Dove Prism

Dove Prism is a form of prism invented by H. W. Dove. It resembles half of a common right-angle prism in which a ray entering parallel to hypotenuse face is reflected internally at that face and emerges parallel to its incident direction. One of the incident rays emerges along a continuation of its incident direction, and if the prism is rotated about that ray through some angle, the image rotates through twice that angle. A Dove Prism must be used in parallel light.