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