Prism sight
A prism sight or prismatic sight, sometimes also called prism scope or prismatic scope, is a subtype of telescopic sight that uses a reflective prism (typically roof prism) mechanism for its image-erecting system, instead of the series of relay lenses found in traditional refractive telescope-based scopes. The use of prisms, which fold and lengthen the optical path via internal reflections, makes it possible to construct a much shorter and lighter sight, or one with an offset between the optical axes of the eyepiece and objective.