68 Mr. J. H. Baldock on 
angle greater than its critical angle, and undergoes total 
internal reflection, while the extraordinary ray, striking the bal- 
sam at less than its critical angle, can traverse it. Hence, 
since the Nicol’s prism allows only the extraordinary ray to 
pass, it may be used, like a tourmaline, either as a polarizer, 
or as an analyzer. 
At this point it may be as well just to state what is meant by 
ordinary and extraordinary rays. All transparent crystals which 
do not belong to the regular or cubical system are doubly refract- 
ing; when a double refracting crystal, such as the one we are 
considering, t.e. Iceland spar, is placed over a dark dot on a piece 
of white paper, and looked through, not one dot but two are seen; 
and if the crystal is rotated, keeping the eye in the same line, one 
of these dots will appear to revolve round the other, the ordinary 
image, corresponding to the ordinary ray, being fixed; while the 
extraordinary image, corresponding to the extraordinary ray, de- 
scribes a circle round it. 
Unfortunately, there is now such a scarcity of crystals of Ice- 
land spar, especially of any size and purity, that a perfect crystal 
of, say, 2 in., would cost £200 or £300, even if it could be 
obtained at all, which is doubtful. Opticians have therefore 
been obliged to resort to some other means for polarizing light. 
In the instrument before you, this has been obtained by means of 
a number of flat, thin, colourless glass plates, the last one at the 
back being made of black glass, or blackened, and the whole so 
arranged that the light from the condenser in the lantern falls 
on the plates at an angle of fifty-six degrees from the normal, 
this being what is called the polarizing angle. When, therefore, 
a ray of light encounters such a bundle, part of it is reflected, 
and this reflected light is in part polarized, With one plate 
polarization is only partial, but with ten or twelve plates polari- 
zation is tolerably complete. 
There are two disadvantages to this form of polariscope, 7. e. 
that the lantern has to be turned sideways consequent on the 
elbow ; and the other is that the polarizer cannot be rotated ; but 
these are not very serious objections. 
We have now got polarized light, and, with the addition of a 
second polarizing arrangement called an analyser, which in this 
case, being much smaller, may be a Nicol’s prism constructed as 
already described, our apparatus is complete. 
When the polarizer and analyser are so arranged that the plane 
of polarization of the two coincides, the light which has passed 
through the polarizer, which is rather less than half the original 
light, passes through the analyser in the same plane with little 
further loss. But when the analyser is rotated around the axis 
of the beam of light, the plane polarized light which falls on it 
is resolved into two components, one parallel with, and the other 
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