188 MINERALOGY 



of quartz in such a manner that one flat side is parallel to a plane 

 containing the optic axis, i.e., the vertical crystallographical axis ; 

 the long edge of the wedge, in most cases, is inclined at an angle of 

 45 to the optic axis. Some wedges are cut with their long edge 

 parallel to the optic axis.. In all cases the vibration plane of the 

 slow or extraordinary ray is always indicated on the wedge by an 

 arrow or mark, as in Fig. 334. 



In the tube of all petrographical microscopes, just above the ob- 

 jective, is a slot, into which the quartz wedge slips back and forth, 

 in such a position that the vibration planes of the wedge are fixed 

 at 45 to the vibration planes of the nicols. 



A section in which the vibration plane of the slow ray is to be 

 determined is placed on the stage of the microscope and revolved 

 to extinction, then placed at 45 from this position, when the vi- 

 bration planes of the section will lie at 45 to the vibration planes 

 of the nicols and will be parallel to those of the quartz wedge when 

 in position. At this 45 position the section will be evenly colored. 

 Minerals in rock sections are evenly ground to approximately 

 .03 mm. in thickness, and when interference colors of individual 

 species are given they refer to sections of about this thickness. The 

 color will depend upon the thickness of the section, the direction 

 of the section in the crystal, and the double refraction of the sub- 

 stance. As an illustration let it be supposed that the section yields 

 a red of the first order. First order red may be obtained by using 

 a quartz wedge as a section on the microscopic stage, pushing it 

 under in the 45 position until the first red is obtained. A second 

 wedge is now pushed in the slot of the microscope above the objec- 

 tive ; as the edge of the second wedge enters the field of vision there 

 will be a change of color noted. Whether the change of color goes 

 up the scale, from red of the first order to purple, blue, green, etc., 

 of the second order, or down the scale to yellow and grays of the 

 first order, will depend upon whether the difference between the 

 vibrations of the slow ray and the fast ray is still increased by the 

 second quartz wedge or decreased. If the slow ray of the section 

 or the first quartz wedge used as a section is parallel to the slow 

 ray of the second wedge (the direction of each is marked on the 

 wedge) which is inserted in the tube of the microscope, the color 

 change is up the scale, or the effect is that of thickening the section. 

 When the vibration plane of the slow ray of the section is at right 

 angles to that of the quartz wedge, upon pushing the wedge in 

 slowly the colors will go down the scale, from red to yellow and gray, 



