TEE EYE AS A SENSORY APPARATUS. 553 



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 unconsciously 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 de- 

 pends 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 

 distance of the object. The presence of an object of toler- 

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

 conceptions (by comparison) as to size; artists for this pur- 

 pose 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 flat 

 object with both eyes we get a similar retinal image in each. 

 Under ordinary circumstances we see, however, not two ob- 

 jects but one. In the habitual use of the eyes we move them 

 so that the images of the object looked at fall on the two 

 yellow spots. A point to the left of this object forms its 



