ACCESSORY INSTRUMENTS. 



121 



denser; upon this plate the tube carrying the prism is made to 

 revolve, by turning the large circular plate at the bottom with 



Fig. 66. 



a milled edge; this lower prism is termed the polarizer, in 

 contradistinction to another fitted to the top of one of the 

 eye-pieces, and termed the analyzer. An end view of one of 

 the prisms is seen at fig. 67, and a vertical section at fig. 68. 



When applied to the mi- 

 croscope, it is necessary 

 that the axes of both crys- 

 tals should coincide with 

 each other and with the 

 optical parts of the mi- 

 croscope, as in the case 

 of the achromatic conden- 

 ser; this may be known 

 by revolving either of 

 the prisms after the light 

 has been sent through 

 them by the mirror. If 

 they are properly ad- 

 justed, it will be found that there are two positions in which 

 no light will pass through the prisms at aU ; if this does not 

 take place, and only part of the field of view is darkened, 

 then, either ^by turning the arm carrying the compound 

 body or the screw in the plate bearing the polarizer, the 

 two can be made to obscure each other ; they are then in a 

 condition to be used. If now a crystalline plate of sulphate 

 of lime be placed in the focus of the object-glass, it will be 



Fig. 67. 



Fig. 68. 



