222 



PSYCHOLOGY. 



a simple law by which to connect the seen direction and 

 distance of objects with the retinal impressions they pro- 

 duce. Two principal theories have been held of this mat- 

 ter, the ' theory of identical points,' and the ' theory of pro- 

 jection,' — each incompatible with the other, and each 

 beyond certain limits becoming inconsistent with the 

 facts. 



The Theory of Identical Points. 



This theory starts from the truth that on both retinae 

 an impression on the upper half makes us perceive an ob- 

 ject as below, on the lower half as above, the horizon ; and 



Fig. 54. 



on the right half an object to the left, on the left half one 

 to the right, of tlie median line. Thus each quadrant of one 

 xetina corresponds as ?j whole to the similar quadrant of 



its character, or in its immediacy fully homogeneous and consubstantial 

 "with the feeling of literal distance between two seen termini ; in short, 

 that there are Uco sorts of optical .sensation, each inexplicably due to a 

 peculiar neural process. The neural process is easily discovered, in the 

 case of lateral extension or spreadoutness, to be the number of retinal 

 nerve-ends affected by the light ; in the case of protension or mere farness 

 it is more complicated and, as we have concluded, is still to seek. The 

 two sensible qualities unite in the primitive visual bigness. The measure- 

 ment of their various amounts against each other obeys the general laws 

 of all such measurements. We discover their equivalencies by means 

 of objects, apply the same units to both, and translate them into each other 

 so habitually that at last they get to seem to us even quite similar in kind. 

 This final appearance of homogeneity may perhaps be facilitated by the 

 fact that in binocular vision tvro points situated on the prolongation of the 

 optical axis of one of the eyes, so that the near one hides the far one, are by 

 the other eye seen laterally apart. Each eye has in fact a foreshortened 

 lateral view of the other's line of sight. In The London Times for Feb. 8, 

 1884, is an interesting letter by J. D. Dougal, who tries to explain by this 

 reason why two-eyed rifle-shootmg has such advantages over shooting with 

 one eye closed. 



