THE EYE AS A SENSORY APPARATUS 287 



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 distance of the 

 object. The presence of an object of tolerably well-known height, 

 as a man, also assists in forming conceptions (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 conclu- 

 sions from the distribution of light and shade on an object, and 

 so that amount of knowledge as to the relative distance of dif- 

 ferent 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 ordi- 

 nary circumstances we see, however, not two objects 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 fovese. A point to the left 

 of this object forms its image on the inner (right) side of the left 

 eye and the outer (right) side of the right. An object vertically 

 above that looked at would form an image straight below the 

 fovea of each eye; an object to the left and above, its image to 

 the inner side and below in the left eye and to the outer side and 

 below in the right eye; and so on. We have learned that similar 

 simultaneous excitations of these corresponding points mean single 

 objects, and so interpret our sensations. When the eyes do not 

 work together, as in the muscular incoordination of one stage of 



