The liminal brightness increment for white light for different conditions of the foveal and parafoveal retina

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Abstract
The results of measurements of the smallest perceptible intensity of a white light test spot (liminal brightness increment for white) are given, for foveal and parafoveal vision under conditioning stimulations of the following types (a) uniform background brightness (b) uniform surround brightness with dark centre field (c) an isolated bright patch or glare source vertically above the test spot. The variation of the l.b.i. with conditioning stimulation intensity is shown to be of different character for foveal and parafoveal vision, the curves for the latter exhibiting a division into a high intensity range associated with cone vision and a low intensity range associated with rod vision. To a first approximation, the l.b.i. for foveal vision is found to be independent of the color of the conditioning stimulation. For parafoveal vision, however, a type of Purkinje effect is observed. Experiments on day to day variations of l.b.i. measurements and other minor investigations are briefly described.

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