SEC. 10. ON SOME FEATURES OF 

 VISUAL PERCEPTIONS AND ON VISUAL JUDGMENTS. 



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

 differences between the features of external objects and our 

 perception 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, sup- 

 posing the comparison to be made before the sensations are 

 moulded into psychical perceptions, the two groups would 

 appear to belong to very unlike objects. Moreover, 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 heterogeneous med- 

 ley. But in the field of vision of which we are actually aware, 

 that in which the crude sensations have by psychical oper.itions 

 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 

 that objects on the far left or far right hand are producing 



959 



