386 



SirC. Wheat stone on the 



[Mar. 23, 



The various homogeneous rays being accelerated differently in their 

 transmission through the two sections of the crystallized plate, this dif- 

 ference is preserved after reflection from the silver plate, and the oppositely 

 circularly polarized beams are reflected with the same difference of phase 

 as the two plane-polarized rays are when emerging fiom the crystallized 

 lamina. The composition of two circular waves, one right-handed, the 

 other left-handed, gives for resultant a plane wave the azimuth of which 

 varies with the difference of phase of the two components. 



When the plane of polarization does not lie equally between the two 

 rectangular sections of the laminae, these still remaining 45° from the plane 

 of reflection of the silver plate, the beam is resolved into two unequal 

 portions, the amplitudes of which are as sin a to cos a. Each therefore 

 gives rise to a circular undulation of different amplitude. The resultant of 

 two opposite circular undulations of different amplitudes is an ellipse of 

 constant form, the axes of which vary in position according to the difference 

 of phase. The same phenomena of successive polarization are therefore 

 exhibited, in whatever azimuth the lamina is turned in its own plane ; but 

 the tints become fainter and fainter until ultimately, when the principal or 

 perpendicular section is parallel to the plane of reflection of the polarizing 

 plate, all colour disappears. 



VI. 



By means of the phenomena of successive polarization it is easy to 

 determine which is the thicker of two films of the same crystalline substance. 

 Place one of the films on the diaphragm {a) of the instrument (fig. la) in 

 the position to show, say, right-handed polarization, then cross it with the 

 other film ; if the former be the thicker, the successive polarization will be 

 still right-handed ; if both be equal there will be no polarization ; and if 

 the crossed film be the thicker, the successive polarization will be left- 

 handed. In this manner a series of films may be readily arranged in their 

 proper order in the scale of tints. 



VII. 



In the experiments I have previously described the planes of re- 

 flection of the polarizing mirror and of the silver plate were coincident ; 

 some of the results obtained when the azimuth of the plane of reflection of 

 the silver plate is changed are interesting. 



I will confine my attention here to what takes place when the plane of 

 reflection of the silver plate is 45° from that of the polarizing reflector. 



When the principal sections of the film are parallel and perpendicular 

 to the plane of reflection of the polarizing mirror, as the whole of the 

 polarized light passes through one of the sections, no interference can 

 take place, and no colour will be seen, whatever be the position of the 

 analyzer. 



When the principal sections of the film are parallel and perpendicular 



