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Proceedings of Boyal Society of Edinburgh. [sess. 
the same thing would happen were the surfaces BD in optical 
contact, that is, were they united by a substance having its index 
of refraction greater than or equal to the greater of the two indices 
for calcareous spar. But the index for Canada Balsam is less than 
that for carbonate of lime, and on this account the rays may not 
proceed uninfluenced. 
If, indeed, the ray FG fall very obliquely on the surface of the 
varnish it may be totally reflected, no portion of it passing into the 
second wedge DBC. So it may also happen with the ray Yg ; but 
as the two rays fall with different obliquities on the varnish, their 
limits of total reflection are different, and between these, the extra- 
ordinary light alone will find a passage. 
The ray E : F falls at such an angle that the ordinary pencil would 
barely suffer total reflection, while the extraordinary pencil would 
not. Any ray incident between the directions AF and EjF would 
transmit both its pencils through the whole instrument. The ray 
E 2 F, on the other hand, is so placed that both of its pencils suffer 
total reflection ; and, hence, all rays within the angle E X FE 2 will 
transmit to the eye only that portion which has experienced the 
extraordinary refraction, while no ray incident in the angle E 2 FD 
will send any light through. 
Such is the true action of the polarising eye-piece : it does not 
depend, as has been thought, on the separation of the images, for 
in truth there is never more than one image formed, and the virtual 
place of that image is not affected by the film of balsam. The 
perfection of its action depends on the magnitude of the angle E^FEg, 
which magnitude regulates the extent of the field of view; and on 
the transmitted light passing without being at all deflected in 
its path. For the attainment of the latter object, the surfaces AD, 
BC must be so placed that a ray of extraordinary light, passing in 
the interior parallel to the sides AB, may not suffer refraction in 
escaping at either surface. By this arrangement the light proceeds 
from the object to the eye, so that the first and last portions of its 
path are not only parallel to each other, but actually in one straight 
line. If this adjustment be not effected, there is created a parallax 
similar to that occasioned by the transmission of light obliquely " 
through a thick plate of glass : that parallax affecting the apparent 
positions of near, but not those of distant, objects. 
