A forward-oblique viewing optical system for endoscopes comprising an observing-direction changing prism which has a surface for refracting rays from an object to be observed and is arranged in front of an objective, said optical system as a whole thereby being made compact....http://www.google.es/patents/US4138192?utm_source=gb-gplus-sharePatente US4138192 - Foward-oblique viewing optical system
