Figure 1.

Mach bands. (A) Diagram of the painted disk used by Mach to elicit the Mach band illusion. When the disk is spun, a luminance gradient is established between points 2 and 3, which links the uniformly lighter center of the stimulus (1) and the uniformly darker region at its periphery (4). (B) Enlargement of a portion of the stimulus in A, indicating the nature and position of Mach bands. A band of illusory lightness is apparent at position 2, as is a band of illusory darkness at position 3. (C) Because the portion of the black sector between points 2 and 3 in A is a segment of an Archimedean spiral, the luminance gradient generated between the corresponding points on the spinning disk is linear, as indicated by this photometric measurement along the line in B. (D) Diagram of the perception of the photometric profile in C, indicating the illusory lightness maximum at the initiation of the linear gradient 2, and the illusory minimum at its termination 3. Mach ruled out the possibility that these illusions arise from movement of the disk by observing that the light and dark bands are equally apparent in photographs, as is evident here.