PRINCIPLE OF THE POLARISCOPE, The polariscope used with the micro scope is in two parts ; the " polariser " mounted in a sleeve to slide into the tube below the stage or into the substage (Fig. 714), and the " analyser " mounted in a tube to fit over the objective or eyepiece (Fig. 715). Both the polariser and analyser con sist of what is known as a Nicol's prism, that is, a natural crystal of Iceland spar which has been cut in a plane lying be tween its obtuse angles, ivr, x, 0, P (Fig. 716), and the faces polished and cemented together in their original position with is therefore known as the " extraordinary refracted ray " ; it has a refractive index of 1'486. When the Nicol's prism is used the extraordinary ray, having a lower re fractive index than the Canada balsam, (1'53), passes through the prism a ray of plane polarised light, while the ordinary refracted ray having a higher refractive index than that of the balsam strikes the surface of the latter at too high an angle, and it is reflected and submerged by the fitting (Fig. 718). If two Nicol's prisms
be placed in the same plane, so that the principal axis of the second lies in the same direction as the first, the ray of polarised light issuing from the latter will pass through the second prism un altered, and the field of view will appear fight ; but as the second prism is rotated, less and less light is transmitted until the chief section is at right angles, when the field appears quite clark. If one of the prisms be rotated through 360° there will be two positions of maximum brightness and two positions of maximum darkness, at equal intervals of 90°.