224 PSYCHOLOGY. 



spot on the glass, and then by alternate shutting of them 

 let it be noted what objects behind the glass the spot 

 covers to the right and left eye respectively. Now with 

 both eyes open, both these objects and the spot will 

 appear in the same place, one or other of the three becom- 

 ing more distinct according to the fluctuations of retinal 

 attention.* 



Now what is the direction of this common place ? The 

 only way of defining the direction of an object is by point- 

 ing to it. Most people, if asked to look at an object over 

 the horizontal edge of a sheet of paper which conceals their 

 hand and arm, and then to point their finger at it (raising 

 the hand gradually so that at last a finger-tip will appear 

 above the sheet of paper), are found to place the finger not 

 between either eye and the object, but between the latter 

 and the root of the nose, and this whether both eyes or 

 either alone be used. Hering and Helmholtz express this 

 by saying that we judge of the direction of objects as they 

 would appear to an imaginary cyclopean eye, situated be- 

 tween our two real eyes, and with its optical axis bisecting 

 the angle of convergence of the latter. Our two retinae act, 

 according to Hering, as if they were superposed in the 

 place of this imaginary double-eye ; we see by the corre- 

 sponding points of each, situated far asunder as they really 

 are, just as we should see if they were superposed and could 

 both be excited together. 



The judgment of objective singleness and that of identi- 

 cal direction seem to hang necessarily together. And that 

 of identical direction seems to carry with it the necessity of 

 a common origin, between the eyes or elsewhere, from which 

 all the directions felt may seem to be estimated. This is 

 why the cyclopean eye is really a fundamental part of the 

 formulation of the theory of identical retinal points, and 

 why Hering, the greatest champion of this theory, lays so 

 much stress upon it. 



It is an immediate consequence of the laiv of identical prO' 



* These two observations prove the law of identical direction only for 

 objects which excite the foveas or lie in the line of direct looking. Ob- 

 servers skilled in indirect vision can, however, more or less easily verify the 

 law for outlying retinal points. 



