CONSTRUCTION OF THE MICROSCOPE. 53 



ment 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. Eepeat 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 divi- 

 sions 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- 

 hundredth of an inch. This is the common mode of 

 expressing microscopical measurements ; but I am of 

 opinion that a decimal notation would be preferable, if 

 universally adopted. 



" Take the instance supposed, and let the microscope 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 mea- 

 sure ten ; which, divided by 2,000, gives the decimal 

 fraction '005. 



" Enter in a table the length to which the tube is drawn 



