PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 341 excitation is high. This is a special case of the reciprocal in- hibition that we have to study in the next section. Assuming this relation to obtain between the two tracts, it remains to explain the alternate predominance of either tract in turn. That this is due to fatigue of the tracts rapidly induced during activity and rapidly passing away during rest seems to be proved by the following considerations : (1) If one tract, say R, is more strongly excited than the other by rendering the intensity of the red light falling into the right eye greater than that of the blue light falling into the left eye, the periods of dominance of red become longer than those of blue, but do not altogether exclude them. (2) If the two tracts are equally intensely excited the periods bf dominance of red and of blue are approximately of equal duration, unless the domi- nance of one is favoured in some way at the expense of the other. These periods in my own case are seldom of more than two seconds' duration and usually are shorter, and in this respect individuals differ markedly. The predominance of one colour may be prolonged a little by a voluntary con- centration of attention upon the corresponding field, but much more effectively by the appearance of movements in one field, movements of objects in the visual field being, as is well known, very effective in drawing the attention. In order to favour the dominance of one colour a four- rayed wheel is made to rotate in the field of one eye (the right eye) so that its spokes pass in rapid succession across the coloured field (red). This causes the red field to dominate completely for some seconds, after which the blue field begins to make momentary and partial appearances which increase in extent and duration as the observation is con- tinued, without however equalling in duration the periods of dominance of the red field. After one minute the wheel is removed, so that the red field is no longer favoured by its movements. The blue field then predominates, totally ex-
- luding the red field for many seconds (in some cases as
tany as 15 seconds), after which the rapid alternation of the o fields begins again. In this case the artificially prolonged
- tivity of the red-exciting tract induces in it a degree of
'atigue which enables the rival blue-exciting tract to inhibit its activity for a considerable period as soon as the move- ments in the red field no longer favour its predominance. (3) If the left eye is kept closed for some seconds while the right eye is exposed to its red field, and the left eye is then exposed to the equally bright (or to a less bright) blue field, I+he blue invariably predominates at once over the red, totally xcluding it for some seconds, i.e. the rested tract inhibits i
