Abstract
THE interesting visual phenomenon described by Mr. Dartnall in NATURE recently1 was observed by Helmholtz many years ago2. The effect—the appearance of a purple after-image in a patch of light alternating at a frequency just below the critical frequency—no doubt arises from the differential rate of development of after-images of different colour, but some observers whom I tested a year or two ago were unable to see the phenomenon, at least under conditions when it could be readily observed by myself and others. I then found that those who could not see the effect were those who gave a delayed type of recovery curve after light-adaptation3 as recorded by the binocular matching method.
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