PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 331 eye from a to b. But, as I have already pointed out, although these eye -movements favour these changes, it is easy after a little practice to associate each change with the opposite movement of the eyes. In the case of Necker's cube (fig. 8) Prof. Loeb l has pointed out that when one of the angles a or b is fixated, that angle tends to appear as the nearest or the most remote point of a cube, according as the accommodation of the eye is increased or diminished. 2 Loeb finds that these effects are still produced if the accommodatory muscles of the eyes are completely paralysed with atropine. He is therefore unable to regard the effect in this case as the result of afferent impulses to the brain initiated in the eye-muscles, and falls back on the suggestion that the essential factor determining one or other mode of perception is the current of innervation passing out from the brain to the eye-muscles. This modified form of the " muscular view " seems how- ever to be no more tenable than the cruder form of it, in face of the following observations : (1) The changes of form of the cube may by a little practice be made to accompany changes of accommodation the reverse of those which they most naturally accompany, e.g. the angle b being fixated, it may be made to recede as accommodation is increased, and to become the most prominent point of the figure as accom- modation is relaxed. (2) A point midway between a and b may be fixated and, while this fixation is maintained, either of the principal forms suggested by the figure may be called up at will, and its presence to consciousness may be prolonged by an effort of attention. The influence of voluntary effort was studied by the method applied to the study of the struggle of two differ- ently coloured fields presented to the two eyes (p. 478 3 ). The results of one experiment may be quoted : Necker's cube (fig. 8, p. 483). Point midway between a and b continuously fixated during 118" in each case. 1. Passive : 27a = 11", 216 = 11", flat = 96". 2. Holding a : 29a = 28", 156 = 7", flat = 83". 3. Holding b: 22a = 9", 246 = 21", flat = 88". 4. a and 6 held in turn as long as possible on each appear ance : 24a = 19", 156 = 14", flat = 85". 1 Pfliiger's Archiven, Bd. xl. 2 Ibid. 3 MIND, N.S., No. 48, vol. xii.
