ADJUSTMENT OF THE FOCUS. 147 



moved a little in either direction without affecting the body ; thus 

 occasioning a great diminution in the sensitiveness of the adjust- 

 ment. This fault may sometimes be detected in Microscopes of 

 the best original construction, which have gradually worked loose 

 owing to the constancy with which they have been in employ- 

 ment ; and it may often be corrected by tightening the screws that 

 bring the pinion to bear against the rack. And by ' twist ' it is 

 intended to express that apparent movement of the object across 

 the field, which results from a real displacement of the axis of 

 the body to one side or the other, owing to a want of correct fitting 

 in the working parts. As this last fault depends entirely on bad 

 original workmanship, there is no remedy for it ; but it is one 

 which most seriously interferes with the convenient use of the in- 

 strument, however excellent may be its optical performance. In 

 the use of the coarse adjustment with an Objective of short focus, 

 extreme care is necessary to avoid bringing it down upon the 

 object, to the injury of one or both ; for although the spring with 

 which the tube for the reception of the object-glass is furnished, 

 whenever the IFine Adjustment is immediately applied to this, 

 takes off the violence of the crushing action, yet such an action, 

 even when thus moderated, can scarcely fail to damage or disturb 

 the object, and may do great mischief to the lenses. Where no 

 such spring-tube is furnished, the fine adjustment being otherwise 

 provided for, or being not supplied at all, still greater care is of 

 course required. It is here, perhaps, well to notice, for the guidance 

 of the young Microscopist, that the actual distance between the 

 Object-glass and the object, when a distinct image is formed, is 

 always considerably less than the riominal focal length of the 

 object-glass : thus, the distance of the 1 inch or 2-3rds inch object- 

 glass may be little more than half an inch : that of the 4-10ths inch 

 may be but little more than one-tenth of an inch ; that of a l-4th 

 or a l-5th inch may scarcely exceed one-twentieth ; that of a l-8th 

 inch may not be one-fortieth ; and that of a 1-1 2th or a l-16th inch 

 may be so close as not to admit the intervention of a piece of glass 

 more than one-hundredth of an inch thick. One more precaution 

 it may be well to specify ; namely, that either in changing one 

 object for another, or in substituting one Objective for another — 

 save when powers of such focal length are employed as to remove 

 all likelihood of injury — the Body should be turned to one side, 

 where the construction of the Microscope admits of this displace- 

 ment, or (where it does not) should have its distance from the 

 Stage increased by the 'coarse adjustment.' This precaution is 

 absolutely necessary when Objectives of short focus are in use, to 

 avoid injury either to the lenses or to the object ; and when it is 

 habitually practised with regard to these, it becomes so much like 

 an ' acquired instinct,' as to be almost invariably practised in other 

 cases. 



112. In obtaining an exact Focal Adjustment with Object-glasses 



l 2 



