THE ORGANS OF SENSE. 139 



downwards, 65 to the nasal side, and 50* upwards. The field on the 

 nasal side is obstructed by the bridge of the nose, but the area of the 

 retina on which the obstructed rays would fall is insensitive. When 

 both eyes are in use, the fields of vision overlap, so that the combined 

 field extends to 65 on either side of the central point, that is, the 

 point which is focussed on the fovea. 



VISUAL JUDGMENTS. 



The flat picture formed on the retina gives rise to sensations of 

 light, colour, and shade. These sensations are conveyed to the associa- 

 tion areas of the brain, where the interpretation of the picture, or visual 

 judgment, takes place. If, for the sake of simplicity, the image of a 

 single object be considered, judgments are formed as to its position in 

 space, its distance from the eye, its size, form, and solidity. These 

 judgments are based partly on the visual sensation, partly on previous 

 experience derived not only from vision but also from the other senses. 

 A new-born infant is unable to interpret its visual sensations, but it 

 gradually learns to correlate these with tactile and other impressions, 

 until finally the visual sensation alone conveys impressions which at 

 first were dependent on other senses as well as that of sight. 



An object can be localised as the result of experience that an image 

 on a given part of the retina corresponds with a definite position in 

 space. The image on the retina is inverted, but the object is seen in 

 the upright position because the interpretation of the image is again 

 the result of experience. The retina, in fact, acquires "local sign." 



Experience also enters largely into judgments of size and distance, 

 and the latter are closely related to each other. If the size of an 

 object is known, its distance is estimated by the visual angle which it 

 subtends ; in other words, by the size of its image on the retina. On 

 the other hand, if the distance of an unfamiliar object is known, its 

 size can be judged in the same way. Other factors, however, enter into 

 judgments of distance. If the object is close at hand, the degree of 

 convergence of the eyes and of accommodation required to see it distinctly 

 are of assistance. The importance of convergence can be shown by 

 holding a pencil vertically about 40 cm. from the face and attempting 

 to touch it from one side with another pencil, first with one eye closed, 

 and then with both eyes open. If the object is distant, its outline is 

 more or less indistinct, owing to the fact that the atmosphere is never 

 perfectly transparent, and the degree of blurring varies with the distance. 

 The estimation of the size and distance of an unfamiliar object at an 

 unknown distance is assisted by comparison with other objects which 



