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Monday , 20th February 1860. 
Dr CHRISTISON, Vice-President, in the Chair. 
The following Communications were read : — - 
1. On the Action of Uncrystallised Films upon Common and 
Polarised Light. By Sir David Brewster, K.H., F.R.S. 
Since the discovery of the polarisation of light by refraction, the 
action of a pile of transparent plates upon common and polarised 
light has not been studied by any of the writers on physical optics. 
It was believed that a pencil of common light was completely 
polarised in the plane of refraction when the plates were sufficiently 
numerous, no special notice having been taken of the light thrown 
back by reflexion into the transmitted and polarised beam. Sir 
John Herschel, indeed, had referred to it ; but he remarks that “ it 
mixes with the transmitted beam, and, being in an opposite plane, 
destroys a part of its polarisation.”* So long ago as 1814, Sir 
David Brewster had shown that this reflected light is distinctly 
visible as light polarised by reflexion ;f but owing to the difficulty 
of procuring very thin plates of glass with perfectly parallel surfaces, 
it was impossible to ascertain the true character of the oppositely 
polarised pencils. 
Having obtained, however, films of decomposed glass of great 
thinness, and perfectly colourless, the author was enabled to prove 
that the transmitted beam consisted of two pencils oppositely polar- 
ised, and that when polarised light was incident obliquely on 
such a pile, and subsequently analysed, the pile of films exhibited all 
the properties of a plate cut perpendicular to the axis of a negative 
uniaxal crystal, the tints produced by the interference of the pencils 
rising to the blue of the second order of Newton’s scale of colours, 
by increasing the obliquity of the incident pencil. 
A line perpendicular to the plates or films at the point of inci- 
dence corresponds with the axis of the uniaxal crystal; and the dif- 
ferent azimuths in which the polarised ray may be inclined to this 
axis correspond with the principal sections of the crystal. 
* Treatise on Light, Art. 868. 
t Phil. Trans., 1814, p. 226. 
