164 ELEMENTARY CHEMICAL MICROSCOPY 



ration of some salt, exhibiting parallel extinction and crystal- 

 lizing in long prisms with straight edges. 1 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 corre- 

 spond to the planes of vibration of the nicol 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- 

 ference passes through the intersection of the cross-hairs but 

 whose center is off to one side. Slowly rotate the stage until 

 the mark has made a complete revolution, fixing in your mind 

 the position of the center about which the particle has rotated. 

 Now turn the stage until the particle or mark reaches its maxi- 

 mum distance from the intersection of the cross-hairs and by 

 means of the stage centering screws bring the particle to the 

 center about which it has rotated. Move the slide on the stage 

 with the fingers until the particle or mark again falls directly 

 under the cross-hairs. Rotate the stage. It will now be found 

 that the stage is nearly but not quite centered. Rotate again, 

 noting as before the path of the mark or particle, and the position 



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. 



