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423.—Improved Transit with Adjustable Axis.—This instrument, Fig. 176, in general, resembles that last described, except that it has a larger telescope and it is mounted on a sliding stage with screw adjustments, which is particularly described below. It is frequently provided with a tacheometrical eye-piece for giving horizontal distances by subtense taken on the incline, which will be described in Chapter XII.
Fig. 176.—Stanley's patent new model theodolite, with mechanical stage.
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424.—The Mechanical Tribrach Stage.—This important addition to the theodolite above described permits exact adjustment over a station. The upper plate of the tribrach with the movable stage is shown in Fig. 177. A dovetail slide is fitted upon the base of the stage adjustable for wear by a slip-piece with two screws at the narrow part. The slide is adjusted to position in the direction of its dovetail fitting by a large milled screw so as to move the whole instrument above it for centring in this direction. An upper slide acting in the same manner, with dovetail fitting pieces at sides moves for an equal distance for centring transverse to the lower slide by a milled head. This gives the same kind of motion of displacement that we have in the slide rest of a lathe or the mechanical stage of a microscope, except that in this case we have a kind of three-point bearing surface. The motion given to the screws permits the perfect adjustment of the theodolite over a point on the ground corresponding with the suspended plummet, after the instrument is set up to nearly its true position by movement of the tripod legs. The range of motion is from ¾ to 1 inch, a quantity quite sufficient for final adjustment, but which does not materially affect the equilibrium of the instrument upon its rigid tripod, as it has in this case a broad solid base even in the extreme positions of the slide.
Fig. 177.—Stanley's patent tribrach mechanical stage.
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This movement being above the levelling screws, the adjustment of the instrument for level is not affected by its use, as in the case of all sliding arrangements below the levelling screws. Suitable means are provided for taking up any wear that might occur in the slides.