On the Action of Unctystallized Films upon Light. 269 



exerts upon a similar mass the absolute unit of force), if R is a 



millimetre, and F', N', R' the measures actually used, we have 



_ FN 'N'RR 



^""FNNR'R'' 



hence using the absolute measure itself, 77 = 1 



Every electric force can act, however, as electromotive force ; 

 aod this latter e is represented, according to the general funda- 

 mental law of electric action, as an expression of the electric 

 mass v, of the length of the element in which is contained the 

 quantity of electricity acted upon ; further, of the distance r of 



dr 

 both from each other, of their relative velocity -j-, and their 



ddr dt 



change -p-, and of the angle 9 which ds forms with r ; that is, 



if the constant whose value is to be determined from the mea- 

 sures chosen is called k, we have 



vdsV _ \(dr i _ ddr\l 



The constant k has the following meaning : — If E is the abso- 

 lute unit of measure of electromotive forces, N the absolute unit 

 of measure of the electric fluid, C the absolute unit of velocity 

 (a millimetre in a second), R a millimetre, and E', N', C, R' the 

 measures actually used, we have 



1 EN'C'R 

 2^2E'NCR' ; 



hence using the absolute measure, 



2^2 



XXXV. On the Action of Unciystallized Films upon Common and 

 Polarized Light. By Sir David Brewster, K.H., F.R.S.* 



IN a paper "On the Polarization of Light by Refraction," 

 published in the Philosophical Transactions for 1814, 1 have 

 shown that when a pencil of light is incident on a number of 

 uncrystallized plates, inclined at the same or different angles to 

 the incident ray, all their surfaces being perpendicular to the 

 plane of the first incidence, the transmitted pencil will be wholly 

 polarized when the sum of the tangents of the angle of incidence 

 upon each plate is equal to a constant quantity depending upon 



* Communicated by the Author. This paper was read at the meeting 

 of the British Association held at Aberdeen in Sept. 1859. 



