162 MANAGEMENT OF THE MICROSCOPE. 



be, not, what is its greatest magnifying power, but, what is 

 the least magnifying power under which it will show objects of 

 a given degree of difficulty. 



81. In making the Focal Adjustment, when low powers are 

 used, it will scarcely be necessary to employ any but the coarse 

 movement ; provided that the rack be well cut, the pinion work 

 in it smoothly and easily, without either "spring," "loss of 

 time," or "twist," and the milled head be large enough to give 

 the requisite leverage. All these are requisites which should be 

 found in every well-constructed instrument ; and its possession 

 of them should be tested, like its freedom from vibration, by the 

 use of high powers, since a really good coarse adjustment should 

 enable the observer to "focus" an objective of l-8th inch with 

 precision. What is meant by "spring" is the alteration which 

 may often be observed to take place on the withdrawal of the 

 hand ; the object which has been brought precisely into focus, 

 and which so remains as long as the milled head, is between the 

 fingers, becoming indistinct when the milled head is let go. The 

 source of this fault may lie either in the rack-movement itself, 

 or in the general framing of the instrument, which is so weak 

 as to allow of displacement by the mere weight or pressure of 

 the hand ; should the latter be the case, the " spring" may be in 

 great degree prevented, by carefully abstaining from bearing on 

 the milled head, which should be simply rotated between the fin- 

 gers. By " loss of time" is meant the want of sufficient readiness 

 in the action of the pinion upon the rack, so that the milled head 

 may be moved a little in either direction, without affecting the 

 body ; thus occasioning a great diminution in the sensitiveness 

 of the adjustment. This fault may sometimes be detected in 

 Microscopes of the best original construction, which have gradu- 

 ally worked loose, from the constancy with which they have 

 been in employment ; and it may often be corrected by tighten- 

 ing the screws that bring the pinion to bear against the rack. 

 And by "twist" it is intended to express that apparent move- 

 ment 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 upon bad original workmanship, there 

 is no remedy for it ; but it is one which most seriously interferes 

 with the convenient use of the instrument, however excellent 

 may be its optical performance. In the use of the coarse adjust- 

 ment 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 althgugh the spring with which the tube for the 

 reception of the object-glass is furnished, whenever the fine 

 adjustment is immediately applied to this ( 31), 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- 



