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 Hghting mechanism, and above is the 

 arm carrying the optical parts. 



The Hghting 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 Ught from the mirror 

 by means of an Abbe condenser, a large lens placed 

 below the stage. 



The stage itself in the simpler microscopes is merely a 

 fiat plate with an opening in the center, and with clamps 

 for holding the shde upon which the object is mounted. 

 For deHcate 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 

 accompHshed by a revolving disc with openings of dif- 



