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. [Illustration: FIG. 8.--Compound Microscope Eyepiece Draw Tube Body Tube Coarse Adjustment Revolving Nosepiece for three Objectives Fine Adjustment Stage Objectives Limb Abbi Condenser Iris Diaphragm Hinge for Inclining Substage Attachment Mirror Pillar Foot] 2. The =Abbé condenser= (Fig. 9) is a combination of two or more lenses, arranged so as to concentrate the light on the specimen placed on the stage. The condenser is located in the opening of the stage, and its uppermost surface is circular and flat. [Illustration: FIG. 9--Abbé Condenser] 3. =Objectives= (Figs. 10, 11, and 12). There are low, medium, and high-power objectives. The low-power objectives have fewer and larger lenses, and they magnify least, but they show more of the object than do the high-power objectives. There are three chief types of objectives: First, dry objectives; second, wet objectives, of which there are the water-immersion objectives; and third, the oil-immersion objectives. The dry objectives are used for most histological and pharmacognostical work. For studying smaller objects the water objective is sometimes desirable, but in bacteriological work the oil-immersion objective is almost exclusively used. The globule of water or oil, as the case may be, increases the amount of light entering the objective, because the oil and water bend many rays into the objective which would otherwise escape. [Illustration: FIG. 10.] [Illustration: FIG. 11.] [Illustration: FIG. 12. Objectives.]