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CHESTER K. WENTWORTH 



the string of screw-nut pairs by a spring, is determined by the total 

 length of the string. The latter is adjustable by rotating any one 

 of the several nuts. Thus, the sledge carrying the rock section may 

 be driven across the field of view by turning the appropriate dial 

 in crossing the part of the section occupied by each of the several 

 minerals. Thus, as many minerals as desired may be measured, 

 recorded, and the results added, in one operation. The instrument, 

 as constructed for the writer, carries five dials, but any number may 

 be added by increasing the length of the machine. In order to 

 avoid the necessity of having an exceptionally strong backlash 

 spring, it is found desirable to traverse the sections in the same 

 direction by increasing the length of the string of screw-nut pairs. 

 With this precaution, which is good practice with any instrument 

 of this sort, the micrometer as shown gives excellent results. The 

 Bausch and Lomb Optical Company has the necessary data for 

 constructing duplicates of this instrument and will furnish estimates 

 on request. The following measurements of the minerals of a 

 thin section of granite will serve as an example of its performance. 



TABLE I 

 Instrument Readings — Trial I 



I 



