258 RECENT PROGRESS OF THE THEORY OF VISION. 



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 mo- 

 ment 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 or failing 

 to fix the attention. Indeed there is scarcely any phenomenon 

 so well fitted for the study of the causes which are capable of 

 determining the attention. It is not enough to form the 

 conscious intention of seeing first with one eye and then with 

 the other ; we must form as clear a notion as possible of what 

 we expect to see. Then it will actually appear. If, on the 

 other hand, we leave the mind at liberty without a fixed inten- 

 tion to observe a definite object, that alternation between the 

 two pictures ensues which is called retinal rivalry. In this 

 case, we find that, as a rule, bright and strongly marked objects 

 in one field of vision prevail over those which are darker and 

 less distinct in the other, either completely or at least for a time. 

 We may vary this experiment by using a pair of spectacles 

 with different coloured glasses. We shall then find, on looking 

 at the same objects with both eyes at once, that there ensues a 

 similar rivalry between the two colours. Everything appears 

 spotted over first with one and then with .the other. After a 

 time, however, the vividness of both colours becomes weakened, 



