554 THE HUMAN BODY. 



image on the inner (right) side of the left eye and the outer 

 (right) side of the right. An object vertically above that 

 looked at would form an image straight below the yellow 

 spot of each eye; an object to the left and above, its image 

 to the inner side and below in the left eye and to the 

 outer side and below in the right eye; and so on. We 

 have learned that similar simultaneous excitations of these 

 corresponding points mean single objects, and so interpret 

 our sensations. This at least is the theory of the experi- 

 ential or empirical school of psychologists, though others be- 

 lieve we have a sort of intuition on the subject. When the 

 eyes do not work together, as in the muscular incoordination 

 of one stage of intoxication, then they are not turned so that 

 images of the same objects fall on corresponding retinal 

 points, and the person sees double. When a squint comes 

 on, as from paralysis of the external rectus of one eye, the 

 sufferer at first sees double for the same reason, but after a 

 time he makes new associations of corresponding retinal 

 points, and this is in favor of the empirical theory. 



When a given object is looked at, lines drawn from it 

 through the nodal points reach the fovea centralis in each 

 eye. Lines so drawn at the same time from a more distant 

 object diverge less and meet each retina on the inner side of 

 its fovea; but as above pointed out the corresponding points 

 for each retinal region on the inside of the left eye, are on 

 the outside of the right, and vice versa. Hence the more 

 distant object is seen double. So, also, is a nearer object, be- 

 cause the more diverging lines drawn from it through the 

 nodal points lie outside of the fovea in each eye. Most 

 people go through life unobservant of this fact; we only pay 

 attention to what we are looking at, and nearly always this 

 makes its images on the two foveae. That the fact is as 

 above stated may, however, be readily observed. Hold one 

 finger a short way from the face and the other a little farther 

 off; looking at one, observe the other without moving the 

 eyes; it will be seen double. For every given position of the 

 eyes there is a surface in space, all objects on which produce 

 images on corresponding points of the two retinas: this sur- 

 face is called the horopter for that position of the eyes: all 

 objects in it are seen single; all others in the visual field, 

 double. 



The Perception of Solidity. When a solid object is 



