JUDGMENTS OF SIZE AND DISTANCE 



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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. 



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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 

 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 

 a judgment of the size and distance of the former by comparison. Many 

 visual deceptions are based on these comparisons, a fact that is often taken 

 advantage of by photographers. It is also well known that people living in 

 a moist, hazy climate are utterly unable accurately to estimate distances 

 when suddenly transferred to a clear mountain climate. 



