JUDGMENTS OF SIZE AND DISTANCE 



73 



When an object is placed so near the eyes that to view it the optic axes 

 must converge, a different perspective projection of it is seen by each eye, 

 these perspectives being more dissimilar as the convergence of the optic axes 

 becomes greater. Thus, if any figure of three dimensions, an outline cube, 

 for example, be held at a moderate distance before the eyes, and viewed with 

 each eye successively while the head is kept perfectly steady, A, figure 479, 

 will be the picture presented to the right eye, and B that seen by the left eye. 

 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. 



Judgments of 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 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 . 



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



Obtained. 



While the size 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 out- 

 lines 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 



