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A monocular is essentially half of a pair of binoculars, or a very simple telescope. Using prisms and lenses, a monocular allows the user to view objects at a distance as though they were much nearer. Through the use of two prisms, the resulting image appears in the correct orientation. This is in contrast to most telescopes, in which the image is inverted.
A monocular is both lighter and smaller than an equivalent pair of binoculars, making it ideal for certain uses. The fact that, like a telescope, it must be used with only one eye introduces a number of problems that make binoculars preferable in many situations. The stereoscopic effect, which is achieved by using both eyes to view an object, allows for much better tracking of moving objects. It is for this reason that binoculars are preferred for following races or for tracking aircraft or birds in the sky.
Monoculars are perfectly suited for astronomy, however, where the objects being viewed are relatively stationary. Many astronomers use a monocular as a spotting telescope, to identify a portion of the sky to focus in on with a larger, more highly-powered telescope.
While there exist accounts of ancient monoculars in Arabia and other parts of the world, the earliest recorded patent for a telescopic monocular dates from the early 17th century. The primary purpose of undocumented monoculars in earlier ages was as a spotting device during war and for defensive measures. While tracking with a monocular is difficult, it was sufficient to spot riders at great distances and make out their colors or banners to determine whether they were friend or foe.
Hans Lippershey is the first documented telescope maker, both of binocular and monocular types, though he was undoubtedly not the first to actually create such a device. His early prototypes were monocular, but after the urging of members of the patent-granting institution of the time, he developed binoculars as well.
It is easy to find monoculars ranging from the very cheap to the very expensive. Cheap toy telescopes intended for children are of the monocular variety, with magnifications typically of only three or four times. More expensive monoculars may be found with much better lenses and prisms, allowing for magnification of 30x or greater.