MICROMETER EYE-PIECE. 



53 



extent of half its diameter, there is no perceptible 

 distortion ; and the clearness of the definition gives a 

 precision to the measurement which is very satisfactory. 

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

 34; 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 prevent 

 the plumbago from being wiped out. The slip of glasf 

 thus prepared is secured in a thin brass frame, so thai 



FIG. 34. Jackson's Micrometer Eye-piece. 



it may slide freely ; it 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, 

 #, 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 adjustment for different eyes. The brass frame is 

 retained in its place by a spring within the tube of the 

 eye-piece ; and in using it the object is brought to the 

 centre of the field by the stage movements ; and the 

 coincidence between one side of it and one of the long 

 lines is made with great accuracy by means of the 



