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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 



(Fig- 2). 



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 ob- 

 ject 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 I will explain more fully. 



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-micro- 

 meter, 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 di- 

 visions would measure the two-hundredth of an inch. This is the 

 common mode of expressing microscopical measurements ; but T am 

 of opinion that a decimal notation would be preferable if universally 

 adopted. Take the instance supposed, and let the microscope be fur- 

 nished with a draw-tube marked on the side with inches and tenths. 

 By drawing this out a short distance the image of the stage mi- 

 crometer may be expanded until one division is covered by twenty in 

 the eye-piece. These will then have the value of two-thousandths of 



