1 76 ELEMENTARY CHEMICAL MICROSCOPY 



One of the best examples of the every-day practical applica- 

 tion of the polarizing microscope and selenite plate by chemists 

 is in the differentiation of pure fresh butter from very old, or 

 process butter or oleomargarine. The fat of fresh, unmelted 

 butter thus examined yields a uniform red field. Process 

 butter, melted butter and oleomargarine on the other hand 

 yield a field mottled in many colors. 



For use with the chemical microscope the selenites are 

 usually obtained as disks with two black dots at opposite 



ends of a diameter, Fig. 104. 

 These dots locate the direction 

 of vibration of the test plate as 

 shown in the figure by the dotted 

 arrow. These selenite disks are 

 employed as follows: After cen- 

 tering and focusing the prepara- 

 tion, the selenite disk is laid upon 

 the eye-lens of the ocular in such 

 a position that its direction of 

 vibration bisects the angles of 



FIG. 104. Selenite Disk. The Arrow ,, , . , . , 



Indicates the Direction of Vibration. the ^OSS-hairs, as shown "in the 



diagram. Petrographic micro- 

 scopes are generally supplied with test plates mounted in a 

 metallic carrier arranged to slide into the tube of the micro- 

 scope in a slot provided for this purpose. The direction of the 

 vibration is in this case indicated upon the mount by an arrow. 



Absorption. Pleochroism. Many compounds have the power 

 of absorbing part of the light rays vibrating in certain planes 

 and therefore if viewed through the polarizing microscope with 

 the analyzer removed will exhibit .a change of light intensity, 

 in certain positions. This property of crystals known as ab- 

 sorption should not be confused with a change of color. 



All anisotropic substances to a greater or lesser extent remove 

 the rays of certain colors in certain planes from white light sent 

 through them. This property when sufficiently pronounced 

 to be observable with the normal human eye is termed pleo- 

 chroism. Substances are tested for pleochroism by placing 



