254 SUMMARY OF CURRENT RESEARCHES RELATING TO 



different degrees of hardness. One plate is for emery, the second for 

 Vienna chalk, and the third for iron oxide or diamentin. These three 



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Fig. 40. 



Fig. 41. 



Fig. 43. 



Fig. 44. 



glass plates are kept in a wooden block, each in a separate compartment, 

 the compartments being lined with cloth (fig. 40) ; the intention being 

 to prevent soiling the plates with dust or with foreign material, which 



