874 THE SENSE OF SlGHT 



(6) If the right eye is made to receive a figure composed of horizontal lines and 

 the left eye one composed of vertical lines, the result is a struggle between these 

 impressions. Sometimes the former and sometimes the latter gains the upper 

 hand. 



The sum total of the corresponding points in the binocular field 

 of vision producing a single impression, forms the so-called horopter.^ 

 It differs with every new position of the eyes and may be a 

 straight or a curved line, a plane or a curved surface. 



Visual Judgments. — It has been pointed out repeatedly that our 

 visual impressions in consciousness are the result of experience. Like 

 other sense-organs, our eyes are the mere recipients of stimuli which 

 are moulded into concepts within the cortical realm of vision and these 

 concepts are acquired gradually by constant repetition. To begin 

 with, the infant receives these stimuli without being able to interpret 

 them, because its association areas are as yet incompletely developed. 

 In the course of a few months, however, it begins to form simple con- 

 cepts. It follows the course of a moving light with its eyes and also 

 responds in other ways to stimuli of this kind. A few months later 

 it has learned to associate objects in space in their proper relations, 

 irrespective of the fact that the images upon the rods and cones are 

 inverted. 



The adult being, therefore, is guided by the associations thus grad- 

 ually acquired and does not concern himself with the manner in which 

 the images are formed in the sense-organ, i.e., the fact that the objects 

 in space are presented to him inverted he has overcome by experience 

 and proper psychic interpretation. The visual concepts thus formed 

 are gradually brought into relation with concepts of a different nature, 

 so that, for example, the visual concept of a certain object is 

 subsequently correlated with its taste and odor or with the sound 

 which it may produce. A similar expansion of our concepts enables 

 us to form judgments not only regarding the general outline of objects 

 but also regarding their depth or solidity. Although the most perfect 

 results are obtained by binocular vision, one eye is quite sufficient to 

 obtain correct relationships in space, and to rate objects in accordance 

 with their height, breadth and depth. Obviously, the judgment of 

 the size of an object is cniefly dependent upon the size of its imag3 

 upon the retina and hence, upon the angles which its luminous rays 

 form with the visual angles of the eyes. This requires accommodation 

 by the ciliary mechanism as well as variations in the position of the 

 visual axes of the eyes. Since an object at a distance of 5 m. emits 

 a large number of parallel rays, practically no accommodation is 

 required. Beyond this point, we must rely chiefly upon the visual 

 angle, while within this distance, this factor is augmented by the degree 

 of contraction of the ciliary muscle as well as by that of the orbital 

 muscles used in converging the visual axes. Lastly, our associations 

 are based upon certain outside factors, for example, the character 



1 Johannes Miiller, Beitr. zur vergl. Physiol, der Sinnesorgane, 1826. 



