. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. 400 PHYSIOLOGICAL PHYSICS. [Chap. and extraordinary rays. The effect of this prism is shown in Fig. 180, where the line IIC/H is the line in which the cut was made. A ray of light ab falling on the prism undergoes double refraction into the extraordinary ray bd, and the ordinary be. The extraordinary ray passes on through the Canada balsam junction, and emerges at c in a direction parallel to the entering ray ab. The ordinary r
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. 400 PHYSIOLOGICAL PHYSICS. [Chap. and extraordinary rays. The effect of this prism is shown in Fig. 180, where the line IIC/H is the line in which the cut was made. A ray of light ab falling on the prism undergoes double refraction into the extraordinary ray bd, and the ordinary be. The extraordinary ray passes on through the Canada balsam junction, and emerges at c in a direction parallel to the entering ray ab. The ordinary ray meets the balsam at c and is totally reflected. Only one of the two rays, therefore, tra- verses the prism. This ray is, however, found to be of a character different from an ordinary beam of light. If two ^ic°l's prisms be taken and the one placed in a line with the other so that the extraordinary ray which passes through the first is able to enter the second, it would be expected that the ray from the first prism would undergo double refraction oil entering the second, that the ordinary ray would be totally reflected as in the first, but that the extraordinary ray would pass 011 and a circle of light would appear on looking through the second Nicol. In one position of the prisms, namely, when they are in such a position that their principal planes are parallel, the circle of light is seen, and at its greatest intensity. If, however, one of the prisms be rotated on the other, the circle of light becomes less brilliant, and as the rotation is continued it becomes more and more dim, till, when the prism has passed through a right angle, the light is extinguished. If the rotation be carried on the light returns slowly, till, after going through another right angle the light is a second time at its greatest intensity \ and, if one continues turning, the light will again disappear, and again be restored. In two positions opposite to one another the light is most intense, and in
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