MANAGEMENT OF THE MICROSCOPE. 13T 



without such an increase of its angle of aperture as unfits it for its own 

 proper work, but the ' deep eye-piecing ' required cannot be had recourse- 

 to habitually without exposing the eyes to severe overstrain. The advan- 

 tage of loiv Eye-pieces and deep Objectives, as compared with deep Eye- 

 pieces and low Objectives, has been very well put by likening it to the 

 comfort of reading large print without spectacles, or with spectacles 

 suited to the sight, and reading small print with a magnify ing-glass. 



137. In making the Focal Adjustment, when low powers are used, it 

 will scarcely be necessary to employ any but the coarse adjustment, or 

 'quick motion;' 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 7 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 pres- 

 sure of the hand: should the latter be the case, the 'spring' may be in a 

 great degree prevented by carefully abstaining from bearing on the 

 milled-head, which should be simply rotated between the fingers. 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 slightly 

 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 gradually worked loose owing to the constancy with which 

 they have been in employment; and it may often be corrected by tight- 

 ening 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. 1 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 instrument, 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 'fine 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 the fine adjust- 

 ment is otherwise provided for, still greater care is of course required, 



1 In testing either the ' coarse' or the ' fine ' adjustment for ' twist,' care should 

 be taken that the light reflected from the mirror is axial not oblique ; since, if 

 the illuminating rays are inclined to the optic axis, the object, when thrown out 

 of focus, will appear to vanish laterally, "which it does not do (provided the 

 adjustments work well) when illuminated axially. 



