Fig. 57.—Powell’s larger No. 2 Instrument.
Fig. 58.—Powell and Lealand’s Students’ Microscope arranged for direct illumination. A. Secondary or Sub-stage racked up to bring the Achromatic Condenser close to the object.
The cross arm encloses the lever mechanism for the fine adjustment, as originally devised by Andrew Ross. This cross arm is longer than that used by Ross, and carries the body more forward, so as to provide radial space for the complete rotation of the stage and the optic axis, and at the same time the lever of the adjustment is lengthened, and delicacy of motion secured. The stage retains the mechanical movements invented by E. Turrell, and first applied by Hugh Powell. It also rotates completely by means of an obliquely placed pinion acting on a bevelled rack on the inner face of the stage-ring supporting the mechanism. Finders are engraved on the plates, and the main support of the stage-ring is graduated for angle measuring, a pointer on the ring marking the unit of motion in arc.
The sub-stage is carried by rack-work, and has rectangular centring movements, supporting an inner socket that can be rotated by rack and pinion, and which carries the several sub-stage accessories. A fine adjustment, by screw-cone and stud, is applied by means of an extra slide.
The stage is attached to the sheath of the stem by a special arrangement of screws, by which the rotation in the optic axis can be centred; sliding spring clips and a movable and a removable and adjustable angle-piece to hold the slides are applied on the upper surface. The body-tube is pivoted to move laterally on the top of the stem, and an adjustable steel stud beneath serves to stop the movement in the axis. Such is Powell’s present instrument, and it represents the results of sixty years’ steady devotion to secure perfection, and at the same time embody the best ideas of mechanical design by Andrew Ross.
A cheaper form of students’ microscope is furnished by Powell and Lealand, with ¾-inch stage movement, coarse and fine adjustments to body, plane and concave mirrors, revolving diaphragm, two eye-pieces, and Lister’s dark wells. These makers also adopt a gauge of tubing, the size being such that it will take in a binocular body, a Huyghenian 2 inch eye-piece having the largest field-glass possible. The tube of the sub-stage is the same size, so as to secure one gauge of tubing throughout. This allows of a Kellner or other eye-piece to be used as a condenser.
Ross’s Microscopes.
Messrs. Ross have more recently introduced several changes and modifications in the Zentmayer stand, all tending to improve it, so that the Ross-Zentmayer model takes its place as a first-class microscope.
Messrs. Ross have lately manufactured other forms of microscopes; one especially designed for those commencing the study of bacteriology ([Fig. 59]). This instrument is one of the steadiest among those lately constructed for high-class work. The circular foot and short stout pillar support the whole instrument, and a substantial knee-joint sustains the full weight in the upright or inclined positions, while the centre of gravity is by no means disturbed, and absolute steadiness secured. The stage is of the horse-shoe form, which affords convenient space for the fingers to lift the slide up while the oil is placed in contact with the objective. The fine adjustment is extremely sensitive, working smoothly and direct; this is entirely covered, to prevent injury by dust. The micrometer screw works directly in the centre of its fittings, the milled head being divided to read to 1⁄500 of an inch. The sub-stage is fitted with a new centring coarse and fine adjustment, so that when using high powers with the Abbe condenser accurate focus can be secured with the least amount of trouble.