40 CONSTRUCTION OF THE MICROSCOPE. 



former is quite free from distortion, even to the edges of the field ; 

 but the object is slightly coloured. The latter is free from colour, 

 but is slightly distorted at the edges. In the centre of the field, how- 

 ever, to the extent of half its diameter, there is no perceptible dis- 

 tortion ; and the clearness of the definition gives a precision to the 

 measurement which is very satisfactory. For this reason Mr. Jackson 

 gives it the preference. 



Short bold lines are ruled on a piece of glass, a, fig. 34 ; 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, fig. 34, 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 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 the laying a rule across the body to be 

 measured ; and it matters not whether the object be transparent 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 is best performed as follows : 



Lay a slip of ruled glass on the stage ; and having turned the eye- 

 piece so that the lines on the two glasses are parallel, read off the 

 number of divisions in the eye-piece which cover one on the stage. 

 Repeat this process with different portions of the stage-micrometer, 

 and if there be any difference, take the mean. Suppose the hundredth 

 of an inch on the stage requires eighteen divisions in the eye-piece to 

 cover it ; it is quite plain that an inch would require eighteen hundred, 

 and an object which occupied nine of these divisions would measure 

 the two-hundreth of an inch. This is the common mode of express- 



