Changes in the sensitivity of the fovea of one of the eyes (the right eye) under influence of a glaring source of light shining into that eye can be demonstrated by comparing the visual impression obtained by that eye from a dimly illuminated surface with the impression obtained by the other eye (the left eye) from a similar surface. By adjusting the illumination of one of the surfaces the apparent brightnesses can be made to match exactly. In this way a quantitative method of measuring the sensitivity of the fovea under influence of various conditions of illumination is obtained. When a small area of the retina is illuminated the sensitivity of this area and also of the surroundings drops to a considerably lower level within 0.1 second (α-adaptation). The rate of recovery of the sensitivity after exposure is strongly dependent upon the time of exposure (β-adaptation). α-adaptation can neither be accounted for by assuming a strong adaptive influence of the stray light within the eye, nor by assuming a lateral diffusion of photosensitive substances or of photolytic products over the retina. The hypothesis is put forward that α-adaptation is of electrical origin. This hypothesis accounts for many of the phenomena observed with incandescent lamp light but not for those observed with strongly chromatic light. The only hypothesis presenting itself at present to account for these phenomena seems to be that α-adaptation has its seat in the retinal synapses.
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