576 PHYSIOLOGY 



the primary one. This causes impulses to travel to the visual centres un- 

 affected at first by the effects of other images, but these tend to accumulate 

 more and more until the primary image is overcome, and that of the other 

 eye put in its place. But this image in time suffers in the same way, so that 

 the images alternate. The fact that a new image can suppress an old is due 

 to the absence of after images in the first and their presence in the second. 

 The predominance of a bright image can be explained by the longer time 

 required for the after image to reach such a level as shall cause suppression. 

 The preference for an image with contours would seem to be due to the greater 

 ease with which the after image may be removed by small deflections of 

 gaze. 



