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13.—Nores on THE PoLARISCOPE PRESENTED BY THE PrusIpENt. 
By J. H. Batpock, F.C.S8. 
(Read September 20th, 1904.) 
Wiru reference to the very beautiful present just made to the 
Society by its President, Mr. F. C.-Bayard, I have been asked to 
say a few words of explanation as to what polarized light is, and 
its application to the instrument before us, after which I shall, 
with the kind assistance of Mr. Gower, show you some of the 
extremely beautiful results to be obtained by the means of the 
lantern polariscope. 
Most people who possess microscopes which are capable of 
producing polarizing effects, know that by doing certain things, 
with certain parts of the instrument, they are able to produce 
certain effects, but it is very probable that many do not know 
what happens to enable them to do this. 
Distinction between Common and Polarized Light.—Every beam 
of common light appears to consist of an indefinite number of 
systems of waves, undulating in a determinate plane, always at 
right angles to the direction pursued by the ray. These waves 
vibrate in all azimuths around the course of the ray, but may 
be theoretically resolved into two component vibrations at right 
angles to each other. Thus, common light may be regarded as 
composed of two beams of light which are vibrating in planes at 
right angles to each other. 
Polarized light differs from ordinary light in being produced by 
vibrations in a single plane only. Polarization once impressed 
on a beam of light continues permanent, whether the subsequent 
course of the ray is long or short, provided it continue in a 
homogeneous medium. 
What is known as the ‘‘ Nicol’s Prism,’ composed of Iceland 
spar, calcite, or carbonate of calcium, as it is variously called, is 
a convenient means of obtaining a polarized beam of light, 
depending on the principles of double refraction and total 
internal reflection. It is constructed thus:—A rhombohedron of 
Iceland spar is bisected in the plane which passes through the 
obtuse angles; the two halves are then joined again, in the same 
position, by means of Canada balsam. The refractive index of 
Canada balsam, 1:549, is less than the ordinary index of Iceland 
spar, 1°654, but greater than its extraordinary index, 1:483. The 
section of the Iceland spar prism is cut in such a direction that 
_ the ordinarily refracted ray strikes the Canada balsam at an 
