152 MANAGEMENT OF THE MICROSCOPE. 



be moved in order to bring it back to 0°, the position suitable for 

 an uncovered object. The thickness of the glass cover must then 

 be measured by means of the 'slow motion}' this is done by bring- 

 ing into exact focus, first the object itself, and then the surface of 

 the glass cover, and by observing the number of divisions through 

 which the milled head (which is itself graduated) has passed in 

 making this change. A definite ratio between that thickness of 

 glass and the correction required in that particular Objective is 

 thus established ; and this serves as the guide to the requisite 

 correction for any other thickness, which has been determined in 

 like manner by the ' slow motion.' Thus, supposing a particular 

 thickness of glass to be measured by 12 divisions of the milled - 

 head of the 'slow motion,' and the most perfect performance of 

 the Object-glass to be obtained by moving the screw-collar through 

 8 divisions, then a thickness of glass measured by 9 divisions of 

 the milled-head would require the screw-collar to be adjusted to 6 

 divisions in order to obtain the best effect. The ratio between the 

 two sets of divisions is by no means the same for different combi- 

 nations ; and it ought to be determined for each Objective by its 

 maker, who will generally be the best judge of the best 'points' 

 of his lenses ; but when this ratio has been once ascertained, the 

 adjustment for any thickness of glass with which the object may 

 happen to be covered is readily made by the Microscopist him- 

 self. Although this method appears somewhat more complex 

 than that of Mr. Powell, yet it is more perfect ; and when the 

 ratio between the two sets of divisions has been once determined, 

 the adjustment does not really involve more trouble. — Another 

 use is made of this adjustment by Messrs. Smith and Beck, 

 namely, to correct the performance of the Objectives, which is 

 disturbed by the increase of distance between the Objective and 

 the Eye-piece that is occasioned by the use of the Draw-tube 

 (§ 63). Accordingly, they mark a scale of inches on the Draw- 

 tube (which is useful for many other purposes), and direct that for 

 every inch the body is lengthened, the screw-collar of the Objective 

 shall be moved through a certain number of divisions. 



115. Arrangement for Transparent Objects. — If the Object be 

 already ' mounted ' in a Slide, nothing more is necessary, in order 

 to bring it into the right position for viewing it, than to lay the 

 slide upon the Object-platform of the Stage, and to support it in 

 such a position (by means of the sliding ledge or other contrivance) 

 that the part to be viewed is, as nearly as can be guessed, in the 

 centre of the aperture of the stage, and therefore in a line with the 

 axis of the body. If the object be not ' mounted,' and be of such 

 a kind that it is best seen dry, it may be simply laid upon the 

 Glass Stage-plate (§ 96), the ledge of which will prevent it from 

 slipping off when the Microscope is inclined, and a plate of thin 

 glass may be laid over it for its protection, if its delicacy should 

 seem to render this desirable. If, again, it be disposed to curl up, 



