MICROMETER. 215 



by means of the small pushing screw that moves the slip of 

 glass ; the divisions are then read off as easily as the inches 

 and tenths on a common rule. The operation, indeed, is 

 nothing more than laying a rule across the body to be 

 measured ; and it matters not whether the object be trans- 

 parent or opaque, mounted or not mounted, if its edges can 

 be distinctly seen, its diameter can be taken. Previously, 

 however, to using the micrometer, the value of the divisions 

 should be ascertained with each object-glass, the mode of 

 doing which will be subsequently explained more fully. 



Fig. 143. 



Fig. 144. 



