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 
Fig. .43. Selenite Disk. The Arrow diagram. Petrographic micro- 
Indicates the Direction of Vibration. 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 
