GB802918A - Improvements in sighting apparatus for fire-arms and other instruments - Google Patents

Abstract

802,918. Lighting devices. VICKERSARMSTRONGS (AIRCRAFT), Ltd., and JEFFREE, H. Oct. 23, 1956 [Oct. 27, 1955], No. 30833/55. Class 97(1) [Also in Group XXI] A backsight for a firearm is in the form of a zone plate which enables the eye to focus simultaneously on the foresight and on the target. The zone plate may consist of a lens or transparency A, Fig. 1, disposed normal to the sighting line and displaying a series of narrow circular concentric zones A1 and A2 respectively transparent and opaque, the radii of the boundaries between successive zones being substantially proportional to the square roots of the natural numbers. Alternatively, instead of transparent and opaque zones A1, A2, the zones B1, B2, Fig. 2, may be alternately of different optical thickness such that light passing through the thicker zones B2 is caused to lag behind, that passing through the thin zones B1 by an odd number (preferably one) of half wave-lengths. Assuming that the eye focuses the target details, this is viewed by substantially parallel light rays advancing on substantially planar wave-fronts and such light passes through the transparent zones A, Fig. 1, as though the zone plate were in fact absent. Light from the foresight, however, advances with spherical fronts which reach different zones of the plate at different times, the arrangement being such that at any instant, the mean phase of the wave-front then passing has the same value for all transparent zones and although the pulses originally belonged to different wave-fronts, they combine after passing the zone plate to form a planar wave-front which is focused simultaneously with that received from the target. A similar effect is produced by the zone plate of Fig. 2. A lens may be provided at the backsight in addition to the zone plate. As shown in Fig. 9, a positive lens is mounted in conjunction with an index mark, such as cross lines or a graticule, at the foresight position FS, and a zone plate/negative lens combination ZP and L is provided at the backsight position, such backsight combination being chosen so that on the one hand it acts as a negative lens, forming with the foresight lens a Galilean telescope, and on the other hand it enables the index mark to be varied in sharp focus simultaneously with the enlarged target detail. The index mark and the zone plate may be formed on the surfaces of the foresight lens and backsight lens respectively.