26 ZOOLOGICAL EXERCISES, 
The stage is always fixed in a plane, at right angles to 
the axis of the body. It is provided either with a pair of 
spring clips for holding the slide, which can be moved about 
below them, or with a movable object-carrier. In the larger 
microscopes this object-carrier has mechanical movements, 
Below the opening in the stage there is a revolving dia- 
phragm-plate pierced with circular holes of different sizes, 
which, by revolving the plate, can be brought in succession 
under the central opening of the stage. One part of the 
plate is not pierced, and is sufficiently large to cover the 
whole of the stage opening, thus preventing the access of 
light from below. This is used when opaque objects are 
being examined by reflected light. The holes in the dia- 
phragm-plate serve to regulate the amount of light thrown 
on the object. With all powers objects are often more 
distinctly seen when some of the light is cut off by using 
the smaller apertures of the diaphragm plate. 
The MiRRoR is concave, and is fixed in a support that 
enables it to be placed in any position. Ordinarily it 
should be placed so that the axis of the body passes through 
its centre—(direct illumination); but occasionally it is 
useful to turn it on one side, altogether out of the axis of 
the instrument, so that it may throw light obliquely on to 
the object—(oblique illumination). In large microscopes 
there is a sub-stage for carrying an achromatic condenser, a 
polarizer, or other illuminating apparatus, in which case one 
side of the mirror is made flat to be used with them. 
The Stand. The stage is always fixed firmly to the 
stand, and the body is moved to or from it according to the 
magnifying power that is being used. In all good micro- 
scopes there are two movements to the body :— 
1. The coarse adjustment, by which the body can be 
placed nearly in the required position. 
