418 PHYSIOLOGY CHAP. 



objects in the visual field. This is known as visual measurement 

 (Augenmass). 



Judgment of the size of an object is based essentially upon 

 the size of the retinal image. But as objects of very unequal 

 size may produce retinal images of the same size, when they are 

 at such a distance from the eye that they form the same visual 

 angle, it follows that the retinal image can only give us an 

 impression of the apparent size of objects, i.e. their size in relation 

 to their position. In order to estimate their real size, it is 

 necessary to form a judgment of their distance. When the 

 distance at which we regard an object is known by previous 

 experience, we can estimate its real from its apparent size ; when, 

 on the other hand, we know the real size of the object, we are 

 able from its apparent size to form a judgment of its distance. 



Under ordinary conditions our judgments of the relative size 

 of objects are largely aided by the eye-movements in addition to 

 previous experience, practice, and custom as we are able by this 

 means to compare the apparent size of the unknown object with 

 that of the known objects near it. 



Again, the approximate judgment of the distance of objects 

 is facilitated in uniocular vision by what is known as aerial 

 perspective, i.e. the degree of precision and clearness of their 

 retinal images and their relative light-intensity, both of which 

 diminish with distance. Binocular vision is further aided by the 

 sensation of effort in accommodation, particularly that due to 

 convergence, which increases with the nearness of the object, and 

 disappears on gazing into the far distance. 



Comparison of the two spatial magnitudes is generally made 

 by moving the eyes, so that the images of the two objects fall 

 successively on the same elements of the fovea. In these cases 

 the retina, according to Helmholtz, acts as a measuring calipers, 

 the points of which are successively applied to the ends of different 

 lines. The judgment of their equality or inequality varies with 

 the time that elapses between the two ocular measurements, and 

 with the training of the observer's eye. 



When the two spatial objects . do not lie parallel as in the 

 case of two straight lines forming an angle, or two surfaces with 

 different outlines a successive formation of the corresponding- 

 images at the same retinal points is not possible ; comparison by 

 the eye becomes difficult, and judgment as to their equality or 

 difference is more uncertain. 



Generally speaking, a comparative judgment of vertical 

 distances is far less accurate than that of horizontal differences. 

 Consequently, when we attempt to divide vertical lines exactly, we 

 make greater errors than in dividing horizontal lines. Vertical 

 distances, too, are generally estimated as greater than horizontal 

 distances. To many observers the superior angle of an equilateral 



