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 The tripod foot, which has superseded most other forms, is adopted.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. [Illustration: Fig. 78.--Watson’s Portable Microscope. Height of instrument when placed vertically and racked down is 9-3/8 inches.] _Watson’s Petrological Microscope_ (Fig. 79) is a modification of their Edinburgh Students’ pattern, and designed specially for petrological and mineralogical work. [Illustration: 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. [Illustration: Fig. 80.--Swift’s Histological and Physiological Microscope.] Messrs. Swift’s Microscopes.