Abstract
The amount by which the apparent brightness of a visual field is inhibited by a surrounding field depends in the area of the inhibiting field, inter ocular brightness matches showed that, as the size of a surrounding annulus is increased from a thin ring, the degree of inhibition on the brightness of an inner disk increases rapidly at first and then more slowly as the effect approaches an asymptote. The increase of the inhibition with size of annulus can be expressed as an increase in the exponent of the power function that relates the apparent brightness of the disk to its physical luminance.