ILLUMINATION OF OBJECTS; POLARIZED LIGHT 57 



the other until extinction results; read the scale; the reading 

 upon each circle should be the same number of degrees. 



4. Testing the Graduated Circle upon the Circumference of 

 the Stage. — Place at the center of the stage a preparation con- 

 taining long prisms of a salt exhibiting parallel extinction. With 

 the nicols crossed at zero, select a good crystal, center it and 

 bring its long prism edge coincident with a cross-hair. Now 

 turn polarizer and analyzer several degrees, each being rotated 

 an equal distance and therefore maintaining the relative positions 

 of crossed nicols. Read the graduated circle on the analyzer, 

 read the position of the stage and rotate the stage until the crys- 

 tal extinguishes. Read the stage circle. The angular rotational 

 displacement should be the same number of degrees as that of 

 the nicols. In like manner compare a number of different seg- 

 ments of the stage graduations. In all cases several observations 

 should be made at each position, the mean of all the readings 

 being taken. 



Polarization without a Nicol Prism. — When employing the 

 hot-stage microscope it is sometimes essential to obtain polarized 

 light, yet have the substage kept clear. A polarizer of the nicol 

 or other analogous prism type is obviously impossible. Recourse 

 must then be had to polarization by reflection. A variety of 

 devices have been proposed, one of these is illustrated in the 

 microscope shown in Fig. 29. In this type the light is twice 

 reflected below the stage with the result that the object is illu- 

 minated by transmitted plane polarized light. The analyzer may 

 consist of any convenient sort of prism, placed either above the 

 eyepiece or mounted to slide in and out of the body-tube. The 

 best results are obtained from reflections from tourmaline plates 

 but Cheshire l has shown that fair results can even be obtained 

 from a thin plate of glass, ground on one side, and blackened 

 upon the ground surface. Light reflected from such a plate is 

 polarized; the maximum polarization is obtained when the angle 

 of the incident light is 56! degrees. The plate may be mounted 

 permanently at this angle and arranged to slip into the sub- 

 stage ring, or in chemical work involving heating with a flame 



1 J. Quekett Micro. Club, 8, 353. 



