in all cases slight, in some cases hardly appreciable, and none could prevent the appearance of the opposite phase. (4) While the subject observed the wheel binocularly only one phase (the true motion) was seen, but, if at intervals of ten seconds or more he closed one eye for one or two seconds, the illusory phase invariably appeared at once. (5) When binocular observation was prolonged for two minutes (during which the true phase alone was seen) and one eye was then closed, the illusory phase always appeared at once and persisted for an abnormally long period varying from half a minute to more than one minute, after which period the two phases continued to alternate rapidly in the manner characteristic of monocular vision. In several subjects the period of dominance of the true phase of two minutes' duration (secured by binocular observation) was always followed, when one eye was closed, by a long period of dominance of the illusory phase and this by a brief period of the true phase and this in turn by a second period of the illusory phase considerably longer than normal but less long than the former long period; and it was not until the end of this second abnormally long period of dominance of the illusory phase that the balance between the two phases was restored and an alternation of equal periods of the two phases was observed. The following figures given by a single experiment will illustrate these effects:
During binocular observation:
true phase only for 120″
Then right eye closed and then during monocular observation:
illusory phase for 35″
true 2″
illusory 17″
true 3″
illusory 3″
true 2″
illusory 2″
true 2″
illusory 2″
These observations seem to show that the conditions which underlie the fluctuations of attention during the observation of ambiguous figures are similar to those which condition the alternation of colours in the rivalry of two different colour-fields presented to the right and left eyes respectively; they seem to show that in both cases a principal condition of the alternating appearance in consciousness of two objects, while the impression made on the sense-organ remains unchanged, is fatigue of the cortical tract concerned in the perception of either object, a fatigue which is induced during
