e ) ; and of course O 2 = F 2 — E 2 . But in an 



experiment we must not use the direct indication of the multi- 

 plier, when the polarizing and analyzing planes are parallel, for 

 the total quantity or F 2 ; for a large proportion of the heat is not 

 completely polarized, and in order to compare the values of E 2 

 and F 2 , 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 I did by ascertaining 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 analyzing plates parallel and per- 

 pendicular to one another ; whilst the principal section of the 

 interposed mica remained parallel to one or other, so as to exer- 

 cise 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, 



* 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 disper- 

 sion in the case of double refraction is inconsiderable. 



X 2 



