It is a true tripod model, consisting of a triangular base with two pillars rising from a cross-piece, which carries the trunnions. The slow movement is obtained by a second slide close behind the first; but to avoid the friction of rubbing surfaces, hardened steel rollers are inserted between them, which give a frictionless fine motion, amenable to the slightest touch of the milled-head screw situated conveniently at the back of the limb, through which a steel lever passes which actuates the slow motion slide. The body of the instrument is therefore not touched during the fine focussing, so that all lateral movement is avoided. The mechanical stage rotates axially, and the outer edge of the lower plate is divided into degrees, in order to register the angles; a simple mode of adjustment is provided for setting the centre of rotation exactly coincident with the focal point of the objective. As the plates of the stage have no screw or rackwork between them (these are placed externally), they are brought close together, thus affording the advantage of a thin substantial stage, and ensuring rigidity where most required; phosphor-bronze being used in its construction. The stage is attached to the limb by a conical stem, with a screw and clamp nut at the back, so that it can be easily removed for the substitution of a simple plate or other stage; by turning the stem in the socket the stage may be tilted sideways at any angle required. A feature in the Ross-Zentmayer stand is the swinging sub-stage and bar carrying the mirror, having its axis of rotation situated from an axial point in the plane of the object, which consequently receives the light without requiring alteration of focus in any position of the bar; by this means facilities are afforded for the resolution of objects requiring oblique light and for the development of their structure. Rays are thus obtained from any angle and indicated by the graduated circle round the top of the swing-bar, and many troublesome and expensive pieces of sub-stage apparatus dispensed with. The value of this arrangement was long ago recognised in Grubb’s “Sector Stand,” the movement of which was obtained in a far less efficient manner. [Illustration: Fig. 63.--The Improved Ross-Zentmayer Model.] The base or foot of the Ross-Zentmayer instrument is made in one piece. Preference must be given to the double pillar support, as this is firmer, and allows the sub-stage to swing free while the microscope is in a vertical position, as in working with fluid preparations. The sub-stage is provided with screws for centring, and, when determined, secured by a clamping screw. The sub-stage, with its apparatus in place, can be instantly removed, by being drawn out sideways, so as to use the mirror alone, which is a great convenience. The mechanical movements of this instrument are perfect, and well adapted to their purpose.