A TEXTBOOK OF PHYSIOLOGY 



and in the mechanism of accommodation. Objects at the near point 

 are seen clearly, and are judged to be near; those farther off less and 

 less distinctly, and are judged to be correspondingly distant. 



Judgment of size is the resultant of two factors the actual size 

 of the retinal image of the object, and the known or apparent distance 

 from the eye. Similarly, we estimate distance by the size of the 

 retinal image, and the known or presumed size of the object. An 

 object seen through field-glasses appears near at hand, but seen 

 through the wrong end appears far away. The after-image of an 

 object appears large or small, according as the surface on which it is 

 seen is near or far away. 



The relative transparency of the air also plays a part in such 

 judgments. A hazy atmosphere turns a small hill into a large 

 mountain, a very clear atmosphere makes objects appear much 

 nearer than usual. In addition to the size of familiar objects, 

 judgment is also aided by the disposition of lights and shades, accord- 

 ing to the position of the objects in relation to each other. 



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FIG. 3fi3. 



FIG. 364. ZOLLNER'S LINES. 

 The horizontal lines are parallel. 



FIG. 3(io. 



Mathematical perspective also plays a part. Parallel lines, like 

 those of a railway -track, converge more and more in proportion to 

 the distance away. We learn accurately to appreciate this per- 

 spective, but many optical illusions are associated with uncommon 

 combinations of lines. One of the best known is that shown in 

 Fig. 363. The lines of aa. bb, are of equal length. In Fig. 364, the 

 horizontal lines are parallel. 



To sum up, the visual judgment depends upon both monocular 

 and binocular elements. The chief monocular elements are muscle 

 sense, aerial perspective, mathematical perspective, the size of familiar 

 objects, and the disposition of lights and shades. Superimposed on 

 this is the binocular element, due. to the fact that external objects, 

 particularly those close at hand, are not seen exactly the same 

 in both eyes. This add? greatly to our judgment of depth 

 and solidity. Visual judgments are made in the brain. We have 

 no exact knowledge of the seat of such inferences. The judg- 

 ment may be influenced quite independent of any outside cause. 

 Thus, in Fig. 365, the vessel may be imagined to be with its open 



