SIMPLE METHOD OF MICROMETRY. 235 



2ud. The absence or removal of any cause of loss of definition, 

 viz., a slip of glass in the eye-piece. 



3rd. The use of a point for collimation, with a line, instead of 

 one line with another. 



4th. That the draw-tube is discarded for that special use as un- 

 necessary, and likely to impair definition. 



5th. Greater general accuracy and facility even in the hands of 

 the experienced as well as of the student. 



And lastly. Decrease of expense. 



I may conclude by observing that I have said nothing as to the 

 thickness of covering glass, since it is obvious that that must be 

 taken into the account, whatever methods be employed. 



