OPTICAL PROPERTIES OF CRYSTALS 



213 



<>ii' ray is constant, the shadow caused by it will not change on 

 completely rotating the hemisphere with the section. The other 

 ray will increase from a minimum to a maximum according 

 to the position of the section; and these two limiting values 

 will represent the critical angles for the other two indices of 

 refraction. The telescope is fitted with a nicol so that light 

 vibrating only in the plane required may pass and illuminate 

 the field. 



The ray may also be adjusted to enter the section as represented 

 in Fig. 363, and the field a'Ot' will be entirely dark, and the field 

 t'Oe will be illuminated and the contrast between the two fields 

 will be greater. 



III. A convenient refractometer, as constructed by Herbert 

 Smith of the British Museum and illustrated in Fig. 364, is so ar- 



FIQ. 364. Refractometer. 



ranged that the index of refraction may be read directly from a 

 scale in the instrument to the second decimal place and the third 

 estimated. The specimen is placed on the highly refracting glass 

 with a film of liquid between, as in the Abbe instrument. The light 

 entering at O, the shadow is thrown on the scale and the reading 

 taken. In double refracting substances two shadows will be noted, 

 indicating the two indices of refraction ; the section is revolved until 

 these are maximum or minimum values. This instrument is very 

 convenient for jewelers in the determination of the refraction and 

 identification of cut stones. 



IV. Cleavage pieces of transparent minerals may be used to 

 determine the index of refraction with the microscope. The thick- 

 ness of the section S, Fig. 365, is measured with a micrometer cali- 



