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Abstract
In the Purkinje shift, the dark adapted eye becomes more sensitive to blue than to
red as the retinal rods take over from the cones. A striking demonstration of the
Purkinje shift, suitable for classroom use, is described in which a small change in
viewing distance can reverse the perceived direction of a rotating annulus. We measured
this shift with a minimum-motion stimulus (Anstis & Cavanagh, Color Vision: Physiology
& Psychophysics, Academic Press, London, 1983) that converts apparent lightness of
blue versus red into apparent motion. We filled an iso-eccentric annulus with radial
red/blue sectors, and arranged that if the blue sectors looked darker (lighter) than
the red sectors, the annulus would appear to rotate to the left (right). At equiluminance
the motion appeared to vanish. Our observers established these motion null points
while viewing the pattern at various retinal eccentricities through various neutral
density filters.