VISUAL PERCEPTION OF SIZE. 531 



we seem to see distance; it seems at first thought as definite 

 a sensation as color. That it is not is shown by cases 

 of persons born blind, who have had sight restored later in 

 life by surgical operations. Such persons have at first no 

 visual perceptions of distance: all objects seem spread out 

 on a flat surface in contact with the eyes, and they only 

 learn gradually to interpret their sensations so as to form 

 judgments about distances, as the rest of us did uncon- 

 sciously in childhood before we thought about such things, 



The Visual Perception of Size. The dimensions of the 

 retinal image determine primarily the sensations on which 

 conclusions as to size are based; and the larger the visual 

 angle the larger the retinal image: since the visual angle 

 depends on the distance of an object the correct perception 

 of size depends largely upon a correct perception of distance; 

 having formed a judgment, conscious or unconscious, as to 

 that, we conclude as to size from the extent of the retinal 

 region affected. Most people have been surprised now and 

 then to find that what appeared a large bird in the clouds was 

 only a small insect close to the eye; the large apparent size 

 being due to the previous incorrect judgment as to the dis- 

 tance of the object. The presence of an object of tolerably 

 well-known height, as a man, also assists in forming con- 

 ceptions (by comparison)as to size; artists for this purpose 

 frequently introduce human figures to assist in giving an 

 idea of the size of other objects represented. 



The Visual Perception of a Third Dimension of Space. 

 This is very imperfect with one eye; still we can thus arrive 

 at conclusions from the distribution of light and shade on 

 an object, and from that amount of knowledge as to the 

 relative distance of different points which is attainable 

 monocularly; the different visual angles under which 

 objects are seen also assist us in concluding that objects 

 -are farther and nearer; and so are not spread out on a plane 

 before the eye, but occupy depth also. Painters depend 

 mainly on devices of these kinds for representing solid 

 bodies, and objects spread over the visual field in the third 

 dimension of space. 



Single Vision with Two Eyes. When we look at a 



