294 RECENT PROGRESS OF THE THEORY OF VISION. 



Lastly, there are some very curious and interesting 

 phenomena seen when two pictures are put before the 

 two eyes at the same time which cannot be combined so 

 as to present the appearance of a single object. If, for 

 example, we look with one eye at a page of print, and 

 with the other at an engraving,^ there follows what is 

 called the ' rivalry ' of the two fields of vision. The two 

 images are not then seen at the same time, one covering 

 the other ; but at some points one prevails, and at others 

 the other. If they are equally distinct, the places where 

 one or the other appears usually change after a few 

 seconds. But if the engraving presents anywhere in the 

 field of vision a uniform white or black surface, then 

 the printed letters which occupy the same position in the 

 image presented to the other eye, will usually prevail 

 exclusively over the uniform surface of the engraving. In 

 spite, however, of what former observers have said to the 

 contrary, I maintain that it is possible for the observer at 

 any moment to control this rivalry by voluntary direction 

 of his attention. If he tries to read the printed sheet, the 

 letters remain visible, at least at the spot where for the 

 moment he is reading. If, on the contrary, he tries to 

 follow the outline and shadows of the engraving, then 

 these prevail. I find, moreover, that it is possible to fix 

 the attention upon a very feebly illuminated object, and 

 make it prevail over a much brighter one, which coincides 

 with it in the retinal image of the other eye. Thus, I 

 can follow the watermarks of a white piece of paper and 

 cease to see strongly-marked black figures in the other 

 field. Hence the retinal rivalry is not a trial of strength 

 between two sensations, but depends upon our fixing 



' The practised observer is able to do this without any apparatus, but 

 most persons will find it necessary to put the two objects in a stereoscope 

 or, at least, to hold a book, or a sheet of paper, or the hand in front of the 

 face, to serve for the partition in the stereoscope. — Tu. 



