54 THE MICROSCOPE. 



small screw acting upon the slip of glass. The divi- 

 sions 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 its divisions should be ascertained with each 

 object-glass ; the method of doing this is 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 numbrr of divisions in the 

 eye-piece which cover one on tho stage. Repeat this 

 process with different portions of the stage-micrometer, 

 and if there be any difference, take the mean. Sup- 

 pose 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 hun- 

 dred, and an object which occupied nine of these 

 divisions would measure the two-hundredth of an 

 inch. Take the instance supposed, and let the micro- 

 scope be furnished with a draw-tube, marked on the side 

 with inches and tenths. By drawing this out a short 

 distance, the image of the stage micrometer may be 

 expanded until one division is covered by twenty in 

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

 thousandths of an inch, and the object which before 

 measured nine will then measure ten ; which, divided 

 by 2,000, gives the decimal fraction '005. 



Enter in a table the length to which the tube is 

 drawn out, and the number of divisions on the eye- 

 piece micrometer equivalent to an inch on the stage ; 

 and any measurements afterwards taken with that 

 micrometer and object-glass may, by a short process 

 of mental arithmetic, be reduced to the decimal parts 

 of an inch, if not actually observed in them. In 

 ascertaining the value of the micrometer with a 

 deep object-glass, if the hundredth of an inch on the 

 stage occupies too much of the field, then the two- 

 hundredth or five-hundredth should be used, and 



