SIGHT 237 



The retina is capable of judging the size of objects in two dimen- 

 sions only, i. e., in the plane perpendicular to the axis of vision. 

 The perception of distance is closely connected with the fact 

 that the size of the image formed upon the retina varies with 

 the distance. Besides, the distinctness with which objects 

 are seen influences the judgment of distance, because when 

 indistinctly seen, objects are supposed to be farther away. 

 Again, the judgment of distance is further aided by the sense 

 of effort required in accommodation, and also by the change of 

 position of an image on the retina when the eye is moved. 

 The latter depends upon the fact that the change in the position 

 of the image is inversely proportional to the distance of the 

 objects. Many circumstances affect the accuracy of the spatial 

 judgment of the retina. One of these is irradiation. All brightly 

 illuminated objects appear larger than others of the same size. 



Although there are two eyes, each of which furnishes an 

 impression, only one object is perceived. In abnormal positions 

 of the eye the two impressions can be made recognizable. 

 Ordinarily, therefore, the images of objects fall on corresponding 

 points of the retina. A point on the right side of one retina has 

 its corresponding point on the left side of the retina of the other 

 eye. When the images fall on corresponding points, they are 

 blended into one perception. Binocular vision affords a method 

 of judging the solidity of objects, since the image of any object 

 falling on one eye cannot be exactly like that which falls on the 

 other. Thus, the perceptive faculties can judge more correctly 

 of the form and distance of an object. 



From the laws of optics it is known that the image formed on 

 the retina is an inverted image of the object*. Yet it is perceived 

 in its upright position. This is the result of lifelong habit. A 

 baby sees an object; the next step is to touch it; by practice 

 the child finds out which is the top of the object through the 

 touch perception. Very speedily the brain learns to make the 

 correction. It is an act of mental and not physical origin. 

 Thus, objects which are projected upon the left of the retinal 

 surface look to be, as they really are, on the right of the body; 

 and so with all the directions. 



Clearness of vision depends upon the space between the cones 

 in the point of clearest vision, the fovea centralis. It has been 

 calculated that an object must subtend an arc of at least 60 



