196 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
MEASUREMENTS WITH THE DOUBLE-SCREW MICROMETER OCULAR. 
The data given below appear also only in corrected form ready for plotting 
in projection, the actual scale-readings having been reduced to equivalent 
angles in air and these in turn figured to true angles within the crystal by 
means of the refractive index /3. The errors observed above in aragonite 
sections because of strong birefringence apply equally well here. In the 
following tables H indicates the horizontal and V the vertical micrometer 
screw of the ocular. 
Topaz. 
A* 
