The =compound microscope= has undergone wonderful changes since 1667, the days of Robert Hooke. When we consider the crude construction and the limitations of Robert Hooke’s microscope, we marvel at the structural perfection and the unlimited possibilities of the modern instrument. The advancement made in most sciences has followed the gradual perfection of this instrument. The illustration of Robert Hooke’s microscope (Fig. 7) will convey to the mind more eloquently than words the crudeness of the early microscopes, especially when it is compared with the present-day microscopes. STRUCTURE OF THE COMPOUND MICROSCOPE The parts of the compound microscope (Fig. 8) may be grouped into--first, the mechanical, and, secondly, into the optical parts. THE MECHANICAL PARTS 1. The =foot= is the basal part, the part which supports all the other mechanical and optical parts. The foot should be heavy enough to balance the other parts when they are inclined. Most modern instruments have a three-parted or tripod-shaped base. 2. The =pillar= is the vertical part of the microscope attached to the base. The pillar is joined to the limb by a hinged joint. The hinges make it possible to incline the microscope at any angle, thus lowering its height. In this way, short, medium, and tall persons can use the microscope with facility. The part of the pillar above the hinge is called the _limb_. The limb may be either straight or curved. The curved form is preferable, since it offers a more suitable surface to grasp in transferring from box or shelf to the desk, and _vice versa_. [Illustration: FIG. 7.--Compound Microscope of Robert Hooke] 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.