Watson’s Mechanical Draw-tube.
Fig. 73.—Watson’s Mechanical Draw-tube (full-size).
An important feature in connection with the body-tube of Watson’s Edinburgh Students’ Microscope (as, indeed, in all their fully furnished instruments) is that they are provided with two draw-tubes; one moved by rack-work, the other sliding inside the body-tube. The advantage is, that the body can be made very short or extremely long, while sufficient latitude can be given to objectives corrected for either Continental or English tube-lengths, and to adjusting the same for thickness of cover-glass by variation of tube length. Should the cover-glass be thicker than that for which the objective is corrected, a shorter tube-length is necessary; if thinner, the body must be lengthened. This is effected by means of the rackwork draw-tube. The length of the body when closed is 142 millimetres (55⁄8 inches), and when the two draw-tubes are extended, 305 millimetres (12 inches), being, therefore, shorter than the Continental and longer than the English tube lengths. Both draw-tubes are divided into millimetres, and on the rackwork draw-tube a double scale is engraved, reading continuously from the sliding draw-tube when fully drawn out, or giving the body length when the rackwork draw-tube alone is in use. The utility of this mechanical draw-tube is that it permits of quick manipulation with perfect results.
Fig. 74.—Watson’s Histological Microscope. Stand “A.”—Height, when placed vertically and tube pushed home, 9½ inches.
The inside top of the draw-tube is smaller than the remainder, the former making a fitting for the eye-piece about 1 inch long, permitting of the tube being blackened inside up to this fitting, thus minimising reflection. The end of the draw-tube has the universal screw for using the apertometer, &c.
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.
Fig. 77.—Watson’s Improved “Van Heurck Bacteriological” Microscope.
The Stage.—A somewhat new design has been used in building this up so as to reduce vibration to a minimum. The bracket carrying the stage, instead of being screwed on to the front of the limb, as is usually done, is made in a solid casting, taking the sub-stage beneath, and passing into the joint at the top of the foot. The joint bolt goes through the whole (limb and stage bracket), rendering the limb stage and sub-stage as firm as if it were one piece; a point of considerable importance.
The mirrors, which are plain and concave, are mounted on a swing arm, so that they may be turned aside when direct illumination of the object is required. On the right hand side also there is a steel clamping bar for fixing the microscope at any angle of inclination. The tripod foot, which has superseded most other forms, is adopted. At the points of contact with the table the feet are provided with cork pads, which give increased firmness and prevent vibration to some extent.
The sub-stage is provided with a fine adjustment of similar design to that employed for the focussing of the objective. It has become needful to embody such a refinement, in order that sub-stage condensers of large aperture, such as are in constant use for critical high-power work, may be adjusted with the same facility and precision as the objective—they, in fact, require it if the best work is to be got out of them. No pains have been spared by Messrs. Watson to render it absolutely perfect.
Watson’s Portable Microscope.—This instrument is similar in general detail to the Histological Microscope, but the foot, mirror stem, &c., are made to fold up in exceedingly compact form, and when set up for use the stand is perfectly rigid. Portable microscopes are, as a rule, but makeshifts. This, however, is a thoroughly sound, practical instrument and capable of best work with the highest power objectives, having good adjustments and universal size fittings throughout, so that the objectives and apparatus made for the larger instruments can be employed with it.
Fig. 78.—Watson’s Portable Microscope. Height of instrument when placed vertically and racked down is 93⁄8 inches.
Watson’s Petrological Microscope ([Fig. 79]) is a modification of their Edinburgh Students’ pattern, and designed specially for petrological and mineralogical work.
Fig. 79.—Students’ Petrological Microscope.
A polariscope having prisms of large size is supplied with it, the analyser being fitted in the body, and the polariser in the under-stage fitting. The latter has a divided circle and a spring catch at every quarter circle. By removing the polariser and withdrawing the analyser, for which provision is made, the microscope can be used for purposes of ordinary research. A Klein’s quartz plate is fitted beneath the analyser, also in the body of the microscope.
The stage, which has a glass surface, rotates concentrically, and has a divided circumferential edge reading by the verniers. The eye-piece has cross webs to the diaphragm, and when it is desired, an analyser, having a divided circle fitted with a calc-spar plate, can be used above the eye-piece, and condenser lenses attached to the polariser for stereoscopic purposes. All the fittings have the universal thread, and are interchangeable.
Fig. 80.—Swift’s Histological and Physiological Microscope.