OPTICAL pi:opi-:i!Tii:s OF CRYSTALS 



or the center of the diagram. In the quadrants 1 and 3 it will 

 therefore vibrate parallel, or nearly so, in all positions to the 

 arrow or the slow ray of the mica plate. When it emerges from the 

 section, it will have boon retarded and is vibrating in phase behind 

 the other ray, vibrating at right angles to the arrow. In passing 

 the mica plate the slow ray is -till more retarded and the difference 

 between the phases of the two rays is increased by 1/4 X, that due to 

 the mica plate. This is exactly the same as .if the section was 

 increased in thickness, in the quadrants 1 and 3, an amount suffi- 

 cient to produce a difference of phase in the two rays of one quarter 

 wave length, and all color bands in quadrants 1 and^wM move 

 toward the center, a distance 

 due to one quarter wave 

 length. 



In quadrants 2 and 4, the 

 extraordinary ray is now vi- 

 brating at right angles to the 

 arrow, or the slow ray of the 

 mica plate, and the two rays 

 on emerging from the mica 



1 >la te are reduced in phasal dif- 

 ference by one quarter wave 

 length. The effect is exactly 

 the same as if the section 

 had been decreased in thick- 



ness. in the two quadrants 2 and 4, an amount that would equal a 

 difference of phase in the two rays of one quarter wave length. 

 All color bands, therefore, in these two quadrants will be pushed 

 out from the center a distance equal to a phasal difference of one 

 quarter wave length. The interference figure will be displaced 

 along the planes of the nicols, and the colored circles will be 

 interrupted or carried toward the center in quadrants 1 and 



2 a distance equal to one quarter wave length, and in 3 and 4 

 pushed out a distance equal to one quarter wave length. The 

 displacement between two adjacent quadrants is one half wave 

 length. In quadrants 2 and 4 the optic axis is displaced and now 

 two dark spots appear, one in each quadrant, caused by the 

 compensation of the section by the mica plate. In positive crys- 

 tals the extraordinary ray is the slow ray, and these two dark spots 

 will appear in the quadrants not crossed by the arrow, or a line 

 connecting the two dark spots crosses the arrow at right angles, 



4 



