FUNCTION AND PARTS OF THE MICROSCOPE. 19 
and a vertical pillar rising from it, which may or may 
not be jointed. Attached to the pillar is the stage, a 
perforated plate upon which the object may be placed; 
below this is the lighting mechanism, and above is the 
arm carrying the optical parts. 
The lighting apparatus includes first a mirror, usually 
having one plane and one concave surface and so mounted 
on a jointed bar as to set at any angle. For high powers 
it is customary to concentrate the light from the mirror 
by means of an Abbé condenser, a large lens placed 
below the stage. 
The stage itself in the simpler microscopes is merely a 
flat plate with an opening in the center, and with clamps 
for holding the slide upon which the object is mounted. 
For delicate work it is convenient to use a microscope 
equipped with a mechanical stage which can be moved 
forward and backward and from side to side by a microm- 
eter screw. Such an arrangement makes it easier to 
explore the whole of the specimen to be examined, and 
enables one to find any particular portion of it at pleas- 
ure. Under the stage, whether it be of the simple or 
-the mechanical type, there should be some sort of dia- 
phragm for regulating the direction from which light 
will reach the object, and cutting off the peripheral rays 
coming from outside the object itself which tend to 
obscure the image. The diaphragm opening must there- 
fore vary with the size of the object under examination 
and with the power of the microscope, a smaller opening 
being used with a higher power. This adjustment is 
accomplished by a revolving disc with openings of dif- 
