_The Stand._--The general form of the stand still partakes too much of the original sameness of type introduced by Oberhäuser, and modified and improved by Hartnack; the “Babuchin” stand being still in favour with some few makers. The greater firmness and steadiness of Zeiss’s stand (Fig. 95) is secured by the horse-shoe form of foot, which, for the most part, is massive and well adapted to carry the stout uprights, which support a well-balanced, substantial body-tube and a graduated draw-tube, circular stage with a vulcanite disc, 4 inches in diameter; a sub-stage with centring arrangement for Abbe’s illuminating apparatus, and iris diaphragm and other diaphragms for use when the condenser is thrown aside. The mirror is full-sized, plane and concave. The coarse adjustment is regulated by a rack and pillion movement so perfect that objectives of medium power can be focussed by it alone. The fine adjustment is made by micrometer screw, the force exercised by which is transferred to the movable body by a single contact between two hardened steel surfaces. This ensures extremely delicate and uniform motion of the body which carries the tube. The divisions in the milled head of the screw furnish a means for the registration of the vertical movements of the tube. In the latest stands, each division corresponds to an elevation or depression of the tube in the direction of the optic axis of 0·01 mm. By this means measurements of thicknesses may be made with a considerable degree of accuracy, the upper and lower surfaces of the object being successively focussed, and the amount read off on the milled-head, by the fixed index. In doing this, care must be taken to make both adjustments by a rotation of the screw in the same direction. The thickness of an object in air is then equal to the difference between the two readings. By this means the thickness of any other substance may be measured--that, for instance, of the cover-glass of the object. [Illustration: Fig. 95.--Zeiss’s Medium Stand Microscope.] The medium tube-length of the microscope is 160 mm. from the attachment of the objective to the eye-piece end. The draw-tube admits of the length being increased or diminished, and this may be read off by means of the millimetre scale engraved on the tube. My description of this model also applies to the higher class microscopes, which will be found in every way well finished and adapted to biological and scientific research. [Illustration: Fig. 96.--E. Leitz’s Medium-sized Microscope.]