CaL'bou 0-89 per cent. 

 The Edge A was quartz-hard, stripping file teeth. 

 The Edge C was quite soft to the file. 



Fig. 5. 

 Etched S in. diamutuv disc. 

 12 diiimeters after difturcntial licatins; and rapid i 

 (For ln;,di powur majiiiiticatioii sue Fi;;. r>\.) 



