SEC. 10. ON SOME FEATURES OF VISUAL SENSATIONS 

 ESPECIALLY IN RELATION TO VISUAL PERCEPTIONS. 



778. In our previous study of visual sensations we dealt 

 chiefly with the more simple and fundamental characters of 

 sensations ; we considered each sensation by itself and discussed 

 its features irrespective of the influence of other sensations excited 

 at the same time, except so far as it became necessary, in treating 

 of the localisation of sensations, to speak of the circumstances 

 which determined the fusing of two neighbouring sensations into 

 one. It very rarely occurs however that any object or event in 

 the external world gives rise to a simple sensation such as those 

 on which we have dwelt ; each part of the external world, each 

 external object such as a tree, is the source of many distinct 

 sensations differing from each other in intensity and other 

 characters. In looking at a tree we are conscious of many 

 sensations of different colours and intensities, each having a 

 definite localisation ; but these are coordinated in our conscious- 

 ness into a whole and we say we " see a tree." The effect which 

 the whole visible world has upon us is not that of a multitude of 

 single sensations each separate from and independent of the other, 

 but of a smaller though still large number of groups of sensa- 

 tions corresponding to what we call the objects of nature. And 

 we have now to turn our attention to certain facts concerning 

 vision which become especially prominent when we are the subject 

 not of an isolate single visual sensation but of complex groups of 

 simultaneous visual sensations. The sum of visual sensations and 

 groups of sensations which are excited by images falling on the 

 retina at any one time, we call, as we have already said ( 666), 

 the ' field of vision,' or ' visual field.' 



779. Before we proceed any further however it will be well 

 to call to mind that in studying vision as we are now doing by 

 means of an appeal to our own consciousness, we are deserting the 

 ordinary methods of physiology for the methods which are more 

 strictly speaking those of psychology. Or rather in our study of 

 vision we are using both methods, suddenly turning from one to 

 the other. We are using ordinary physiological methods when 



