ILLUMINATION OF OBJECTS; POLARIZED LIGHT 55 
at their zero point. Screen the stage (i.e., the open space between 
the body tube and stage) and cover the head with a dark cloth. 
Now observe carefully whether the nicols thus, set are in their 
position of maximum extinction. This is done by turning one 
of the prisms the laast amount possible and noting whether the 
field becomes darker or lighter. Make a number of observations, 
closing the eyes for a few seconds each time just before looking 
into the microscope. 
2. Testing the Cross-hairs. — Having adjusted the polarizer 
and analyzer to the proper position of crossed nicols as ascer¬ 
tained above, attach a low power objective, insert a cross-haired 
eyepiece and place upon the stage previously centered a prepa¬ 
ration of some salt, exhibiting parallel extinction and crystal¬ 
lizing in long prisms with straight edges.^ Center a good crystal 
and turn the stage until the crystal extinguishes — i.e., attains a 
maximum darkness;' its edges in this position should be exactly 
parallel to one of the cross-hairs. Turn the stage through 90 
degrees; the edge of the crystal must now be exactly parallel 
with the other cross-hair. If in either case exact parallelism 
has not been obtained, the cross-hairs of the ocular do not cor¬ 
respond to the planes of vibration of the nigol prisms. 
Centering the Stage. — Before it is possible to make obser¬ 
vations relative to the behavior of crystals or other substances 
toward polarized light or to measure crystal or extinction angles, 
it is essential that the rotating stage of the microscope be accu¬ 
rately centered. 
Place a half slide upon the stage of the microscope, holding 
it securely in place with a stage spring clip. Focus with a i inch 
or 32 millimeter objective upon the upper surface of the glass 
slide, moving it about until a tiny defect or mark is found. Move 
the slide with the fingers until this mark or tiny particle is 
brought directly under the intersection of the cross-hairs of the 
eyepiece. Rotate the stage. If the stage is centered the mark 
or particle will remain under the intersection of the cross-hairs. 
If not centered, the particle will move in a circle whose circum- 
1 For this purpose allow a drop of a saturated solution of mercuric chloride, or 
of ammonium sulphate to crystallize very slowly upon an object slide. 
