SENSE OF DIRECTION : SINGLE VISION. 433 



inverted, and that this error is only corrected by experience. 

 The cases alluded-to in the last paragraph, however, satisfac- 

 torily prove this assumption to be incorrect ; since, although 

 the individuals saw everything upon the same plane, as in a 

 picture, the representation was erect from the first. The fact 

 now appears certainly to be, that we have an intuitive sense 

 of direction, which guides us in our appreciation of the actual 

 situations of objects and parts of objects ; so that, when a visual 

 impression is made upon any part of the retina, we see the 

 point from which the rays proceed, in the direction of a line 

 drawn from the affected spot of the retina through the common 

 centre (fig. 210, h) through which all the rays pass, this line 

 serving as a true guide to the actual place of the object. 



559. The same may be said of the cause of single vision, 

 that is, of our seeing but one object, although its picture is 

 double, being formed upon both retinse. In animals which, 

 like Man, look straight forwards, the field of vision of the 

 two eyes is nearly the same ; so that most of the objects seen 

 with one eye will be seen with the other also. The objects 

 at the right and left sides of the field of vision, however, are 

 seen with the right and left eyes singly ; yet we perceive no 

 difference in the aspect of these from that of the objects 

 towards which both our eyes are directed. It is evident, 

 then, that the pictures formed on the two retinae are blended, 

 as it were, by the mind, into a single perception. This seems 

 to be, in part at least, the effect of habit. "When the images 

 do not fall upon parts of the two retinas which are accustomed 

 to act together, double vision is the result. Thus if, when 

 looking steadily at an object, we press one of the eyeballs 

 sideways with the finger, the object is seen double. In the 

 same manner, if an affection of the nerves or muscles of one 

 eye (as happens temporarily in intoxication) prevent it from 

 being directed to the same point with its fellow, double vision 

 of all objects is the result. This, when it does not soon pass 

 away, is a not unfrequent symptom of serious disorder within 

 the brain. If it continue long enough, however, the indivi- 

 dual becomes accustomed to the double images, or rather 

 ceases to perceive that they are double, probably because the 

 mind becomes habituated to receive them, or rather perceives 

 only one of the impressions on the two parts of the retinae 

 which now act together. For if, after the double vision has 



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