220 



USB OF THE MICEOSCOrE. 



It should be borne in mind that, when it is required to 

 ascertain the value of the divisions in the eye-piece micro- 

 meter with the highest powers, the division into hundreds 

 of the stage micrometer wiU occupy too much of the field of 

 view : some smaller parts of an inch, such as the two-hundredth 

 or five-hundredth, should then be used, and the number of the 

 divisions corresponding to that quantity be multiplied by two 

 hundred or five hundred, as the case may be. 



To find the Value of the Divisions in the Positive Eye-piece 

 Micrometer. — This instrument, before described at page 212, is 

 used in the same manner as the above-mentioned. Mr. Eoss^ 

 who always adopts this form in preference to the negative, 

 does not employ the draw-tube with his microscopes. The 

 micrometers, as shown in fig. 139, are ruled in squares, and 

 one or more of them, of different degrees of minuteness in 

 their ruling, may be employed; but it has been found in 

 practice that divisions ruled about ■^\-^ of an inch apart wiU 

 suit nearly all the powers. The method of finding the value 

 of the divisions, with each of the object-glasses, is performed 

 by means of a stage micrometer, in the manner previously 

 described. The positive eye-piece gives a much better view 

 of the micrometer than the negative one, but the definition 

 of an object to be measured is not quite so good. Mr. Ross 

 generally rules his micrometers in squares, but Mr. Jackson 

 prefers lines, on account of the greater facility afforded for 

 counting. As with the negative eye-piece micrometer, so with 

 t'lis, it becomes necessary to put down in a tabular form the 



