SEC. 12. ON SOME FEATURES OF VISUAL 

 PERCEPTIONS AND ON VISUAL JUDGMENTS. 



795. We may now turn our attention to some of those 

 differences Between the features of external objects and our per- 

 ception of them which are more distinctly of psychological origin ; 

 but since the purpose of this work is physiological and not 

 psychological we must be content to treat them very briefly. 



Taking first of all the general features of the field of vision, we 

 find psychical processes entering largely even into these. As we 

 have incidentally seen, the sensations which an object excites are 

 very different according as the object is in the central or in the 

 peripheral region of the field of sight. Two parts of the object 

 sufficiently far apart to give rise to two sensations in the former 

 case may give rise to one sensation only in the latter case ; and 

 the colour sensations excited by the same object may be widely 

 different in the two cases. If we picture to ourselves the group of 

 sensations excited by the image of an object, such as a flower, when 

 the image falls on the fovea, and compare that group with the group 

 of sensations excited by the same flower when the image of it falls 

 on the periphery of the retina, supposing the comparison to be made 

 before the sensations are moulded into psychical perceptions, the 

 two groups would appear to belong to very unlike objects. More- 

 over, when we use both eyes, the images of some of the objects 

 in the field of sight are falling on both retinas, while others are 

 falling on one retina only, and of those which fall on both retinas, 

 some lie on corresponding points, so that the sensations of the 

 two eyes are blended, while others, not lying in the horopter, 

 give rise to sensations in one eye different from those in the 

 other. Could we become aware of the crude sensations which go 

 to make up our field of vision, they would appear as a heteroge- 

 neous medley. But in the field of vision of which we are actually 

 aware, that in which the crude sensations have by psychical 

 operations been moulded into perceptions, we do not recognize the 

 various discrepancies of which we are speaking ; the field of vision 

 .is homogeneous. When we look at a landscape we are not aware 



