of Binocular Vision. 



195 



Let R (fig. 11) be an observer and I, II his two eyes, A the 

 point of sight, B an 



object beyond and B' Fig. 11. 



an object nearer than 

 the point of sight, but 

 all in the same line, 

 joining the root of the 

 nose and the point of 

 sight. Of course both 

 B and B' will be seen 

 double. If, now, while 

 the sight is still fixed 

 upon A, B be elevated, 

 its two images, ac- 

 cording to Meissner, 



will approach until at some point, W, they coalesce. If, on the 

 contrary, B be depressed, its images separate more and more. If, 

 now, B' be elevated, its images separate ; but if it be depressed, 

 its images approach and coalesce at 0. The line W A is, 

 therefore, the horopter or line of single vision. It is not at 

 right angles, but inclined to the plane of vision. Again, accord- 

 ing to Meissner, if instead of points we have vertical lines like 

 threads, WB and OB' (fig. 11), then B' will double at B', 

 the images being crossed, and they will approach one another and 



B '\ | B ' 

 meet at O, in other words, will appear thus, \ I ; while B W will 



also double at B but not cross (i. e. each image will have the 

 same name as the eye), and will be seen to converge and meet 



at W thus, /\. Lastly, if the vertical line pass through the 



point of sight A, the images will cross one another like an X. 



Meissner accounts for these phenomena by supposing that, in 

 converging the optic axis, the eyes rotate on the optic axis out- 

 ward, so that the vertical lines of demarcation C D (fig. 12) no 

 longer coincide perfectly with the vertical meridians A B, as 



Fig. 12. 



C A 



they do when the eyes 

 are in the primary direc- 

 tion (the axis parallel), 

 but cross them at a small 

 angle. In the primary 

 direction of the eye the 

 image of a vertical line, 

 according to Meissner, 

 falls on the vertical line 

 of demarcation C D in both eyes (for these lines then coincide 



BB 



DB 



