Incomplete reflecting telescope of the Gregorian type. The eyepiece consists of two lenses, one closer to the primary mirror, the other closer to the eye. The primary mirror measures 65 mm in diameter and has a hole 17 mm in diameter. The secondary mirror is missing. Although the instrument is signed by Domenico Selva, the attribution cannot be made with certainty. His son Lorenzo, in a pamphlet of 1761 dedicated to Francesco Algarotti, states that all their instruments—even those that he had made himself—were signed with the name of his father. Lorenzo gives a precise description of this telescope: the secondary mirror was concave and made of metal; by adjusting the small steel bar parallel to the tube, the secondary mirror could be moved closer to or farther from the primary. The secondary mirror received the image of the objects from the primary, which were then magnified by the eyepiece. The telescope was supported by a wooden column (now missing) and could be oriented in any direction thanks to a brass knob attached to the column, "therefore it is practical and equally useful for terrestrial and celestial objects."