RAW QUARTZ, ITS DEFECTS AND INSPECTION 347 



the great concentration of light which is necessary for detecting fine-textured 

 defects, and makes it easier to determine just where in the crystal (in depth) 

 the defect lies. Further, with the large angle of illumination available, 

 those defects which require a specific angle of illumination may be found with 

 less hunting. It should be noted that in figuring the object and image dis- 

 tances, the refractive index n of the immersion fluid must be taken into 

 account (the window to image distance with fluid present is about n times 

 that with fluid absent). 



The color quality of the light used in the polarized light system has a 

 considerable effect on the ease of observing the light patterns obtained when 

 inspecting for twinning and optic axis. A typical light pattern of a piece of 

 raw quartz viewed along the optic axis is shown in Fig. 4.8. The broad dark 

 and light contours, "thickness-contours," are the ones used in locating the 

 optic axis. The finely "toothed-patterns" at A, B, and C show twinning. 

 The conditions that make the former most pronounced are not necessarily 

 the same as those that make the latter most pronounced. The broad thick- 

 ness-contours are most pronounced in monochromatic light, but barely 

 visible in white light. This is due to the large variation of rotatory power 

 with color, which, in all but the smallest stones, causes such overlapping of 

 the white-light color contours as to result in practically no appearance of 

 contours at all. This effect does not apply to the twinning regions, since 

 in most cases the thickness of oppositely handed material is too small to 

 develop overlapping. The result with white light is that the stone appears 

 mainly white, except for regions of twinning where the toothed pattern is 

 seen in color. For twinning detection, then, the advantage of white light is 

 largely due to removal of the extraneous thickness contours. This possible 

 advantage for the novice is not obtained without some loss of factors neces- 

 sary for complete identification. 



On the other hand, in determining the direction of the optic axis, the 

 thickness contours are essential, and hence monochromatic (or a restricted 

 spectrum) light is necessary. This illumination is most easily achieved with 

 a mercury arc and color filters. The mercury arc emits a restricted spectrum 

 (mainly .578/i yellow, .546)u green, .436^ far-blue lines and weak red and 

 blue-green bands), and is very efficient. Even without filtering it gives 

 far better thickness-contours for axis determination than does white light. 

 The insensitivity of the eye to blue leaves mainly yellow and green. The 

 yellow may be largely removed without appreciable loss of green by using 

 filter Fo and the blue and blue-green may be removed with filter Fi. How- 

 ever, the red can be removed only with considerable loss of green by filter 

 F3. The use of Fi and F2 alone are recommended as giving sufficient re- 

 striction of spectrum and yet high illumination. (All three filters, as used 

 in the conoscope, give a fairly monochromatic green.) The filters need not 



