462 SPECIAL PHYSIOLOGY. 



wards, sideways, or obliquely, i. e., of their apparent position, can be 

 formed by one eye alone ; but for the appreciation of their real position, 

 it is necessary to be able to judge of their distance behind one another, 

 or their depth in space. In the case of near objects, we, to a certain 

 degree, estimate their depth in space, in the same manner as we judge 

 of their solidity, by means of a stereoscopic action of both eyes. In 

 this way, we become conscious of the intervention of a certain depth of 

 space between near objects ; but in order to estimate more accurately, 

 their relative distance behind each other, it is supposed that we are 

 able, by looking from one object to another, to obtain important data 

 from the muscular movements by which the two eyes are made to con- 

 verge, and probably also from the changes which ensue in their focal 

 adjustment; this is especially the case in regard to near objects, in 

 looking at which, these actions require a greater effort. The knowl- 

 edge thus obtained is not a mere sensation, but a mental notion, founded 

 on the judgment, and helped by experience. Infants are entirely un- 

 able to judge of distances. 



In estimating the real positions of distant objects, beyond the range 

 of the binocular parallax, however, there is no convergence of the eyes, 

 and frequently no focal adjustment is required ; here, we are guided 

 by movements of the head, by the effects of different distances on the 

 apparent size of objects, such as are explained by linear and plane 

 perspective ; by the changes in the distinctness of outline, shadow, de- 

 tail, and color, due to aerial perspective ; by the manner in which ob- 

 jects intercept, or are intercepted by each other ; by comparison with 

 known objects, and, if these are in motion, by their absolute and rela- 

 tive rates of movement. It is by the perception of objects in space, 

 that we obtain, through the eye, as well as through touch, materials 

 for the mental conception of space itself. 



An inexact estimate of relative distances, leads to certain errors as 

 regards the sizes of objects. Thus, the idea that the sky at the hor- 

 izon, is more distant than the sky directly above us, an idea dependent 

 on the number of known and near objects between us and the horizon, 

 leads to an erroneous estimate of the size of the rising or setting sun 

 or moon, which thus appear to us larger than when they are seen above 

 our heads. According to another view, the image of the sun or moon, 

 seen near the horizon, is compared, by the mind, with the smaller 

 images of intervening distant objects known to be of great size, and so 

 those luminaries are interpreted to be of larger dimensions than when 

 seen in the open sky. Through a telescope, these bodies appear nearer 

 than they are, and hence the magnifying effect is underrated by the 

 observer. Convergence of the eyes, also influences our judgment of 

 distance ; for, and this is remarkable, a fixed object appears smaller, 

 when the optic axes are made to converge in front of it. In railway 

 travelling, objects passed near the side of the road, appear larger and 

 nearer, than they are, because their rapid movement past each other, 

 suggests, as w r e shall see, large angular motion, and therefore unreal 

 proximity. Objects seen through a fog, seem larger than they are, 

 because their indistinctness suggests the notion of great distance. 



We are only able directly to perceive the movements of objects, 



