SEC. 8. ON SOME FEATURES OF VISUAL SENSATIONS 

 ESPECIALLY IN RELATION TO VISUAL PERCEPTIONS. 



§ 580. 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 localization of sensations, to speak of the cir- 

 cumstances 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 Avhich 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 localization ; but these are coordinated 

 in our consciousness 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 sensations corresponding to what we call 

 the objects of nature. And we have now to turn our attention 

 to certain faces concerning vision which become especially prom- 

 inent when we are the subject not of an isolated single visual 

 sensation but of complex groups of simultaneous visual sensa- 

 tions. 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 (§ 494), the ' field of 

 vision,' or ' visual field.' 



§ 581. 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, sud- 

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