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The effects of varying the duration of a stroboscopic flash are considered. If the disk in Fig. 1(a) is rapidly rotated and viewed under continuous illumination which is periodically interrupted for short intervals, then by adjusting the frequency of the inter-ruptions, the stationary negative image in Fig. 1(b) is produced. Standard texts in stroboscopy' and visual percep tion2 do not mention this effect,and we have been unable to find anyone who is familiar with it. There is, however, an early paper by Faraday,3 cited by Flelmholtz,4 which des-cribes related but not identical phenomena. The observed patterns can be predicted using optical arguments. Two time-varying illumination schemes are called complementary if the sum of the intensities of both schemes gives a constant illumination in time. While the disk in Fig. 1(a) is rotating rapidly, it appears a uniform gray under constant illumination, due to time averaging in the visual :system.4 ence, if the spatial pattern of intensity observed on the rotating disk under some illumination scheme is summed with the pattern observed under the complementary scheme, the result must be a uniform gray. Consequently, since illumination with periodic short light flashes gives a positive, image (narrow dark bars on a bright field), illumination by the complement must give a negative image (narrow bright bars on a dark field), Fig. 1(b).
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