VISION 397 



in the brain. Pressure on button A or button B in the retina 

 causes bell A' or bell B' to ring in the brain, but it does not 

 follow that perception A" or perception B" will be heard in 

 the mind. It will be heard if this is the association established 

 by custom, since mind is the product of experience. But the 

 new sensation is creating precedent as well as being judged 

 by it. 



Point A in the right retina is associated by experience with 

 point a in the left, and point B with b. These are termed 

 corresponding points, because they are similarly stimulated in 

 binocular vision. The mind, therefore, judges that it receives 

 the same information from each pair of corresponding points. 

 The position of corresponding points will be understood if the 

 right retina is imagined as put inside the left, precautions being 

 taken to make the yellow spots coincide, and to avoid twisting 

 the retinal cups in taking them out of the eyeballs. Great 

 care is taken to maintain the points in correspondence during 

 the various movements of the two eyeballs. In addition to 

 the four recti muscles which move the eyeball upwards, down- 

 wards, to right and left, two oblique muscles give it the re- 

 quisite amount of rotation. We have learned to give the 

 same value to the impulses from two corresponding points. 

 But under changed conditions the correspondence changes. 

 When a squint develops in childhood, it follows one of two 

 courses ; either the obliquity of one of the eyeballs increases 

 until it looks towards the nose, and its images cease to inter- 

 fere with the images in the dominant eye they are ignored by 

 the mind or a fresh correspondence is established between 

 points in the oblique eye and points in the eye which looks 

 straight forward. If we are severely critical, we find, from a 

 study of the form of the eyeball, that it is impossible that the 

 same rods and cones should occupy corresponding points in 

 different positions of focus and with different degrees of con- 

 vergence of the eyeballs. To permit of this the retinal cups 

 would need to change in shape. But again mechanical cor- 

 respondence is of little consequence. In the light of experience 

 the mind judges that points correspond. When we are gazing 

 at a flat surface, the mind judges that corresponding points are 

 giving it similar information. It does not see a flower on a 

 wall-paper twice as bright or twice as red with two eyes as 



