ADJUSTMENT OF THE OBJECT-GLASS. 167 



84. Although the most perfect correction required for each par- 

 ticular 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 de- 

 vised by Mr. Powell, will suflice. The object-glass, adjusted to 

 "uncovered," is to be "focussed" to the object; its 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 striae by which it 

 may be marked ; the object is then to be again brought into 

 focus by the "fine movement." The edge of the screw-collar 

 being now usually graduated, the particular adjustment which 

 any object may have been found to require, and of which a 

 record has been kept, may be made again without any difificulty. 

 By Messrs. Smith and Beck, however, who first introduced 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 ob- 

 ject. The thickness of the glass cover must then be measured 

 by means of the " fine movement :" 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 "fine movement." Thus, supposing a 

 particular thickness of glass to be measured by 12 divisions of 

 the milled head of the fine movement, and the most perfect per- 

 formance of the object-glass to be obtained by moving the screw- 

 collar through 8 divisions, then a thickness of glass measui'ed by 

 9 divisions of the milled head, would require the screw-collar to 

 be adjusted to 6 divisions in order to obtain the best efiect. 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 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 himself. Although this method appears some- 

 what more complex than that of Mr. Powell, yet it is more per- 

 fect ; and when the ratio between the two sets of divisions has 

 been once determined, the adjustment does not really involve 



itient is thus performed within a third part of a revolution, with scarcely any friction, 

 and with such an imnnediate transition from good to bad definition, that the best point 

 is made readily apparent. 



