MOTION OF OBJECT. 



rod tt, and the other by turning the head rr fixed upon tke 

 hollow rod #, through which 1 1 passes. 



Another and more simple form of moveable stage is shown in 

 fig. 21, where a a a represents a circular brass disc, having a 

 circular aperture in its 

 centre. Upon this a 

 second disc b b b is 

 placed, which is moved 

 within certain limits in 

 two directions, at right 

 angles to each other, 

 by the screws v v', 

 against which the spring 

 n I p L reacts. The 

 entire stage is in this 

 case moveable round its 

 own centre. 



By these expedients 

 the observer has com- 

 plete command over the 

 object, so as to be able 

 to move it at pleasure 



in any direction, with a motion which will be smooth, slow, and 

 free from jerks and starts, even when magnified with the highest 

 powers. 



To centre the object, that is, to place it on the stage so that its 

 centre shall be in the centre of the field, is not so easy as it 

 might appear to the unpractised in microscopic manipulation. 

 To accomplish this, let the slide be first laid across the aperture 

 of the stage, the object being as nearly as possible concentric with 

 the aperture. Let the stage and object-glass be brought nearly, 

 but not actually, into contact by the coarse adjustment. Let the 

 slide be then again centred, so as to render the object concentric 

 with the object-glass. Let the stage be then moved from the 

 object-glass until the instrument is focussed as nearly as it can 

 be by the coarse adjustment. Let the object be then more 

 exactly centred by the stage -screws, and more exactly focussed 

 by the fine adjustment. 



It must not, however, be supposed that this elaborate process is 

 necessary in the case of every class of objects. The larger sort 

 can be easily enough centred by the hand, and focussed by the 

 coarse adjustment; and in the cheaper description of microscopes 

 no other means are provided. For a smaller sort, the centring 

 may be effected by proximity with the object-glass, and rendered 

 more exact with the fingers when no stage -screws are provided. 



39 



