RAW QUARTZ, ITS DEFECTS AND INSPECTION 345 



immersion tank, Fig. 4.3. A raised perforated table T rests on the bottom of 

 the tank. For polarized light inspection of twinning and of optic axis direc- 

 tion, the right to left optical system is used. This comprises an AH-4, 

 100 watt, mercury-vapor lamp (requiring special transformer) isolated in a 

 well ventilated housing with window IIFi, a set of color filters Fi, Fo, (and 

 possibly Fs), a polarizing filter Pi with ground surface G (for light dififusion), 

 the immersion tank, the polarizing filter Po (crossed to Pi), and the mirror 

 (mounted at 45° to the vertical) to reflect light vertically up through a 

 window in the drain pan (not shown in Fig. 4.3), to the eye. For inspection 

 of defects other than twinning, a high powered projection system is used. 

 This comprises a projection lamp (isolated in a housing by window IJF2, 

 and with forced draft), condenser lenses CL, and the immersion tank. Here 

 one looks down directly into the tank at the stone. Reflectors R may be 

 added to both systems to increase the illumination. 



This instrument resulted from a restudy of long-used "inspection tanks" 

 and methods, and includes some features not originated by the author. 

 Since this inspectoscope is believed to be superior to many inspection-tank 

 equipments now in use, the more important design features will be described. 



The tank should be large enough to allow easy handling of the stones in 

 the fluid, with allowance for positioning any portion of the stone in the pro- 

 jection beam, and allowance for rise in level of the fluid as the stone is im- 

 mersed. However, the size should not be made larger than necessary, for it 

 has been found in practice that the fluid very rapidly collects lint and dirt, 

 which (being kept in suspension by agitation) scatters or diffuses the light. 

 This, besides scattering the light from its proper paths, interferes with the 

 polarized light inspection by depolarizing the beam. In large tanks with 

 dirty oil the light becomes almost completely depolarized and no patterns 

 can be seen. If, however, the crystal is large enough to nearly fill the length 

 of the tank (along the polarized light beam), this depolarization is small.'' 

 Thus, a tank as small as is consistent with the size of the stones to be ex- 

 amined should be used. The smallest convenient size and shape of tank 

 would be about 8 inches in the polarized light direction, 8 inches in the pro- 

 jection light direction, and 5^ inches high (all elements of Fig. 4.3 are drawn 

 in proportion to these tank dimensions). This permits easy examination of 

 two to three pound stones (pintsized), and six-inch long stones may be used 

 without great difficulty. 



Isolation of lamp heat is an important consideration in both optical sys- 

 tems. In the projection system the high wattage lamp would dangerously 

 heat up the whole instrument if the heat were not properly dissipated. In 



■* In an emergency small stones can be examined in a large tank with polluted fluid by 

 placing the stone at the mirror end of the tank and [introducing a polarizing filter directly 

 in the fluid, close to the stone, on the lamp side (Polaroid J-Film is only slowly attacked 

 by many of the immersion fluids). 



