
A prismatic binocular (or prism binocular) is composed of a pair of telescopes in which the objective lens and the eyepiece are those of an astronomical telescope and the vehicle is an inverting prism. In the exemplar we present the vehicle is a prism created by Ignazio Porro (18011875) topographer and professor of tacheometry at the Politechnic of Milan starting from 1863; he was also a manufacturer of optical and mechanical instruments, with wellknown factories first in Turin, then in Paris and finally in Milan. Porro's vehicle is composed of two rectangular isosceles total reflection prisms, arranged to cross with the corners at right angles and the 'hypotenuse' faces that receive the light face each other and are orthogonal to the axis of the objective lens. The light is reflected four times (twice in each prism) and the one that propagates along the axis of the objective lens emerges in a direction parallel to that of the incidence along the optical axis of the eyepiece. The first prism produces a tilting of the image supplied by the objective lens around an axis parallel to its corner passing through the radius; if for example the corner is horizontal, the points at the top of the image given by the prism correspond to those at the bottom of the image given by the objective lens and viceversa; the positions of the points to the right or to the left for the observer of the image given by the objective lens is unaltered. The second prism, which tilts the image around a vertical axis, gives an image in which only the points to the right and to the left are inverted with respect to the image given by the first. The result of these two 180degree turns around two axes orthogonal to each other and to the beam, is a complete inversion of the image that is observed through the eyepiece with respect to the image supplied by the objective and therefore a righting with respect to the object. The exemplar we present has a magnifying power of 6 and a diameter of the objectives of 32 mm. The two telescopes of which it is composed can be rotated around the central axis to which they are attached by varying the distances between the optical axes of the two eyepieces and between those of the two objective lenses from 5 to 7 cm; they are equal when the planes found by the optical axes of the objective lens and the eyepiece lens of each telescope are parallel to each other, but when one is at the maximum the other is at the minimum. This allows the observer to set the two eyepieces at the same distance that there is between the irises of the observer. Each eyepiece must be focused separately. The two objective lenses are protected by a lens hood formed by a 1.5 cm extension of the tubes. The eyepiece lens hoods have the usual shape to fit the observer's eyes and are made of ebanite. The binocular is completely covered with leather and also has a leather carrying case. Its state of conservation is good.
Bibliografia: Persico (1932), p. 333 Perucca (1949), Vol. II, pp. 46, 50, 158

