MANAGEMENT OF THE MICROSCOPE. 14:1 



140. Although the most perfect correction required for each particu- 

 lar object (which depends not merely upon the thickness of its glass 

 cover, but upon that of the fluid or balsam in which it may be mounted) 

 can only be found by experimental trial, yet for all ordinary purposes, 

 the following simple method, first devided by Mr. Powell, will suffice. 

 The object-glass, adjusted to ' uncovered/ is to be 'focussed' to the ob- 

 ject; the screw-collar is next to be turned until the surface of the glass 

 cover comes into focus, as may be perceived by the spots or stride by 

 which it may be marked; the object is then to be again brought into fo- 

 cus by the * slow motion.' The edge of the screw-collar being graduated, 

 the particular adjustment which any object may have been found to re- 

 quire, and of which a record has been kept, may be made again without 

 any difficulty. By Messrs. Smith and Beck, however, who first intro- 

 duced this graduation, a further use is made of it. By experiments such 

 as those described in the last paragraph, the correct adjustment is first 

 found for any particular object, and the number of divisions observed 

 through which the screw-collar must 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 bringing 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 mo- 

 tion.' 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 Objective to be obtained by moving the screw- 

 collar through 8 divisions, then a thickness of glass measured by 9 divi- 

 sions of the milled-head would require the screw-collar to be adjusted to 

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

 sets of divisions is by no means the same for different combinations; and it 

 ought to be determined for each Objective by its maker, who will generally 

 be the fittest 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 Micro- 

 scopist himself. Although this method appears somewhat more complex 

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

 tween the two sets of divisions has been once determined, the adjustment 

 does not really involve more trouble. Another use is made of this ad- 

 justment by Messrs. Smith and Beck; namely, to correct the disturbance 

 in the performance of Objectives, which is made by the increase of dis- 

 tance between the Objective and the Eye-piece, occasioned by the use of 

 the Draw-tube ( 83). 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. 



141. 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 so to support it by means of the spring-clips, 

 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 



