The other microscope stands have square immovable stages, sufficiently large for general microscopic work. For the examination of very large sections two instruments are manufactured: the Nebelthau sliding microscope (p. [50]) and the stand after Dölken (p. [52]). The former is for magnifications up to 120 diam, whereas the latter can be used with any magnification.
3. The sub-stage carries a plane and concave mirror, the condenser and the diaphragm for regulating the illumination of the object.
The Condenser is so adjusted as to concentrate the rays of light about 2 mm above its surface, which is the average thickness of the microscopic slides, under an angle equal to the angular aperture of the highest power objective, which is about 120°.
In the lower power objectives the angular aperture is to be reduced by the iris diaphragm.
To focus the Condenser properly it is adjustable by rack and pinion movement on Stands A, I, Ia, Ib, and by side screw on Stands II and IIa.
4. The body of the microscope carries the tube and is fitted with two adjustments for focussing; a coarse adjustment and a fine adjustment. The smooth working of these adjustments is one of the chief requirements of a good microscope.
The coarse adjustment consists of rack and pinion and moves the tube of the microscope in a vertical groove with the least possible friction. The teeth of the rack and pinion are set obliquely to the axis of the tube and are so adjusted that two teeth of each are always interlocked, thus avoiding all possibility of slipping or loss of motion in the movement.
Figure of the Micrometer-screw of the fine adjustment.
The fine adjustment consists of a micrometer screw which moves the tube and arm of the microscope supporting the coarse adjustment vertically upon a triangular column. This column is virtually a continuation upward of the upright support of the base of the microscope. Around the column is a broad collar so accurately fitted that it moves smoothly upon it with a minimum of friction and still without lateral motion, the movement being controlled by means of a micrometer screw at the top of the column. The head of this micrometer screw, a section of which is shown in the accompanying figure, is milled and is graduated in such a way as to indicate the exact degree of motion of the microscope tube which is accomplished by turning the micrometer screw, each division of the graduation corresponding to a motion of the tube through 1/100 millimeter, and a complete revolution of the screw corresponding to a motion of the tube through ½ millimeter. The accuracy of this adjustment and the smoothness of its motion must be conceded to be a triumph of mechanical skill.