ESTIMATION OF SIZE AND OF DISTANCE. 877 



ESTIMATION OF SIZE AND OF DISTANCE. 



FALSE ESTIMATES OF SIZE AND DIRECTION. 



The judgment as to the size of an object depends, apart from all other 

 factors, upon the size of the retinal image; thus the moon is estimated 

 to be larger than a star. If, while looking at a distant landscape, a 

 fly suddenly crosses the field of vision, close to the eye, its image may 

 give the impression of a large bird, because of the relatively great size of 

 the retinal image. If the image is seen in diffusion-circles, 'on account of 

 a lack of accommodation, it may appear even larger. As, however, 

 objects widely unequal in size yield equally large retinal images, especi- 

 ally when their distance is such that they subtend the same visual angle 

 (Fig. 279), the estimate of the distance is of the greatest importance in 

 estimating the actual size of an object, as opposed to the apparent size, 

 which is determined by the visual angle alone. 



An estimate of the distance of an object is formed from the feeling of 

 accommodation, as a greater effort of accommodation is required for 

 accurate vision of a near object than for seeing distant objects. As, 

 however, of two unequally distant objects forming retinal images of the 

 same size the one that is nearer is found from experience to be the 

 smaller, the object for which greater accommodation is made is esti- 

 mated to be the smaller. 



This fact explains the following observation: beginners in microscopy usually 

 make a strong accommodative effort, while trained observers work without exer- 

 cising their accommodation. Hence, beginners estimate all microscopical images 

 as too small, and in drawing them, make them much too small. The following 

 experiment is a further proof. An after-image appears smaller if the eye accom- 

 modates for near vision, and much larger when the eye comes to rest. If a small 

 object is held as close as possible to the eye, an object behind it, which is seen indi- 

 rectly, appears to be smaller. 



A much more important means of estimating the size of an object, 

 by judging the distance, is given by the degree of convergence of the 

 visual axes. The position of an object that is seen binocularly is re- 

 ferred to the point where these axes cross. The angle that is formed by 

 the visual axes at their point of intersection is called the angle of con- 

 vergence of the visual axes. The larger, therefore, this angle of conver- 

 gence (with equal retinal images), the nearer the object is judged to be. 

 The nearer, however, the object, the smaller it may be in order to subtend 

 the same visual angle as a larger, more distant object. From this it may 

 be concluded that when objects have the same apparent size (equal 

 visual angles, or equal retinal images) that object is judged to be the 

 smallest that requires the greatest convergence of the visual axes during 

 binocular vision. The muscular sense of the ocular muscles gives the 

 information as to the amount of muscular effort that is necessary. 



The following experiments afford the proofs for this statement: i. The 

 tapestry- phenomenon described by Herm. Meyer. If a background with a reg- 

 ular, chessboard-pattern (tapestry or wickerwork) be looked at the squares appear 

 of a certain size when the visual axes are parallel. If now the eyes are converged 

 on an object closer to the eyes, so that the visual axes cross, the pattern appears 

 to move into the same plane as the point fixed, the crossed double images, dis- 

 placed laterally, coincide, and the pattern appears smaller. 2. Rollett looked 

 at an object through two thick glass plates; in one case the plates were so placed 

 (Fig. 308, II) that the apex of the angle between the two plates was turned toward 



