SEEING DOUBLE. 



parallel. Yet as each one sees the object in 

 its line of direct vision, the brain receives the 

 sensation of one object only. The images 

 make two impressions, one on each retina, 

 but only one image is consciously perceived. 

 This state of things is shown in Fig. 47, 

 where R represents the right eye, and L the 

 left in a plan view seen from above. Both are 

 directed to the primary object P, and though 

 they each receive a separate impression of it, 



Fi<3. 47 THE BRAIN'S INTERPRETATION OF THE EYES* VISION. 



as each receives it in the central line of vision 

 the mechanism of sight-sensation is such that 

 the brain only sees one object. 



A secondary object, Sj, at the same dis- 

 tance from the face, is on the same side of the 

 central vision line for each eye, and in the 

 same part of the field of vision of each eye ; 

 so this also, making two similar impressions 

 on the two eyes which are looking at P, is 

 seen by the brain as one object. 



But if there be another secondary object 

 nearer to the face, in the position S 2 , this 

 appears on the right-hand side of the vision 



221 



