Fig. 93.—Pillischer’s “International” Microscope.
The following reference letters serve to explain the general construction of the microscope ([Fig. 93]):—a, the eye-piece; b, the draw-tube; c, the sliding-tube; d, micrometer or fine adjustment; e e, the coarse adjustment; g, the mirror arm and mirror; h, sub-stage carrying Abbe condenser; i, the stage with spring-clips; j, objectives screwed into place and double nose-piece.
The “Kosmos” is Pillischer’s cheaper model. The stand of this somewhat novel and original microscope is framed entirely of brass and gun-metal. The fine adjustment is very sensitive and perfectly steady, admitting of the highest immersion objectives being used. The optical parts are constructed upon principles consistent with the latest improvements. It has a claw-foot stand with a semi-circular arm, which carries the body, with sliding-tube coarse adjustment, and micrometer screw fine adjustment, with a large square stage diaphragm and mirror. The instrument is neatly packed in a mahogany box, together with the A or B eye-piece, 1-inch and 1⁄5-inch objectives of good defining and penetrating power, magnifying from 30 to 380 diameters, in mahogany cabinet, for the moderate sum of £5.
Pillischer’s Binocular Microscope ([Fig. 94]) is constructed on a plan somewhat intermediate between that of Beck’s and Ross’s well-known patterns, and in point of finish is equal to any student’s microscope in use. The semi-circular form given to the arm carrying the body increases the strength and solidity of the instrument, although it is doubtful whether it adds to its steadiness when placed in the horizontal position. The straight body rests for a great part of its length upon a parallel bar of solid brass ploughed into which is a groove for the reception of the rack attached to the body, the groove being of such a form that the rack is held firmly while the pinion glides smoothly through it. A steady, uniform motion is thus obtained, which almost renders the fine adjustment unnecessary. The binocular bodies are inclined at a smaller angle to one another than in most instruments; nevertheless, the range of motion given to the eye-pieces by the rack and pinion enables those whose eyes are widely separated to use the instrument with comfort. The prism is so well set that it illuminates both fields with equal intensity. The stage is provided with rectangular traversing movements to the extent of an inch and a quarter in each direction. The milled heads which effect these are placed on the same axis, instead of side by side, one of them—the vertical one—being repeated on the left of the stage, so that the movements may be communicated either by the right hand alone or by both hands acting in concert. The stage-plate has the ordinary vertical and rotatory motions, but to a much greater extent than usual; and the platform which carries the object is provided with a spring clip to secure the object when the stage is placed in the vertical position. A new form of sub-stage with centring screws is made to carry the Abbe achromatic condenser, diaphragm, polarising and other apparatus.
Fig. 94.—Pillischer’s Binocular Microscope.
Continental Microscopes.
Continental Microscopes.—The better known among continental opticians are Zeiss, Leitz, Seibert, Reichert and Hartnack. All seem to have vied with each other in the attainment of perfection in the manufacture of the most useful forms of microscopes. The late Carl Zeiss did more for the modern microscope than either of the opticians referred to above. I therefore take a medium typical model of his from a long series of highly-finished instruments for my illustration. Zeiss’s successors have of late endeavoured to perfect the mechanical details of their instruments in three or four directions, i.e., fundamental features of the stand, stage arrangements, means of focussing, and illumination.
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.