232 



CHESTER K. WENTWORTH 



of time. In case one wishes to identify a mineral grain between 

 crossed nicols, the instrument can be rotated through more than 



270 degrees of the motion of the stage 

 (Fig. 2). Certain improvements occur 

 to the writer which might be followed 

 by anyone wishing to duplicate the in- 

 strument. The one constructed for the 

 writer weighs about twenty-one ounces. 

 The use of aluminum-zinc alloy instead 

 of brass for certain parts and attention 

 to details of design would probably 

 result in reducing this weight by nearly 

 one-half. The substitution of celluloid 

 dials and black graduations for the brass 

 dials on the instrument shown would 

 greatly increase the ease of reading the 

 instrument. In the present instrument, 

 whole turns are read by counting threads 

 exposed or by measuring with a scale; a 

 thin scale could easily be added to each 

 screw by means of which these could be read at once. The pitch of 

 the screws ideally should be i millimeter and the graduations of 

 the dial .01, though in practice the graduations of the model, which 

 are .008 mm., are equally satisfactory, since computations are 

 wholly relative. 



Fig. 2. — Micrometer mounted 

 on stage of Leitz petrographical 

 microscope. 



