Chap, hi.] SIGHT. 961 



into the region of the blind spot there has been much discus- 

 sion ; but into this we cannot enter here. 



In ordinary vision, the existence of the blind spot is of little 

 moment. Since it lies outside the region of distinct vision, and 

 since moreover in each movement of the eye the image of a 

 fresh part of the external world falls upon it, the errors to 

 which it may lead are not serious even when we use one eye 

 only. The deficiency is further remedied by the use of two 

 eyes, since, the two blind spots being each on the nasal side, 

 the image of an object will not fall on both blind spots at the 

 same time. Other smaller or accidental imperfections in one 

 or both eyes are similarly remedied by the use of two eyes. 



§ 598. Turning now to the psychical processes connected 

 with the perception of particular objects, we find these to be 

 very complex. Some of them relate to the very formation of 

 the perception out of the sensations which the object excites, 

 and are often of such a kind that the perceptions which they 

 influence so distinctly fail to correspond with the actual objects 

 that the lack of correspondence can in many cases be demon- 

 strated : such erroneous perceptions are often spoken of as "illu- 

 sions." In other cases the psychical processes relate to a further 

 mental action by which we form judgments as to the features of 

 external objects. It is not easy however always to draw a line 

 between a ' visual judgment,' such as that involved in forming 

 a conclusion as to the size of an external object, and what may 

 be called a mere "modified perception," as when a line appears 

 to us shorter or longer than it really is. We may be content 

 here to treat them all together. 



The complexity of the psychical processes in question comes 

 about in various ways. On the one hand the characters of a 

 perception are determined not alone by the sensations which 

 actually give rise to it but also by the psychical conditions re- 

 maining as the effect of former like sensations. In the forma- 

 tion of perceptions and judgments, suggestions and associations 

 play their part; so that each perception, while it adds to, is 

 also in part the result of our 'experience.' A simple illustra- 

 tion of this is seen in some of the effects of colour. Blue 

 colours as we have seen predominate in a dim light such as 

 that of evening, of moonlight or of winter, whereas reds and 

 yellows are marked in a bright light such as that of full sun- 

 shine, or of a summer's day. Hence, when a landscape is 

 viewed through a yellow glass, the yellow hue suggests to the 

 mind bright sunlight and summer weather, although the actual 

 illumination which reaches the eye is diminished by the glass. 

 Conversely when the same landscape is viewed through a blue 

 glass the idea of moonlight or winter is suggested. And many 

 other instances might be given in which the appreciation of the 

 present is moulded by the experience of the past. 



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