OPTICAL I'Kol'KKTII.s 



CRY8TALB 



I'.Ki 



intf the concentric circles of light and darkness into four equal quad- 

 rants. These are caused by the nicols extinguishing the compo- 

 nents vibrating perpendicular to their planes of vibration. 



Thus in Fig. 340 any ray whatever, as at R, upon emerging 

 from the section is divided into two rays: One, the extraordinary 

 ray vibrating in the principal optic section, which contains the 

 ray R and the optic axis c; the trace of this plane on the section 

 is Re: The ordinary ray vibrates in a plane at right angles to this, 

 us oo' ; each of these rays will have components vibrating parallel 

 to AA' and will pass the analyzer; the point R on the section 

 will therefore appear illuminated and with a maximum illumina- 

 tion when the angle RCA is 45, as at this angle the component 

 passing the analyzer is the 

 largest. For any point on 

 AA' there is only one com- 

 ponent possible, that parallel 

 to PP', and this is extinguished 

 by the analyzer ; the line AA' 

 is dark. The same conditions 

 hold also for the line PP', which 

 is dark. These two dark re- 

 gions, parallel to PP' and AA', 

 cross each other through the 

 optic axis, Fig. 341. 



When either the analyzer or 

 polarizer is revolved until their 

 vibration planes are in paral- 

 lel position, the interference figure is crossed by a single band of 

 darkness, parallel to the plane of vibration of the nicols. 



When monochromatic light is used, the interference figure is illu- 

 minated with concentric circles of the color used, as yellow, red, 

 or blue; but when white light is used, the concentric rings are 

 colored with the interference colors, as was the case with the 

 quartz wedge. The color nearest the optic axis is violet, then 

 blue, yellow, and on through the colors of the first order to red, 

 after which they are repeated as the second order colors, then the 

 third order colors, and on according to the orders. The dark cross 

 remains the same as in monochromatic light. When the section 

 is cut exactly at 90 to the optic axis, the central dark spot, 

 where the optic axis emerges, will be in the center of the field 

 of the microscope, and the colored rings are also symmetrically 



FIG. 341. Interference Figure of Calcite. 



