[A study on simultaneous and successive Delboeuf illusions from the standpoint of the theory of set].

An attempt was made to examine whether or not the simultaneous Delboeuf illusion and the successive one were based on the same perceptual process. By the method of the fixed set, the subject's inner process (set) during the occurrence of the illusions was investigated in Experiment I. The results indicated that his set toward the Delboeuf illusion figure was not changed when only the inducing circle was presented first, but that his set was changed at the moment the inducing circle disappeared and the two circles of identical size were presented. Experiment II examined his perception when there was a time delay between the presentations of the inducing circle of the Delboeuf illusion figure and the other two circles. In that situation, the simultaneous Delboeuf illusion and the successive one should occur successively. The results of Experiment III suggested that the transitional change from the simultaneous Delboeuf illusion to the successive one, which had been observed in the previous study, was due to a change in the judgement of the subject who experienced these two different phenomena successively.