The research formed part of an investigation of problems of searchlight illumination. The detection of objects illuminated by a searchlight beam is largely a matter of contrast. A screen was illuminated to a known value so that its brightness was comparable with that of the path of a searchlight beam. Upon this screen was projected a rectangular beam, of variable horizontal width. The observer thus saw a rectangular strip of light upon a light background, and the length of the strip was varied from nothing to a length which enabled an observer at a distance of 26.3 mm. just to see the patch. Two series of observations were made: (1) The field illumination and strip illumination were varied by steps in the same ratio, preserving constant contrast; (2) the field brightness was kept

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