In practice the prism is made as shown, in three parts, two of them right angled prisms, the third containing the roof surfaces. The extreme precision required in figuring the roof forms a considerable obstacle to the production of such prisms in quantity and while they have found convenient use in certain special instruments like gunsights, where direct vision is useful, they are not extensively employed for general purposes, although both monocular and binocular instruments have been constructed by their aid.
Fig. 123.—Abbé Roof Prism.
One other variety of prism involving the roof principle has found some application in field glasses manufactured by the firm of Hensoldt. The prism form used is shown in Fig. 124. This like other forms of roof prism is less easy to make than the conventional Porro type. Numerous inverting and laterally reflecting prisms are in use for specific purposes. Some of them are highly ingenious and remarkably well adapted for their use, but hardly can be said to form a material portion of telescope practice. They belong rather to the technique of special instruments like gunsights and periscopes, while some of them have been devised chiefly as ingenious substitutes for the simpler Porro forms.
Most prism telescopes both monocular and binocular are generally made on one or the other of the Porro forms. This is particularly true of the large binoculars which are occasionally constructed. Porro’s second form with the sphenoid prisms seems to be best adapted to cases where shortening of the instrument is not a paramount consideration. For example, some Zeiss short focus telescopes are regularly made in binocular form, and supplied with inverting systems composed of two sphenoid prisms, and with oculars constructed on the exact principle of the triple nose-piece of a microscope, so that three powers are immediately available to the observer.
Fig. 124.—Hensoldt Prism.
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.
Fig. 125.—Clark 4″ Binocular Telescope.