ETCH TECHNIQUE 



31 



to be made from the stone, ma\' be determined from tliese reflecting posi- 

 tions of the stone, and twinning may be partially explored. Thus if the 

 stone appears to be not badly twinned, it may be cut up at once into slabs 

 of proper sense of cut, without previously sectioning for further examina- 

 tion, 



5.53 The Pin-Hole Oriascope 



Figure 5.16 shows a "Basic Pin-Hole Oriascope" with auxiliary attach- 

 ments for shop examination of etched Z-cut sections, and Fig. 5.18 the same 



Fig. 5.16— The BASIC PIN-HOLE ORIASCOPE with matching and marking arms 

 for use on Z-cut sections. Twinning, and the direction and sense of the X (electric) axes 

 may be determined and marked on the section. 



for X-cut sections. The optical principle of this instrument is according to 

 Section 5.43. Light from a concentrated-filament lamp within the central 

 ventilated housing, is projected horizontally forward by a pair of condenser 

 lenses and reflected upward by a mirror in the forward housing, onto a dif- 

 fusion-disk placed directly against the pin-hole. i" The latter is centrally 

 located in the inclined mounting table. Etched quartz sections are placed 

 over this pin-hole and viewed from above. The section may be moved 

 about and examined for twinning boundaries, which are then penciled in. 



1^ See footnote 12. 



