Magnifying lens cassette with illumination system e.g. for map reading - has light from either side of lens focussed via parabolic mirrors

摘要

The magnification cassette has a chamber on either side of the lens (2) containing an illumination circuit comprising a battery coupled to a light bulb via a switch contact. The light from each bulb is focussed onto the image being viewed via a parabolic mirror. The cassette (1) can be used for map reading with one of the chamber incorporating a magnetic compass needle (6) with its accuracy optimised by using a spirit-level (7) to determine the exact horizontal position. The two longer sides of the cassette (1) may each have a distance scale for reading off map distances.