of Binocular Vision. 



97 



Fig. 13. 



with it of every plane passing 

 through the optic centres 0, 0' 

 upward or downward as OBO' 

 and B' 0' is also a circle. It 

 is evident that as these circles 

 would increase in size upward 

 and downward, the horopter, 

 according to Claparede, must 

 be a surface of singular and 

 complex form. 



Finally, Helmholtz arrives at 

 results entirely different from 

 those of all previous observers. 

 He sums up his conclusions as 

 follows : — 



" When the point of conver- 

 gence is situated in the middle 

 [vertical] plane of the head, 

 the horopter is composed of a 

 straight line drawn through the 

 point of convergence [direction 

 not stated, but evidently not at 

 right angles to the visual plane, 

 for see below the sentence marked a ], and a conic section passing 

 through the optic centres and intersecting the straight line." 



" When the point of convergence is in the plane which con- 

 tains the primary visual lines [primary visual plane], the horop- 

 ter is a circle going through that point and the optic centres 

 [Prevost's horopteric circle] and a straight line intersecting the 

 circle [where and in what direction not stated] ." 



"When the point of convergence is situated as well in the 

 middle plane of the head as in the primary visual plane, the ho- 

 ropter is the circle just described [Prevost's horopteric circle] 

 and a straight line going through that point [direction not 

 stated] ." 



" There is but one case in which the horopter is really a plane, 

 viz. when the point of convergence is in the middle plane of the 

 head and at an infinite distance. Then the horopter is a plane 

 parallel to the visual plane and beneath it, at a certain distance 

 which depends upon the angle between the really and apparently 

 vertical meridians, but which is nearly as great as the distance 

 of the feet of the observer from his eyes when he is standing. 

 Therefore, when we look at a point on the horizon, the horopter 

 is the ground on which we stand. a When we look at the ground 

 on which we stand at any point equally distant from both eyes, 

 the horopter is not a plane ; but the straight line which is one of 



