MICROMETRY MICROMETRIC MICROSCOPES 151 



measured rarely covers exactly a whole number of divisions of 

 the ocular micrometer and we are obliged to make a guess as to 



FIG. 93. Micrometer Scales Improperly Adjusted. 



what fraction of a part to add. Thus there are two estimates 

 necessary and any measurements recorded must necessarily be 



FIG. 94. Micrometer Scales Properly Adjusted. 



mere approximations. The second of these errors cannot be 

 eliminated in micrometer oculars with fixed scales having rulings 



