Watson’s Histological Microscope ([Fig. 74]) is a somewhat cheaper form of instrument, designed for the student; although of plainer construction it is quite as well made as the costlier model. It is provided with spiral rack and pinion coarse adjustment, and with this motion the greatest smoothness is preserved. There is no backlash, the teeth of the pinion never leaving the rack; so effective is it that a high power can be perfectly focussed by its means. It is also furnished with their universal pattern of fine adjustment. This can be had for £3 3s.
Fig. 75.—Watson’s Semi-Mechanical Stage.
Messrs. Watson have among other accessories of value introduced in connection with their several microscopes a semi-mechanical stage, whereby they are enabled to reduce the cost of manufacture. [Fig. 75] is an outline sketch of the same.
This stage is of the horse-shoe shape, with cut-out centre, constructed of ¼-inch brass plate, and measures over all 5¼ inches wide by 4 inches deep. Fitting on the edges of the main stage is a frame which is actuated vertically by means of a double rack and pinion from beneath, giving ¾-inch of movement, having controlling heads on either side of the stage; on the edges of this mechanical frame a sliding bar is fitted, consequently movement may be imparted either by rackwork or by hand. The mechanical movement, however is in one direction only; but as the bar carries the object, the worker can easily move the object out horizontally with the finger. The advantage of this stage is that the whole surface is perfectly flush, and the pinion heads are below its level, so that culture plates or continuous sections may be conveniently examined.
Fig. 76.—New Centring Underfitting for Microscope.
Another addition of considerable value is the centring underfitting for students’ microscopes.
This fitting places in the hands of student workers a means of accurately centring the sub-stage condenser, at a low cost. It consists of the usual underfitting tube, having a flange at the top which is fitted in a box between two plates. The centring is effected by means of two screws, which press the flange against a spring, as in the ordinary sub-stage centring movement. The fitting can be adapted to any form of Messrs. Watson’s and most other makers of students’ microscopes.
Watson’s Bacteriological Improved Van Heurck’s Microscope ([Fig. 77]) is in every way a superior instrument, and it at once conveys a favourable impression to the practical worker. When set up for use its many convenient points—its excellence of workmanship and the precision of its movements—seem to imply its special adaptation for the bacteriological laboratory and for other high-class work where absolute reliance has to be placed in the results obtained. Every detail of the instrument is carried out in the best possible manner. The coarse adjustment is effected by means of a diagonal rack and spiral pinion, which ensures the smoothest possible motion; while the fine, the most important movement in the instrument, is made with an extra long lever, a specialty of Messrs. Watson’s, and which imparts an extremely slow action: this is now one of the most delicate and reliable forms of fine adjustment. By its means the entire body is raised or lowered by means of a milled head fixed to a screw having a hardened steel point acting on a lever against a point attached to the body slide, in a dove-tailed fitting about 2½ inches long. Owing to the position of the controlling milled head on the limb, it can be worked with either hand. Another feature of importance is that, in using the fine adjustment the distance between the eye-piece and objective remains unaltered. All the frictional parts of the microscope have spring slots to the dove-tailed fittings, in which compensating screws are fitted. These are some few of the more important points, to which much thought and attention have been given. The body permits also of the use of objectives of any other optician, since its total length when the draw tubes are closed up is only 143 mm.; when extended, a total length of 320 mm. is available. By this means an ample margin is left for the correction for cover-glass thickness, whether the objective used be intended for the 160 mm. or 250 mm. tube length. The height of the microscope when placed in the vertical position is 131⁄8 inches.