Although the optical system is the first consideration in a microscope, the system is valueless if the fittings do not allow its correct use. The optical system must be kept at a certain distance and well centred, and a correct position for the object in relation to the system must be assured.
In the diagram, the microscope is seen to consist of the heavy metal foot A, which rests on the table at three points. The whole microscope is fitted to this foot. The object can be held firmly on the stage plate B by cramps C. On the lower side of the stage plate are the condenser and the diaphragms, and the illuminating mirror j is held by a rod D fixed to the stage plate. Likewise on the stage plate is the support for the tube E. The rough adjustment of the microscope can be made by a rack and pinion F; and the fine adjustment by the screw G. The tube containing the eyepiece and the objective is double. The inner tube H is movable, making a change in the length of the tube possible. As a rule this inner tube has a mark which allows the length of the tube to be set.
It is most important the stand should be free of vibration. A fine adjustment is also necessary, in order to perform conveniently and with certainty the slight motion of the microscope in relation to the object. In cheap stands the rough adjustment was worked by moving the inner tube by hand, but the more convenient rack and pinion is now used almost exclusively.
For slight magnifications rough adjustment is sufficient, but with objectives of a focus below 1/4 inch, a fine adjustment is wanted. Very different constructions are in use, Almost all are such that the whole microscope tube is raised or sunk by the mechanism of the fine adjustment, and not only the objective.