Fig. 19.—Diagonal eye-piece, full size; S G sun-glass.
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It is astonishing that the Americans, who are usually so quick in adopting the most practical appliances, are so slow in seeing the advantage gained by the use of the now almost universal inverting eye-piece.
90.—The Diagonal Eye-piece, Fig. 19, is used upon transit instruments, theodolites, and occasionally upon mining-dials. It permits the telescope to be used by the observer looking at right angles to its axis. Thus, by the natural direction of the eye, stars or the sun may be observed to near the zenith, or the direction of a line cut by two lights at the bottom of a shaft may be observed from above by the telescope of a theodolite having a hollow centre on its ordinary stand, to check the magnetic bearing of the needle below ground, if this is assumed to be subject to local disturbance. The socket of this eye-piece screws upon the telescope and has a free inner tube for rotation, so that the 90° to the axis of the telescope may be placed at any angle to the axis of its cylindrical circumference; as, for instance, instead of being used vertically or for zenith stars, it may be used horizontally, where precipitous ground would not permit direct axial vision through the telescope. The reflecting arrangement of this eye-piece may be adapted either to the Ramsden or the erecting form. In either case the reflector is placed in the central portion of the eye-piece. In surveying instruments the reflector is generally a piece of polished speculum metal for portable instruments, but a prism of glass for larger fixed instruments. The general arrangement is shown in the section of a diagonal Ramsden eye-piece on page 42, full size. A object lens, D eye lens, adjustable for distance from the reflector R, S outer casing which permits adjustment for focusing, SG sun glass, the diaphragm being in front of A.
91.—When a rectangular prism is used for the reflector, it is worked with one plane 45°, as previously discussed, [art. 55, Fig. 3]. In place of one or both the 90° faces these surfaces are sometimes worked convex so as to form a magnifier, dispensing with one of the convex lenses of the eye-piece. A long diagonal eye-piece is necessary, where stars towards the zenith are to be observed, to prevent interference of the limb of a theodolite with the face of the observer.
92.—Reflecting Eye-piece is used to observe small stars, as for instance the circumpolar stars in the southern hemisphere, by illuminating the front of the webs or lines. A strong light thrown down the telescope from a reflector to illuminate the webs would tend to dim the effect of blackness of the sky and render these stars indistinct. In the eye-piece, Fig. 20, a piece of plain parallel glass is placed at an angle of 45° to the axis. This permits the webs to be clearly observed through the glass at the same time that it throws light from a lamp placed at a distance from the glazed aperture L by reflection of the surface of R sufficient for front illumination. The amount of light required is regulated by the distance of the lamp from L. This eye-piece is made to fit into the diagonal eye-piece casing, as S [Fig. 19], E Fig. 20 being the position of the eye, F field-lens.
Fig. 20.—Reflector in eye-piece to illuminate the front of diaphragm.
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