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An optical instrument consisting essentially of two small telescopes, mounted side by side,
one for each eye. The relatively compact size of binoculars is achieved by the use of prisms
to reflect the light internally. At the same time, the reflection by the prisms makes the
image upright, rather than inverted as it is in an astronomical telescope. The size and
magnifying power of binoculars is usually given in the form A×B,
where A is the linear magnification and B is the diameter of each objective lens
in millimetres (e.g. 10×40). Binoculars are popular with amateur astronomers. They
are particularly useful for certain types of observation, such as comet hunting.
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