PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 337 when the sensations excited by the object on which attention is concentrated through the eye that looks into the lens completely banish from consciousness the sensations excited through the other eye. This effect, too, is neatly illustrated by the observation of after-images of figure 2 (p. 335, vol. xi.). The after-sensations of the white discs are apt to disappear from, and return to, consciousness independently of one an- other in an irregular manner. But if one voluntarily throws them into definite grouping, say horizontal rows, then whole rows of discs tend to disappear and reappear together, and by concentrating attention upon any one such row it can be made to reappear and can be held for some time in conscious- ness while others come and go. A similar effect may be still more clearly demonstrated in the following way : Figures like a and b of figure 3 (p. 346, vol. xi.) are looked at through a stereoscope and with a blue glass before the right eye and a red glass before the left eye. The red and blue fields are thus projected on the central regions of the two retinae respectively and there is struggle and alternation of the two fields in consciousness. If now one concentrates attention upon the one field and causes its coloured discs to appear successively in each of the three linear groupings, the sensations of that eye may be held continuously present to consciousness for some little time to the exclusion of the sensations of the other eye. This observation may be repeated still more satisfactorily in the case of after-images of two such figures impressed upon right and left eyes respectively. A still more striking instance of the influence of the higher levels over the processes of the sensory level is afforded by a phenomenon which I propose to call the cerebral induction of light in order to mark its similarity to, and its difference from, the well-known retinal induction of light. I first noticed it under the following conditions. Two squares of red paper were laid side by side on a black ground with a narrow interval (about 1 cm. in width) between them, and a point about the centre of one of them was fixated steadily. At intervals the black space between the red squares became suddenly filled with red colour indistinguishable from that of the red squares, so that the two red areas were joined together to form a single large red rectangle. The red colour of the intermediate space persists only for a moment and disappears as suddenly as it comes. It is thus very different to the red coloration due to retinal induction, which becomes per- ceptible on such a dark ground adjoining a brightly coloured area after a few seconds' fixation and grows slowly and steadily
