Fig. 72.—Sub-stage of Edinburgh Students’ Microscope. This view of underside of stage of students’ instrument shows the mirror set at an angle for oblique illumination, and sub-stage turned aside.
The various sizes of oculars adopted by opticians and at present in vogue cause considerable confusion. A standard size is specially needed for students’ and small microscopes. The standard long used by Continental manufacturers is 0·92 of an inch. The adoption of this size would place the eye-piece in the same position as that of the universal screw for the objective, formulated by the Royal Microscopical Society many years ago. The desirability of using standard sizes has been fully recognised by Messrs. Watson and they are now adapted to most of their microscopes. The English diameter, 1·35 of an inch, known as the “Ross” size, is retained in all their microscopes of large size.
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.