1890-91.] Mr E. Sang on NicoVs Polarising Eye-Piece. 
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Investigation of the Action of Nicol’s Polarising 
Eye-Piece. By E. Sang, Esq. (With a Plate.) 
(Read February 20, 1887.) 
The first announcement of the construction of this important 
instrument appeared almost paradoxical : a piece of calcareous spar 
was to be cut in two, the surfaces of the section polished, and then 
reunited by help of Canada Balsam : and it seemed strange that 
from such an operation there should have resulted any change in 
the optical properties of the mass. Even now that the instrument 
has been in use for some time, the true nature of its performance 
is often misunderstood ; while no investigation has been made public, 
the object of which is to enquire into the laws of the action, and into 
the circumstances which determine the peculiar forms of the parts. 
This investigation necessarily involves operations belonging to the 
higher branches of algebra and geometry ; but this is not to he 
wondered at, when physical science has reached such a degree of 
development as to exhibit many of the laws of its phenomena. 
Before proceeding with the strict investigation, it may be con- 
venient to take a general review of the modifications which light 
undergoes in its transit through the instrument ; as by that proceed- 
ing we shall be better prepared for seizing the full import of the 
analytic results. Let, then, ABCD represent the prime section of 
the eye-piece, BD the thin film of balsam inserted between the 
halves, and EF a ray of light incident on the surface AD : that 
ray will be refracted in two pencils, EH that submitted to the 
ordinary, FA that to the extraordinary, law. Were the rhomb 
entire these rays would again suffer refraction at the surface BC, 
and would emerge in directions HI, hi parallel to the incident ray : 
