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1. The gain of the central response mechanism and the latency of the pure central response of on‐centre ganglion cells were studied by recording from single optic tract fibres the responses evoked by slow square‐wave stimuli applied against some steady background.

2. The concept of effective flux was introduced and defined: if any portion of a stimulus extends beyond Ricco's area of complete summation, then that stimulus has an actual flux, equal to the product of its area and luminance, but it also has an effective flux which is that fraction of its actual flux which equals the actual flux of another stimulus which, when it falls entirely within Ricco's area, evokes an isobolic pure central response or has the same adaptive effect upon the central response mechanism as the first stimulus.

3. The most significant finding was that when the cell responded with a pure central response to the incremental flux (the square …