252 



THE HUMAN BODY 



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 correspond- 

 ing retinal points. 



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, because 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 



a 



FIG. 97. 



on the two fovese. 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 

 surface is called the horopter for that position of the eyes: all ob- 

 jects in it are seen single; all others in the visual field, double. 



The Perception of Solidity. When a solid object is looked at the 

 two retinal images are different. If a truncated pyramid be held 

 in front of one eye its image will be that represented at P, Fig. 97. 

 If, however, it be held midway between the eyes, and looked at 

 with both, then the left-eye image will be that in the middle of the 

 figure, and the right-eye image that to the right. The small sur- 



