UPON COMMON AND POLARISED LIGHT. 609 
These results will be understood from the annexed diagram, in which M N isa 
thin plate and A B a ray of common light incident perpendicularly at B, and 
emerging at C in the direction CD. As a portion of the ray B C is reflected 
at C, and again reflected at B and transmitted at C, the pencil C D will consist 
of two distinct portions, one of which has been twice refracted, and the other 
and much feebler portion twice reflected. As neither of these portions are 
polarised, no physical change is produced by their combination, unless when the 
plate M N is so extremely thin as to produce the colours of thin plates, by the 
interference of the reflected with the refracted portions. 

D 
When a ray R B is incident obliquely at B, it suffers refraction at B and E, 
and the emergent pencil E F contains a portion of light polarised by refraction. 
This ray, in passing through other plates or films parallel to M N, is at last com- 
pletely polarised in one plane, having grown feebler in intensity by the abstrac- 
tion of the light reflected at the two surfaces of each plate. 
The portion of the refracted pencil B E which is reflected at E and G, and a 
portion of it polarised, emerges at K as a pencil, K L, partly polarised by re- 
flection. A portion of G K is again reflected at K and H, and emerges at P as a 
pencil P S more polarised by reflection than K L. Hence the principal or re- 
fracted pencil, E F, is combined with the pencils K L, P S (and others by reflec- 
tions at P, &c.), polarised in an opposite plane, so that with a certain number of 
plates, varying with the angle of incidence, the emergent pencil E F, K L and 
P §, consists of two oppositely polarised portions of light approximately equal. 
When polarised light is incident upon a pile of these thin and colourless films, 
and subsequently analysed, it exhibits all the properties of a plate cut perpen- 
dicularly to the axis of an uniaxal crystal. The line A D corresponds with 
the axis of the crystal; and the different azimuths in which the polarised ray 
may be inclined to this axis corresponds with the principal sections of an uniaxal 
