A binocular, so called because it gives binocular, and thus three-dimensional, vision, consists of two terrestrial telescopes attached to the same frame with the optical axes parallel to each other.
Each of the two telescopes making up the exemplar presented here is composed of a planoconvex lens-objective (diameter 3.2 cm, focal length 12.5 cm) and a terrestrial eyepiece; this in turn is composed of a Huygens eyepiece composed of two planoconvex lenses 2 cm apart with the flat side facing the observer, and of a erector system formed by two lenses, they too planoconvex, 2.4 cm apart.
The two eyepiece lenses have diameters of 12 and 7 mm and focal lengths of 18 and 15 mm respectively.
A diaphragm with a 10 mm hole is placed between the two lenses at a distance of 4 mm from the first.
The two lenses of the erecting system have diameters of 10 and 6 mm and focal lengths of 16 and 23 mm respectively; in front of the second lens, which is the one closer to the observer, there is a diaphragm with a 2 mm diameter hole at a distance of 5 mm.
The objective is protected by a hood surrounding the end of the tube which can slide on it to an extension of four cm.
The two telescopes are attached to a central axis around which they can be rotated, thus varying the distance between their optical axes from 5.5 to 7 cm; the observer can thus make the distance between the eyepieces the same as the distance between his/her pupils. The same axis is equipped with a screw that allows the user to move the two eyepieces simultaneously nearer to or farther from the objectives in order to focus.
Battelli - Cardani (s. d.), Vol. II, p. 1019
Persico (1932), p. 332
Perucca (1949), Vol. II, p. 157