Dr. Brewster on the Polarisation of Light, 221 
number of plates and the angle of incidence at which the 
polarisation was effected. 
For this purpose, I provided myself with forty-seven plates 
of crown glass, each of which was about three inches long 
and one broad, and having formed them successively into 
parcels of 47, 44, 41, 39, &c. down to eight plates, I mea- 
sured by means of a theodolite the different angles at which 
a pencil of light was polarised when transmitted through 
these various parcels. The results which were thus obtained, 
indicated at once a regular progression, and upon comparing 
the number of plates with the angles, it was obvious that the 
number of plates were always to one another, as the cotan- 
gents of the angles of incidence at which they polarised the 
transmitted light ; that is, if n' represent the number of 
plates in any two parcels, and (p, o? the angles at which the 
pencil was polarised, we have 
n \ n' ^ cotang, (p : cotang, p* and 
n X tang, p = ?i' % tang. p\ 
Hence it follows that the number oj plates in any parcel multi- 
plied by the tangent of the angle ^ at which it polarises lights is a 
constant quantity. From a great number of observations, made 
with a parcel of eighteen plates, I have found the constant 
quantity for crown glass to be 41.84, so that we have' 
tang. ?> = 
that is, divide the constant quantity by any given number of 
plates, and the quotient will be the natural tangent of the 
angle at which that number will polarise a pencil of light. 
In this way I have constructed the following table, shew- 
ing the various angles of polarisadon from one plate up to 
