ESTIMATE OF DISTANCE BY VISION. 551 



ting with yellow spotted with blue. Thus, when we have two dissimilar 

 objects before the eyes, our attention cannot be kept upon either, to the 

 exclusion of the other, but is alternately and involuntarily directed, 

 either in part or completely, to one and the other. 



966. Our idea of the distance si near objects is evidently acquired 

 from experience ; and is suggested by the muscular sensations which 

 are produced by the contraction of the adductor muscles of the eyes. 

 When we direct our eyes towards a near object, a certain degree of con- 

 vergence takes place between their axes ; the degree increasing as the 

 distance between the object and the eyes diminishes ; and vice versd. 

 We instinctively interpret the sensations thus produced, in such a 

 manner as to be able to compare, with great accuracy, the relative dis- 

 tances of two objects that are not remote from the eyes. This intuition, 

 however, is evidently one of the acquired kind ; as may be seen by 

 watching the actions of an infant, or of a person who has recently be- 

 come possessed of Vision. When an object is held before the eyes, and 

 an attempt is made to grasp it, the manner in which the attempt is made 

 clearly shows, that there is no power of forming a precise idea of its 

 situation, such as that which exists in many of the lower animals from 

 their first entrance into the world ( 938). The impressions made upon 

 the eyes have to be corrected by those received through the touch, be- 

 fore the power of judging of distance is acquired. How much this 

 power depends upon the conjoint use of both eyes, is evident from the 

 difficulty with which any actions, that require an exact appreciation of 

 distance, are performed by those who have lost the sight of one eye, 

 until they have acquired new modes of judging of it. 



96T. In regard to remote objects, we have not the same guide ; since 

 the convergence of the eyes, in viewing them, is so slight that the axes 

 are virtually parallel. Our judgment of their distance is chiefly founded 

 upon their apparent size, if their actual size be known to us ; and also 

 upon the extent of ground, which we see to intervene between ourselves 

 and the object. But if we do not know their actual size, and are so 

 situated that we cannot estimate the intervening space, we form our 

 judgment chiefly from the greater or less distinctness of their colour and 

 outline. Hence our idea of it will be very much affected by varying 

 states of the atmosphere ; a slight haziness increasing the apparent dis- 

 tance ; whilst a peculiarly clear state of the air will seem to cause re- 

 mote objects to approach much more closely. This want of conver- 

 gence between the axes of the two eyes, has the further effect of causing 

 the pictures upon the two retinae to be nearly identical ; and conse- 

 quently the idea of projection is not so strongly excited ; nor are we able 

 to distinguish with the same certainty between a well-painted picture, 

 in which the lights and shades are preserved, and the objects themselves 

 in relief. 



968. Our notion of the size of an object is closely connected with 

 that of its distance. It is founded upon the dimensions of the picture 

 projected on the retina ; and the dimensions of this picture will vary, 

 according to the laws of optics ( 955) inversely as the distance, 

 being for example, twice as great when the object is viewed at the 

 distance of one foot as when it is carried to the distance of two feet. 

 When we know the relative distances of two objects, the estima- 



