MICROMETER FOR GEOMETRICAL ROCK ANALYSIS 401 



screw does not matter at all, but a pitch of about one-half to one 

 millimeter is convenient. The scales which record the movement 

 of the screws are of course graduated to correspond to the pitch, 

 and the divisions on both scales are numbered from right to left. 



Fig. 5. — The micrometer in position on the stage of an "Allan Dick" petrological 

 microscope (Messrs. Swift & Son, London). 



The ends of the screws may be rigidly attached to the sledges A 

 and B, but can be more simply secured by means of backlash springs. 

 The innermost sledge A carries two slots, one of them (abed) made 

 to take the standard English size of slide (75X25 mm.), the other 



