532 THE HUMAN BODY. 



flat object with both eyes wo get a similar retinal image in 

 each. Under ordinary circumstances we see, however, not 

 two objects but one. In the habitual use of the eyes we 

 move them so that the images of the object looked at fall 

 on the two yellow spots. A point to the left of this object 

 forms its image on the inner (right) side of the left eye 

 and the outer (right) side of the right. An object verti- 

 cally 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 excita- 

 tions of these corresponding points mean single objects, and 

 so interpret our sensations. This at least is the theory of 

 the experiential or empirical school of psychologists, though 

 others believe 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 cor- 

 responding retinal points, and the person sees double. 

 When a squint comes on, as from paralysis of the external 

 rectns of one eye, the sufferer at first sees double for the 

 same reason. 



If a given object is looked at lines drawn from it through 

 the nodal points reach the fovea central is 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 f ovese. 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 



