214 



USE OF THE MICEOSCOPE. 



plumbago is rubbed into the lines to render them visible and 

 they are covered with a piece of thin glass, cemented by- 



Fig. 140. 



Canada balsam, to secure the plumbago from being wiped out. 

 The slip of glass thus prepared is placed in a thin brass frame, 

 as shown in fig. 140, so that it may slide freely, and is acted 



Vis. 141. 



Fig. 142. 



on at one end by a pushing screw, and at the other by a slight 

 spring. Slits are cut in the negative eye-piece on each side, as 

 shown in figs. 141 and 142, so that the brass frame, m, may be 

 passed across the field in the focus of the eye-glass, 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. When 

 the frame is not employed, an inner piece of tube, a, may be 

 drawn across the slits so as to prevent dust from getting 

 between the glasses. The method of using this micrometer 

 is as follows : — the object is brought to the centre of the field 

 by the stage movement, and the coincidence between one side 

 of it and one of the long lines is made with great accuracy. 



