668 



THE SENSES 



Wheatstone has shown that on this circumstance depends in a great measure 

 our conviction of the solidity of an object, or of its projection in relief. If 

 different perspective drawings of a solid body, one representing the image 

 seen by the right eye, the other that seen by the left, for example, the drawing 

 of a cube, A, B, figure 479, be presented to corresponding parts of the two 

 retinae, as may readily be done by means of the stereoscope, the mind will 

 perceive not merely a single representation of the object, but a body pro- 

 jecting in relief, the exact counterpart of that from which the drawings were 

 made. 



Size and Distance. The estimation of the size of an object and its distance 

 away from the observer is based in part upon the visual image and in part 

 upon judgments due to past experience. The two elements are inseparable 

 and mutually dependent. Thus, a lofty mountain many miles away may 

 subtend the same visual angle as a small hill near at hand. While the size 



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FIG. 479. Diagrams to Illustrate how a Judgment of a Figure of Three Dimensions is Obtained. 



and shape of the two images may be identical, yet the image of the hill near 

 at hand is more distinct, its details are perceived, and its outlines are sharper 

 than in the image of the mountain. If the atmosphere be charged with 

 moisture or with dust, the image of the mountain will be still more indistinct 

 and dim. From previous experiences we have learned that the dimness and 

 indistinctness of the one and the definiteness of the other are associated with 

 distance. 



If two objects are very near at hand then there will be a difference in 

 the convergence of the two eyes in binocular vision. It is now well known 

 that the ocular muscles are possessed of a very delicate muscle sense. This 

 muscle sense leaves the impression which enables us to judge that the one 

 object is nearer and the other farther. In the common and familiar objects 

 about us we have from long experience and intimate contact learned their 

 actual size and the character of the retinal image formed at definite, but known 

 distances. When such an object forms an image of the common size and 

 usual distinctness on the retina, the judgment as to its distance is quickly 

 made. 



In the case of unknown objects which are associated with known ob- 

 jects, the judgment of the size and distance, of the latter is used in forming 



