CHAP. IIL] SIGHT. 1295 



into the region of the blind spot there has been much discussion ; 

 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. 



796. Turning now to the psychical processes connected with 

 the perception of particular objects, we find these to be very com- 

 plex. 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 cor- 

 respondence can in many cases be demonstrated : such erroneous 

 perceptions are often spoken of as " illusions." 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 percep- 

 tion," 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 formation 

 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 illustration 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 sunshine, 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. 



On the other hand the visual perception or visual judgment 



