Further observations on the speed of retinal impression.

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Abstract
The author investigated the relation between the size of a visual object and the degree of contrast it makes with its background, on the one hand, and its threshold time of exposure, on the other. The reciprocal of the last is referred to as the "speed." Two groups of test-objects were used 1 drawn in india ink upon white card, the card and ink presenting brightnesses (light-intensities) as 100:4; the other drawn on thin white celluloid and reversed, so presenting brightnesses as 100:73. It was found that the reduction in contrast was roughly equivalent to a reduction in size of 1 step in the scale: 4 1/2, 3, 2, 1 1/2 mm. (2.58, 1.72, 1.15, 0.86 minutes visual angle). For any 1 test object, the "speed" increased about as the logarithm of the absolute light-intensity (background brightness), the deviations from this being characterized as follows: (1) At higher speeds, a relatively slower increase in brightness for equal increase in speed; and (2) for the more difficult (smaller or lower contrast) test-objects, a relatively greater increase in brightness for the identical increase in speed.

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