ETCH TECHS IQVE 



27 



quartz of A) may be used for determining the handetlness of the section, it is 

 usually considered more reliable to use the conoscope for handedness deter- 

 mination. The counter-clockwise rotation of these sjwts in B indicates 

 right-hand quartz. The more complicated etch-figure \^, results from etch- 

 ing a fine ground surface too long," or from using a coarse instead of a line 

 grind. With such figures it is difficult to know which ]-)ortion of the figure 

 is to be aligned with the screen rulings. Hence the sections must be hne 

 ground and the etching time closely controlled. 



The obvious disadvantages of the reflection oriascoj^e (the necessity of |)re- 

 determining handedness and twinning, and the requirements of fine ground 

 surfaces and closely controlled etching time) are largely overcome by the 

 pin-hole oriascope, later described. However, the reflection oriascope is an 



Fig. 5.12 — Etch-figures obtained on the reflection oriascope with Z-cut sections (re- 

 duced from 11 inches square). A is a good usable figure while B is difiicult to use due to 

 its complexity. 



excellent explanatory' instrument for obtaining experimental etch-ligures 

 from surfaces of any orientation, preliminary to devising a special instru- 

 ment to most advantageously utilize the reflection characteristics found. 

 This fact results from the large and symmietrical screen coverage, and from 

 the fact that only one etch surface is encountered by the light beam (thick- 

 ness and back surface shape is of no concern). 



5.52 The Reflection Twinoriascope 



Figure 5.13 shows diagrammatically a reflection "Twinoriascope" designed 

 especially for shop use in detecting and marking twinning boundaries and the 

 sense of orientation in etched AT, BT, CT and DT slabs. When, for ex- 



" It appears that excessively strong etches (hours long) again give a simple, strong, and 

 reliable figure. 



