Still less commonly binocular telescopes of considerable aperture are constructed, primarily for astronomical use, being provided with prismatic inversion for terrestrial employment, but more particularly in order to gain by the lateral displacement of a Porro system the space necessary for two objectives of considerable size. As we have already seen, the practical diameter of objectives in a binocular is limited to a trifle over 2 inches unless space is so gained. The largest prismatic binocular as yet constructed is one made years ago by the Clarks, of 6¼ inches objective aperture and 92¼ inches focal length. So big and powerful an instrument obviously would give admirable binocular views of the heavens and so accurately was it constructed that the reports of its performance were exceedingly good. The same firm has made a good many similar binoculars of 3 inch and above, of which a typical example of 4 inch aperture and 60 inch focal length is shown in Fig. 125. In this case the erecting systems were of Porro’s first form, and were provided with Kellner oculars of very wide field. These binoculars constructions in instruments of such size, however well made and agreeable for terrestrial observation, hardly justify the expense for purely astronomical use. [Illustration: FIG. 125.—Clark 4″ Binocular Telescope.] CHAPTER VIII ACCESSORIES Aside from the ordinary equipment of oculars various accessories form an important part of the observer’s equipment, their number and character depending on the instrument in use and the purposes to which it is devoted. [Illustration: FIG. 126.—Star Diagonal.] First in general usefulness are several special forms of eyepiece equipment supplementary to the usual oculars. At the head of the list is the ordinary star diagonal for the easier viewing of objects near the zenith here shown in Fig. 126. It is merely a tube, _A_, fitting the draw tube of the telescope, with a slotted side tube _B_, at a right angle, into which the ordinary ocular fits, and a right angled prism _C_ with its two faces perpendicular respectively to the axes of the main and side tubes, and the hypothenuse face at 45° to each. The beam coming down the tube is totally reflected at this face and brought to focus at the ocular. The lower end of the tube is closed by a cap to exclude dust. One looks, by help of this, horizontally at zenith stars, or, if observing objects at rather high altitude, views them at a comfortable angle downward. The prism must be very accurately made to avoid injury to the definition, but loses only about 10% of the light, and adds greatly to the comfort of observing.