Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

668.—The Telescope is achromatic, with draw tube for focussing. It magnifies about 2½ diameters. It has a concave eye-glass, and therefore gives an erect image, [Fig. 14]. A sun-glass E screws over the eye-glass when it is required for sun observations. The telescope is attached to the sextant by means of a crank-piece upon the telescope which is fixed by the mill-headed screw T′ and two steady pins. The crank-piece screws in reverse position upon the telescope for portability before putting it by in its case.

669.—By some makers the telescope is made to slide into the body of the sextant and thus become quite portable. This plan is a very neat one, but it requires care to see that the shades do not interfere before it is put by. The weight of the entire sextant with its solid leather case is about 18 oz. only. For close work the telescope is not generally used. A sliding shutter pierced with a small hole covers the telescope opening into the sextant, which is used as a sight hole.

Fig. 298.—Box sextant under the face.

Larger image

670.—The Interior or Optical and Mechanical part of the Sextant is shown Fig. 298. I index glass, fixed over the toothed segment on the same axis. The pinion is shown working into the segment moved by the milled head O of Fig. 297 on the face of the sextant. Fig. 298: horizon glass, cut by ED, adjusts to the vertical by screws CC′, which have square fittings on the face of the instrument, shown Figs. 299 and 300 full size. The differential adjustment between horizon and index glasses is made by a screw with a square fitting at P. This adjustment acts by screwing against a helical spring, shown at Q. The reflected rays enter by a wide window in the side of the box, Fig. 298 d, the direct rays by a small window f. The path of a ray is shown by fine lines from R to E, for the positions in which the index and horizon glasses are placed. The pin-hole opposite which the eye is placed is shown white. S shades with their axis are shown cut off, to prevent confusion of other parts. They are simply round discs of parallel glass on arms which rise from the back of the face by pressure of the nibs at S.

Fig. 299.—Plan of horizon glass.

Fig. 300.—Section of the same.