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78.—Galileo's Telescope, Fig. 14.—In this the eye-piece is a concave glass. This glass is placed inside the focal distance, so that the rays from the object-glass are bent to less convergence, that they may enter the pupil of the eye in a direction possible to reach the retina. The image in this telescope is maintained erect. This principle is used entirely for field and opera glasses, also for sextants and some other instruments where it is desirable to keep the image erect, and small power is required, sufficient only to obtain more distinct vision. The lines aa′ in Figs. 13, 14 are termed the axis of the telescope.
79.—Optical Arrangement of the Huygenian Telescope.—In surveying instruments, where angles and directions are not taken by coincidence of direct and reflected images, it is necessary that the direction of the axis of the telescope should be clearly indicated. In this case the focus of a distant object—that is, its exact image—is projected upon a plane termed the diaphragm, Fig. 15, SS′ upon which a visible object or index is placed, the position of which is picked up by a secondary telescopic arrangement, or eye-piece as it is technically termed.
Fig. 15.—Diagram of arrangement of lenses.
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80.—The arrangement of lenses in a surveying telescope is shown in the illustration above, where OG is the object-glass or objective, E the eye-glass, F the field-glass. The two lenses E and F, in their mountings, form the eye-piece EP. The dotted line a is the axis of the telescope, SS′ is the focal plane of the object-glass, where a metal disc is placed with an opening in its centre—this is termed the diaphragm or technically, the index-stop. Across the opening in the disc, spider's webs or other fine visible objects are placed, to be described further on.
81.—Both the object-glass and the eye-piece are fitted in sliding tubes, which will be described presently, in such a manner that they may be made to approach or recede from the focal plane SS′. The nearest distance of the object-glass to this plane is the solar focus, or the distance at which a sharp image of the sun or a star placed in the axial line would be formed. The greatest distance of the object-glass from the focal-plane in most instruments is such that a clear image will be given on this plane SS′ of an object placed at about twenty feet from it.
82.—The Ramsden Eye-piece, the optical arrangement of which is shown in Fig. 16, is also known as a positive eye-piece. It consists of two plano-convex lenses, the convex surfaces of which are turned towards each other. They are separated by a distance equal to two-thirds the focal length of either glass, and placed so that the diaphragm is one-fourth this focal length from the field-glass.
83.—This eye-piece is considered not to be quite so achromatic as another form known as the Huygenian eye-piece, but its spherical aberration is less than any other, and it gives what is necessary in all measuring instruments—a flat field of view, requiring no change of position to see the centre and border of the field with equal distinctness.