226 psychology: 



eye's image will now appear to tlie left, the left eye's to the 

 right — the double images will be ' heteronymous.' 



The same reasoning and the same result ought to apply 

 where the object's place with respect to the direction of the 

 two optic axes is such as to make its images fall not on non- 

 similar retinal halves, but on non-similar parts of similar 

 halves. Here, of course, the directions of projection will 

 be less widely disparate than in the other case, and the 

 double images will appear to lie less widely apart. 



Careful experiments made by many observers according 

 to the so-called haploscopic method confirm this law, and 

 show that corresponding points, of single visual direction, exist 

 upon the two retinae. For the detail of these one must con- 

 sult the special treatises. 



Note now an important consequence. If Ave take a 

 stationary object and allow the eyes to vary their direction 

 and convergence, a purely geometrical study will show that 

 there will be some positions in which its two images impress 

 corresponding retinal points, but more in which they im- 

 press disparate points. The former constitute the so-called 

 horopter, and their discovery has been attended with great 

 mathematical difficulty. Objects or parts of objects which 

 lie in the eyes' horopter at any given time cannot appear 

 double. Objects lying out of the Jioropter icould seem, if the 

 theory of identical points icere strictly tru£, necessarily and al- 

 ways to appear double. 



Here comes the first great conflict of the identity-theory 

 with experience. Were the theory true, we ought all to 

 have an intuitive knowledge of the horopter as the line oi 

 distinctest vision. Objects placed elsewhere ought to seem, 

 if not actually double, at least blurred. And yet no living 

 man makes any such distinction between the parts of his 

 field of vision. To most of us the whole field appears single, 

 and it is only by rare accident or by sj)ecial education that 

 we ever catch a glimpse of a double image. In 1838, Wheat- 

 stone, in his truly classical memoir on binocular vision and 

 the stereoscope,* showed that the disparateness of the 



* This essay, published in the Philosophical Transactions, contains the 

 germ of almost all the methods applied since to the study of optical percep- 

 tion. It seems a pity that England, leading off so brilliantly the modem 



