Binoculars, also known as "binoculars". A telescope consisting of two monoculars juxtaposed. The distance between the two eyepieces can be adjusted so that both eyes can observe at the same time, thus obtaining a three-dimensional feeling. If two Galilean telescopes are used, they are called "spectators". It has a shorter lens barrel with a smaller field of view and magnification. If two Kepler telescopes are used, the mirror is longer and inconvenient to carry; Therefore, a pair of total reflection prisms are often installed between the objective lens and the eyepiece, so that the incident light is fully reflected in the lens barrel for many times to shorten the length of the cylinder, and at the same time, the inverted image formed by the objective lens can be reversed to become a positive image. This device is called "prism binocular telescope" or simply "prism telescope", which has a large field of view and is often used for navigation, military snooping and field observation.