The distances at which a series of rectangular test objects are visible were determined experimentally. These test objects were black opaque rectangles having variable length-width ratios and were viewed against a trans-luminated background. Using these data, and an assumed nystagmic motion of the eyeball, the summated temporal illuminance gradients on the retina were computed. These values were found to be essentially constant, even though the length-width ratios of the test objects varied from 1 to 4050. Assuming a motionless eyeball, that is, perfect fixation, the summated values of ΔI were computed for the same observational conditions, ΔI being defined as the difference between the average illuminance incident on adjacent cones in the retinal mosaic. The values of summated ΔI are not constant. Since all of these test objects were just visible at the distance measured, they may be considered as being equal to each other with respect to their ability to excite a threshold neural response. It seems reasonable to conclude, therefore, that since the values of the summated temporal gradients are constant they are the most significant indices of the relative magnitudes of the neural responses. In our opinion these results lend considerable support to the assumption that the perception of inhomogeneities of luminance in the visual field is dependent directly upon the temporal illuminance gradients to which the retinal receptors are subjected by virtue of the movement of the image with respect to the retinal mosaic.
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