Page:Mind (New Series) Volume 15.djvu/357

 PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 343 continuance of one phase seems to involve a rapidly increas- ing fatigue of the corresponding brain-tract which very soon turns the balance in favour of the unfatigued rival tract, when the latter enjoys a prolonged period of dominance during which the fatigue of the other tract is passing away. This effect may be observed in case of figure 2, by voluntarily maintaining an alternation of the appearance of two of the rectilinear groupings in either the figure itself or the after- image of the figure. I find that it is not possible completely to exclude the third rectilinear grouping for any long period. After a few alternations of the two oblique groupings volun- tarily determined the horizontal grouping forces itself mo- mentarily into consciousness, and if the voluntary alternation of the two oblique groupings is made to continue, excluding the horizontal grouping save for these occasional brief appear- ances for thirty seconds or more, then, when the voluntary efforts are relaxed and a passive attitude is assumed, the horizontal grouping distinctly predominates over the other two groupings for some few seconds, until the balance is once more restored. I have found it impossible to obtain any striking evidence of fatigue in the perception of any of the ambiguous figures described in the earlier articles of this series, because I find it impossible to secure the dominance of any one mode of perception for more than a brief period of some few seconds. But there is a familiar object of ambiguous perception which enables us to observe the effects of fatigue in a manner exactly parallel to the observation of fatigue in the 'case of rivalry of two colour- fields. If one looks at the sails of a windmill in motion, standing at a distance of fifty yards or more and at a point almost, but not quite, in the plane of their motion, so that the sails are seen very obliquely, it is often difficult to be sure in which direction the sails are rotating, because as one continues to gaze the sails seem suddenly from time to time to reverse their motion and at the same moment to change the plane of their motion. Let A, B, C, D, in figure 11, represent four sails of a windmill rotating in a plane which cuts the plane of the paper at an angle of about 75. Then at one moment the extremities of A and B will seem to be rotating towards a point a little to the left of the observer while C and D move away and towards the right, the whole wheel seeming to lie in a plane cut by the line of vision at an acute angle opening towards a point on the observer's left hand ; a few seconds later the plane of rotation seems to jump through an angle of 30, to such a position that it is cut by the line of vision