262 



ELEMENTARY CHEMICAL MICROSCOPY 



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 diam- 

 eter, Fig. 143. 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 centering and 

 focusing the preparation, 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 

 the cross-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 microscope 

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

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



The selenite plate is also employed to determine the sign of 

 elongation, or sign of double refraction, of crystals, fibers, etc. 

 The object is 'placed upon the stage and rotated until it extin- 

 guishes ; it is then rotated until it displays its maximum polariz- 

 ation colors, which will be 45 from the position of extinction. 

 If now a selenite plate be inserted so that its direction of vibra- 

 tion (as indicated upon the disk) lies parallel to that of the object, 

 the image of the latter will probably change in color. If the 

 color resulting is an addition color, the double refraction is posi- 

 tive, but if the color is a subtraction color, the double refraction 

 is negative. 



The character of the double refraction of a substance may 

 often prove of considerable value in its identification or in trac- 

 ing changes which may have taken place if the substance has 



FIG. 143. Selenite Disk. The Arrow 

 Indicates the Direction of Vibration. 



