Optical appts. with focusing screen for observer

The light broken down by the lens (25) leaves the focusing screen to a point intended to be in the position of the eye of an observer, The lens takes the form of a plate of even thickness, one side of which has reciprocally concentric, ring-shaped ribs (26,27) in cross-section, forming a surface transversal to the main plane of the lens, and a surface inclined to that plane. The inclination of the ribbed surface corresp. to that for tangents within the rib dia. area to the surface on a theoretical lens with the focal distance possessed by the actual lens (25). The ribs are of two types - the first kind (26) in which its inclination evolving from a first theoretical lens with a focusing distance gives a radiation path from the focusing screen directed towards the position fon the eye of an observer, and a second kind (27) in which the inclination evolves from a second theoretical lens with a focal distance intended to focus light in a path directed towards a separate magnifying glass.