. Elementary chemical microscopy. Microscopy; Microchemistry. 7? -,7-. // 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 areem
. Elementary chemical microscopy. Microscopy; Microchemistry. 7? -,7-. // 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 areemployed ,,, 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. 143. 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 chang
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