430 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



in the same direction, so that the extraordinary rays underwent total 

 reflection long before reaching the plane of section, and if the field 

 gained in extent on the side of the ordinary rays, it lost much more 

 on the side of the extraordinary rays. In fact the field was reduced. 



From this it will be seen that, so long as the direction of the 

 plane of section relatively to the axis of the crystal remains the 

 same, it would be useless to resort to any other cementing substance 

 than balsam. Before insisting further on the efi"ect of the direction 

 of section, it is desirable to consider the result of the obliquity of the 

 faces of incidence and emergence of the prism relatively to its axis, 

 and to the direction of the luminous rays which traverse it. 



Fig. 79 shows that the rays which pass from air into the spar on 

 the side of the limit of the field A, traverse the face A C nearly 

 normally, and that in proportion as they approach the other limit 

 they incline more and more to the face of entrance; the same 

 phenomenon being produced identically on the emergence of the rays 



Fig. 79. 



Fig. 80. 



Path of tlio ordinary and 

 extraordinary rays in a Nicol 

 prism. 



Patli of the rays with different cement- 

 ing media. C A limit of the field for 

 Canada balsam, K O A for balsam of co- 

 paiba, 10 A for linseed oil, POP' for 

 poppy oil ; mm. direction of the axis of the 

 crystal. 



at the opposite face. This progressive increase of the obliquity of 

 the incident rays produces an increasing partial reflection and a pro- 

 portionate diminution of the transmitted light ; so that the field 

 whilst very luminous on one side becomes darker and darker towards 

 the other. 



This obliquity of the faces of incidence and emergence gives rise 



