Chap, xxx.] ROTATION OF PLANE. 411 



doubly refractive plate is not parallel to either of the 

 prisms. If a plate of quartz (rock-crystal) be substi- 

 tuted for Iceland spar, the light will not be extin- 

 guished in any position of the prisms, but will pass 

 through various colours from red to orange, yellow, 

 etc., to violet as the rotation is continued. It appears 

 that the colours are due to the extinction of some 

 particular rays of white light. Thus, in one position 

 of the rotating prism red is extinguished ; as the prism 

 is farther rotated it is the orange rays that are not 

 transmitted, and so on, the colour visible in the field 

 being due to the remaining rays of white light trans- 

 mitted. For the extinction of the red rays the 

 Nicol's prism must be turned 60 out of the crossed 

 position, in which position the red rays must be 

 vibrating at right angles to the principal plane of the 

 prism (for this is necessary for their extinction). 

 That is to say, before the quartz plate was introduced 

 the second prism required to be placed at right angles 

 to the first in order to extinguish the rays from the 

 first, but after the quartz plate is interposed it re- 

 quires to be turned through 60 to meet the polarised 

 ray at right angles, and thus to extinguish it. Conse- 

 quently the quartz has rotated the plane of polarisa- 

 tion to this extent. It also appears that the rotation 

 is different for the different colours of the spectrum, so 

 that the white light is dispersed, and thus on rotating 

 the prism farther one colour after another is extin- 

 guished. There are two kinds of quartz, one of which 

 rotates the plane to the right, another to the left. 

 The rotation of the plane of polarisation by the quartz 

 plate is due to CIRCULAR POLARISATION. The ray 

 on entering the quartz plate is broken up into two 

 rays, not polarised at right angles, as in the case of 

 Iceland spar, but polarised circularly, that is, the 

 vibrations are not rectilinear, do not describe straight 

 lines, as in plane polarised light, but describe circles, 



