389.—The Telescope, [Fig. 153], DD′ has been described [art. 94]. Its general construction is also shown in partial sections in the figure. Its body tube passes through the transit axis in which it is soldered.
390.—The Principal Level Tube is generally mounted on the telescope upon two stiff screws which rise from plates attached to the telescope body by pairs of screws. Each level screw has a pair of capstan nuts. The level is mounted in a brass tube with stop-pieces at the ends, each of which carries a tenon with a hole in its centre through which the level screw passes to be clamped top and bottom by the capstan nuts. These nuts give adjustment to the level, so that the centre of its inner upper surface may be placed parallel with the optical axis of the telescope.
Fig. 156.—Stanley's new model of 4-screw transit theodolite.
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391.—Until 1898 the author was unwilling to attempt to remodel the old form of transit theodolite, believing the 4-screw adjustment would soon become a method of the past, but as a small demand continued from the Colonies and United States for this form of instrument he felt bound to make it of more solid construction to bring it somewhat up to date. The illustration shown, Fig. 156, is of an instrument, following in construction the transit theodolite already described in many details, the marked exception being that the standards are in one casting with the compass-box and axis, these being entirely shaped out in the solid metal. The upper parallel plate is of special design, being far stronger, yet lighter, and gives a much longer bearing to the levelling screws. The lower parallel plate is also shaped with three feet so that the instrument may be set up without its stand when required. It has also modern spring tangent adjustments with covered screws. The limb is covered, and the readers are jointed across the axis to turn up without separation. It has a floating aluminium compass read by a microscope, so that the instrument, except in the four-screw arrangement for setting up, embraces many modern improvements formerly applied only to special high-class theodolites. The improved construction permits greater rigidity with fifteen per cent. less weight.
Fig. 157.—The plummet.
Fig. 158.—Gurley's plummet.
Fig. 159.—Loop.