POLARIZED LIGHT. 105 



Having endeavoured to describe as plainly as 

 possible the nature of polarized light, we will now 

 proceed to describe the methods usually adopted 

 for the purpose of applying polarized light to the 

 examination of microscopic objects. 



The micro-polariscope usually consists of two 

 Nichol's prisms, mounted in appropriate fittings. 

 A Nichol's prism is composed of a crystal of Ice- 

 land spar. It will be remembered that a beam 

 of light, in passing through a rhomb of Iceland 

 spar, becomes doubly refracted, and both polarized 

 beams are visible ; for polarizing purposes this is 

 not by any means desirable, but the difliculty has 

 been overcome in the following manner. A 

 rhomb is divided, as shown in fig. 24, and the two 

 halves cemented with Canada balsam ; the re- 

 fractive power of the film of balsam being different 

 to that of the spar, throws the second image out 

 of the field. 



Pig. 24. 



a, section of Nichol's prism ; b, film of balsam ; c, ray of 

 light; d, ditto passing out parallel to that of incident ray ; 

 c, refracted ray. 



a represents a section of a Nichol's prism, b the 

 cementing film of Canada balsam, c a ray passing 

 into the prism, d the same passing out parallel 

 to the incident ray, e the refracted ray. 



One of these prisms is mounted, as shown in 

 fig. 25, and is made to slide in the short tube 

 attached to the under side of the stage ; a is a 



