Abstract
ON starting an electric fan with the observer so placed that he can see the sky (or other bright background) through the vanes of the fan, then at a certain speed of rotation a hazy violet patch of colour can be seen in the plane of the fan. At higher rates of rotation the effect disappears, but can again be observed on decreasing the speed to the critical value. The speed in question appears to correspond with the frequency of flicker. The effect was actually observed in the case of a fume cupboard extractor fan.
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DARTNALL, H. A Visual Phenomenon. Nature 142, 1000 (1938). https://doi.org/10.1038/1421000a0
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DOI: https://doi.org/10.1038/1421000a0
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