224 Brewster on the Polarisation of Light, 
By means of the preceding table, we may readily determine 
the quantity of light that is polarised at any angle <p, by a 
given number of plates n. Let m be the number of plates 
necessary to polarise the whole of the transmitted light, as 
found either from the table, or from the formula 
4.1.84 
m = r ^ 
tang. 9 
L the quantity of light transmitted through the plates n, then 
L nL 
: — 
m 
% 
the quantity of light required. If (p = 43® 33'; L=in = 
2 and m = 44, then we shall have ^ = o 04545 the quan- 
tity of light polarised by two plates at an angle of 43° 33'. 
The law of polarisation being thus ascertained, my next 
object was to determine the law of the variations which the 
angles experienced by changing the refractive power of the 
plates. From the difficulty of procuring ground plates of any 
substance of a high refractive power, I have made this experi- 
ment only in a rude manner with a parcel of twelve plates of 
green glass, which is nearly equal to flint glass in refractive 
power. The result of this experiment indicated that as the 
refractive power increased, the transmitted light was polarised 
at a less angle of incidence; but I cannot yet state the precise 
law till I have performed a series of experiments with a parcel 
of plates of flint glass which I am now preparing. 
If a series of plates of glass is employed in the experiments 
on the elliptical coloured rings produced by topaz, either in po- 
larising the incident rays in place of the reflecting plane, or in 
examining the rings in place of the agate and Iceland,spar, the 
phenomena retain the same character; but are more distinct and 
