CHAP. XVII.] DOUBLENESS OF THE ORGAN OF VISION. 59 



uniform distance, as in a picture ; and a species of education can 

 alone undeceive him. He learns, through touch, that all objects 

 are not equally near to him, and gradually familiarizes himself with 

 the changes in their apparent size, distinctness, and colours, pro- 

 duced by the movements of his whole body with regard to 

 them. The adjusting faculty is an additional source of correct 

 knowledge. 



Visual Perception of Motion in Objects. When an object 

 moves in a direct line, to or from the eye, its motion is inferred 

 chiefly by the change effected in the size of its image on the retina, 

 as when a locomotive engine, at full speed, approaches the ob- 

 server. When the object moves in an arc, of which the eye is the 

 centre, its motion is known, if the eye be fixed, simply by the move- 

 ment of its image across the retina. But most motions occurring 

 around us are known in both these ways. When, too, the atten- 

 tion is excited to the moving object, the eye is naturally moved in 

 concert with it, in order to keep its image near the axis of the 

 organ where vision is most perfect. Our appreciation of the direc- 

 tion and velocity of the motion is thus heightened by the exercise 

 of the musculary sense. 



J)oubleness of the Organ of Vision. The preceding account has 

 been almost confined to the phenomena of vision with a single eye ; 

 it remains to be explained how the doubleness of the organ affects 

 the sense. 



In some animals the eyes are so placed as to look in different 

 directions, and in these the images formed are, doubtless, inde- 

 pendently recognized by the animal, just as are those thrown on 

 different parts of the retina of a single eye in ourselves. But where 

 the eyes are both directed the same way, it is manifest that a double 

 image of each object must be received, and that the singleness of 

 the resulting sensation must depend either on our noticing only one 

 of these images, or else on our forming a single conception from 

 both conjointly. It is easy to prove that the latter is generally the 

 case, although we sometimes derive our information from the affec- 

 tion of only one eye. 



The eyes are moved in concert by the muscles attached to them, 

 so that their axes always converge towards the object to which 

 they are adjusted. The consequence of this is, that the correspond- 

 ing points of the two images are made to occupy corresponding 

 points of the two retinae, or very nearly so, and single vision is 

 produced. If the two images are unsymmetrically placed on the 

 retinae, as where the optic axes do not converge to the object, a 



