MOTION OF OBJECT. 
rod ttj and the other by turning the head rr fixed upon the 
hollow rod v , through which 11 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 
u 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 
