and its apjilication to the observation of the Shy. 



23 



and 1 inch diameter, pass through a small NIcol's prism, and 

 then fall upon the eye, which is placed in the focus of the 

 lens. Fixed or movable micrometer wires may be introduced 

 just beyond the end of the quartz, for the purpose of measur- 

 ing the displacement of the bands, an indication of the direc- 

 tion of the plane of polarization ; but I have not introduced 

 these. The region indicated, however, is in focus, and the 

 focus is deep. 



T* 



4 



© 



The quartz is adjusted for the axial direction by two pairs of screws. 

 The one pair is seen in profile ; the other pair is at right angles to the 

 first, and has one of the round heads indicated. 



Scale &. 



I remarked at the outset that the difficulty in connexion 

 with Savart's polariscope might be expressed by saying that 

 it passes from positive to negative polarization through a zero 

 value. The polariscope now described, on the other hand, may 

 be regarded as passing under the same circumstances from 

 functions of a certain angle to functions of an angle differing 

 by 90° by simple progression, the intensity of the appearances 

 manifested beino- the same throughout. For the effect of 

 passing through a certain thickness of the crystal is to turn 

 the plane of polarization through an angle proportional to the 

 thickness ; and the same difference of thickness will always 

 correspond to 180° of rotation, or to the interval of one band 

 of a given colour. Hence the appearance will always be that 

 of a succession of bands. 



To determine what appearance of motion in the bands is 

 produced by given rotations of polarizer or analyzer, it will 

 be convenient to imagine a section mapped out with lines 

 drawn at intervals corresponding to 1, 2, . . . millims. thick- 

 ness of right-handed and left-handed action respectively. As 

 an example we will take polarized homogeneous yellow light, 

 in which the millimetre lines correspond to rotations of 24° ; 

 and we will suppose that the analyzer is turned round through 

 successive angles of this amount in the direction of the hands 

 of a watch facing the observer, i. e. left-handed in the direction 

 of the ray. In the left-handed quartz, after each such rota- 

 tion, the corresponding light is that which comes through 

 1 millim. of increased thickness ; consequently the bands 

 appear to move from the centre. In the right-handed quartz, 



