Plastic Crystals of Ammonium Nitrate. 15 



curvature of the crystal (if a is constant for this part of the 

 crystal). 



Fig. 15. 



Planes of the nicols. 



Or (9, this change in direction in crossing the crystal 

 radially from B to 0, is given by 



6= ^tan a. 



(Approximately.) 



Experiments were carried out with the slide shown in 

 fig. 13 (PL I,) in order to determine the direction of the plane 

 containing the optic axes at different points. The "eyes" 

 were observed by employing convergent polarized light and 

 an auxiliary lens in the microscope tube which enabled the 

 back focal plane of the objective to be viewed. Fig. 16 a 



Fig. 16. 



shows the direction of the plane at different points. This 

 was determined by moving the slide over some graph paper 

 to the d 'sired positio is ascertained by means of a scale 

 drawing of the slide prepared previously. A high-power 



