228 PSYCHOLOOY. 



identity-theory, with its Cyclopean starting point for all 

 lines of direction, gives by itself no ground for the distance 

 on any line at which an object shall appear, and has to be 

 helped out in this respect by subsidiary hypotheses, which, 

 in the hands of Hering and others, have become so complex 

 as easily to fall a prey to critical attacks ; and it will soon 

 seem as if the laiv of identical seen directions by corresponding 

 points, although a simple forTnula for expressing concisely many 

 fundamental phenomena, is by no means an adequate account of 

 the ivhole matter of retinal perception. * 



The Projection- Theory. 



Does the theory of projection fare any better? This 

 theory admits that each eye sees the object in a different 

 direction from the other, along the line, namely, passing 

 from the object through the middle of the pupil to the 

 retina. A point directly fixated is thus seen on the optical 

 axes of both eyes. There is only one point, however, 

 which these two optical axes have in common, and that is 

 the point to which they converge. Everything directly 

 looked at is seen at this point, and is thus seen both single 

 and at its proper distance. It is easy to show the incom- 

 patibility of this theory with the theory of identity. Take 

 an objective point (like O in Fig. 50, when the star is looked 

 at) casting its images R' and L' on geometrically dissimilar 

 parts of the two retinse and affecting the outer half of each 

 eye. On the identity-theory it ought necessarily to appear 

 double, whilst on the projection-theory there is no reason 

 whatever why it should not appear single, provided only 

 it be located by the judgment on each line of visible direc- 



* Professor Joseph Le Conte, who believes strongly in the identity- 

 theory, has embodied the latter in a pair of laws of the relation between 

 positions seen single and double, near or far, on the one hand, and con- 

 vergences and retinal impressions, ou the other, which, though compli- 

 cated, seems to me by far the best descriptive formulation yet made of the 

 normal facts of vision. His account is easily accessible to the reader in his 

 volume 'Sight ' in the International Scientific Series, bk. ii. c. 3, so I say 

 no more about it now, except that it does not solve any of the difficulties 

 we are noting in the identity-theory, nor account for the other fluctuating 

 perceptions of which we go on to treat. 



