An optical system for alternative or simultaneous direction of light from two scenes to the eye of a viewer comprising (a) a first lens having a first focal length; (b) a second lens having a second focal length, the first and second lenses being positioned aside one another in front of one of...http://www.google.com/patents/US5724163?utm_source=gb-gplus-sharePatent US5724163 - Optical system for alternative or simultaneous direction of light originating from two scenes to the eye of a viewer 
