THE MICROSCOPE AND ITS REVELATIONS. 



and it is brought accurately into focus by unscrewing the eye-glass until 

 the lines of the scale are clearly seen. The value of the divisions of this 

 scale must be determined by means of a ruled Stage-micrometer, as in 

 the former instance, for each Objective employed in micrometry, the use 

 of the Draw-tube enabling the proportions to be adjusted to even and con- 

 venient numbers) ; and this having been accomplished, the scale is brought 

 to bear upon the object to be measured, by moving the latter as nearly as 

 possible into the centre of the field, and then rotating the Eye-piece in 

 such a manner that the scale may lie across that diameter which it is de- 

 sired to measure. The pushing screw at the extremity of the scale being 

 then turned until one edge of the object appears to be in exact contact 

 with one of the long lines, the number of divisions which its diameter 



occupies is at once read-off by 

 directing the attention to the 

 other edge the operation be- 

 ing nothing more than laying 

 a rule across the body to be 

 measured. 1 This method of 

 measurement may be made 

 quite exact enough for all ordi- 

 nary purposes, provided, in the 

 first place, that the Eye-piece 

 scale be divided with a fair de- 

 gree of accuracy; and secondly, 

 that the value of its divisions 

 be ascertained (as in the case of 

 the Cobweb-Micrometer) by sev- 

 eral comparisons with a ruled 

 scale laid upon the Stage. 

 Thus if, by a mean of numer- 



Jackson's Eye-piece Micrometer. oug observations, WC establish 



the value of each division of the eye-piece scale to be 1-12, 500th of an inch, 

 then, if the image of an object be found to measure 3J- of those divisions, 

 its real diameter will be 3^4- TSTTTO- or WTT inch. 2 With an Objective of 

 l-12th-inch focus, the value of the divisions of the Eye-piece scale may 

 be reduced to 1-25, 000th of an inch; and as the eye can estimate a fourth 

 part of one of the divisions with tolerable accuracy, it follow that a mag- 

 nitude of as little as 1-100, 000th of an inch can be measured with a near 

 approach to exactness.- Even this exactness may be increased by the 

 application of the diagonal scale (Fig. 67) devised by M. Hartnack. The 



l 



1 Dr. Royston-Pigott (loc. cit.) prefers to introduce into the aperture of the 

 diaphragm a plano-convex lens of very long focus, with the lines engraved upon 

 its flat surface. The advantage of the screw-movement is sacrificed, but a greater 

 distinctness of the lines is obtained. 



2 The calculation of the dimensions is much simplified by the adoption of a 

 Decimal scale; the value of each divison being made, by the use of the Draw-tube 

 adjustment, to correspond to some aliquot part of a ten-thousandth or a hundred- 

 thousandth of an inch, and the dimensions of the object being then found by 

 simple multiplication: Thus (to take the above example) the value of each divi- 

 sion in the decimal scale is .00008, and the diameter of the object is .00028. The 

 Metric system being now universally employed on the Continent, many British 

 and American Microscopists prefer to record their observations in parts of a Milli- 

 metre; and with a view to their convenience Messrs. Beck supply Stage-Microme- 

 ters ruled on one side of a median line to lOOths and lOOOths of an Inch, and on 

 the other side to lOOths of a Millimetre. 



