3. The stage is either stationary or movable, round or square, and is attached to the limb just above the hinge. The upper surface is made of a composition which is not easily attacked by moisture and reagents. The centre of the stage is perforated by a circular opening.
4. The sub-stage is attached below the stage and is for the purpose of holding the iris diaphragm and Abbé condenser. The raising and lowering of the sub-stage are accomplished by a rack and pinion.
5. The iris diaphragm, which is held in the sub-stage below the Abbé condenser, consists of a series of metal plates, so arranged that the light entering the microscope may be cut off completely or its amount regulated by moving a control pin.
6. The fine adjustment is located either at the side or at the top of the limb. It consists of a fine rack and pinion, and is used in focusing an object when the low-power objective is in position, or in finding and focusing the object when the high-power objective is in position.
7. The coarse adjustment is a rack and pinion used in raising and lowering the body-tube and in finding the approximate focus when either the high- or low-power objective is in position.
8. The body-tube is the path traveled by the rays of light entering the objectives and leaving by the eye-piece. To the lower part of the tube is attached the nose-piece, and resting in its upper part is the draw-tube, which holds the eye-piece. On the outer surface of the draw-tube there is a scale which indicates the distance it is drawn from the body-tube.
9. The nose-piece may be simple, double, or triple, and it is protected from dust by a circular piece of metal. Double and triple nose-pieces may be revolved, and like the simple nose-piece they hold the objectives in position.
THE OPTICAL PARTS
1. The mirror is a sub-stage attachment one surface of which is plain and the other concave. The plain surface is used with an Abbé condenser when the source of light is distant, while the concave surface is used with instruments without an Abbé condenser when the source of light is near at hand.
