An automatic angle compensator comprising a liquid sealing container 4 having transparent liquid to form a free liquid surface 1 sealed therein, light projecting systems 21 and 22 for projecting light at a given angle to the free liquid surface, and an optical system for guiding the light beam,...http://www.google.co.uk/patents/US5847824?utm_source=gb-gplus-sharePatent US5847824 - Automatic angle compensator
