THE STEREOSCOPE AND VISUAL INTERPRETATION 1 87 



finally everything would have seemed as in normal vision. That is, 

 instead of being inverted would seem in its normal position. 



In Fig. 11 is illustrated why, when we hold up two fingers in line 

 before us and look at the nearer one, the farther one seems double. 

 The farther one B is seen in the same transverse plane (horopter), and 

 seems at B' and B'. Fig. 12 shows the nearer finger double when the 

 farther one A is looked at. The horopter has shifted its position, and 

 each eye projects its source of stimulus forward upon it. To get the 

 tri-dimensional effects in looking at an object, or objects which do not 

 lie in the same transverse plane the eye makes rapid excursions over 

 the different parts, and as a result of the shifting varies the amount of 

 convergence and gets the full feeling of the relative depth positions of 

 the different parts. 



That the convergence and accommodation strains are the cues we 

 have in judging size and distance of objects can be verified by such 

 arrangements as Figs. 5 to 8, where one factor can be kept constant 

 while the other varies and its influence is noted. The size of the retinal 

 image can be kept the same while convergence shifts, or convergence 

 can be kept the same while the size of the retinal image shifts. 



All these experiments show that the cues by which we judge position, 

 size, and depth are seized upon by experience, and in cases where we 

 are deceived these cues are not valid, as in the stereoscope, and the 

 result is an illusion. 



