52 



CONSTRUCTION OP THE MICROSCOPE. 



ment which is very satisfactory. For this reason Mr. 

 Jackson gives it the preference. 



Fig. 35. — Mr. Jackson's Micrometer eye-piece. 



Short bold lines are ruled on a piece of glass, a, fig. 35 ; 

 and to facilitate counting, the fifth is drawn longer, and 

 the tenth still longer, as in the common rule. Very finely 

 levigated plumbago is rubbed into the lines, to render 

 them visible ; and they are covered with a piece of thin 

 glass, cemented by Canada balsam, to secure the plumbago 

 from being wiped out. The slip of glass thus prepared is 

 placed in a thin brass frame, so that it may slide freely ; 

 and is acted on at one end by a pushing-screw, and at the 

 other by a slight spring. 



Slips are cut in the negative eye-piece on each side, b, 

 so that the brass frame may be pressed across the field in 

 the focus of the eye-glass, as at m ; the cell of which 

 should have a longer screw than usual, to admit of adjust- 



