The Microscope. 173 



ordinary ray, the other as the extraordinary. While the other 

 waves of ordinary light are probably of various forms, princi- 

 pally circular, the waves of polarized light are in straight lines 

 at right angles with the path of the ray, and lying in a plane or 

 parallel plane. In case of the two rays resulting from double 

 refraction the vibrations of the one are at right angles to those 

 of the other. 



Light is polarized also by reflection from smooth surfaces not 

 metallic. The angle of incidence, or polarizing angle varies for 

 different reflecting substances. This is polarization by reflec- 

 tion ; the other, polarization by refraction. 



The most convenient and desirable form of polariscope for the 

 microscopist consists of two Nicol prisms. A Nicol prism is a 

 rhomb of Iceland spar cut by a diagonal j^lane and re-cemented 

 with Canada balsam. The effect of this oblique stratum of bal- 

 sam is to reflect to the side, and eliminate entirely the ordinary 

 ray ; so that only the extraordinary passes through, all uni- 

 formly polarized. Now, to the unaided eye, this light appears 

 just like ordinary light. How, then, can we distinguish polar- 

 ized light? If we take one of the mounted prisms, and look at 

 the clear sky, avoiding, however, the direction of about 90° from 

 the sun, little if any difference in brightness will be observed on 

 rotating the prism while looking through it. But if the other 

 prism is imposed upon it, and one of them is rotated, there will 

 be found two positions 180° apart, when all light is cut off, and 

 the field of view is black ; and the light is brightest half way 

 between these two points. One of these prisms is made to go 

 under the stage, and is the polarizer. The other, the analyzer 

 generally goes between the objective and tube ; or in the Griffith 

 Club microscope it may be fastened to the lower end of the 

 draw tube, in which case it is more convenient to rotate the ana- 

 lyzer than the polarizer, as is usual. In this position, the fore- 

 going experiment may be more exactly made. And polarized 

 light from any source may be detected in a similar way, by ex- 

 amining with one Nicol prism. 



Such a prism, then, may be imagined to be a sieve with all 

 the wires parallel, the cross wires being left out ; so that only 

 those vibrations continue which are in the direction of the spaces 

 between the wires. If now the other prism is placed over it with 

 the faces parallel, the polarized rays will still pass through un- 



