. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . , which can be apprehended in severalways, or out of which several features can be isolated—all equallyeasy and of equal significance—gives rise to a succession of differ-ent percepts. A similar instance of the shifting of attention, or of perception,is found in binocular rivalry.^ Ordinarily, both eyes receiveonly slightly differing views of an object, and these views are com-bined or blended in a single percept (compare p. 425). But if,artificially,
. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . , which can be apprehended in severalways, or out of which several features can be isolated—all equallyeasy and of equal significance—gives rise to a succession of differ-ent percepts. A similar instance of the shifting of attention, or of perception,is found in binocular rivalry.^ Ordinarily, both eyes receiveonly slightly differing views of an object, and these views are com-bined or blended in a single percept (compare p. 425). But if,artificially, and most easily by aid of the stereoscope, very differentviews are brought before the two eyes, the conditions suitable forsingle vision are not fulfilled, and there is likely to be an oscilla-tion, in consciousness, between the deliverance of one eye and thatof the other. If a plain red field, for example, is held before oneeye, and a plain green field before the other, we see red and green,not usually together, but alternately. When one of the fields is Mind, 1902, N. S., XI, 316. » Compare Helmholtz, Physiologische Optik, p. Fia. 153.—The Dot Figure (McDougall). SHIFTING OF ATTENTION 599 plain, and the other contains a figure, the figure remains seen formost of the time, though the background may oscillate as in thecases of the other figures. Voluntary attention devoted to one fieldgives it some advantage over the other, by bringing it back morequickly and thus abridging each appearance of the other is true to some extent even if both fields are plain; but volun-tary attention has much more effect in holding a field which pre-sents considerable detail and so allows of a succession of percepts. It is clear that binocular rivalry belongs in large measure to thelevel which we are apt to distinguish as physiological, in contrastwith the mental; but it is equally clear that the value of each of thetwo opposing stimuli is important in giving the advantage
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