156 J. LeConte on Binocular Vision. 
optic axes, if the amount of convergence be great, the horizon- 
tal lines of the two images are distinctly observed to cross each 
5. 
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other at small angle. After my attention was once directed t0 
this fact, I could see slight crossing of the horizontals for every 
degree of convergence, but the verticals seemed to coalesce per 
fectly. By placing, however, both the diagram and the head 
perfectly perpendicular, looking straight forward at a point 
exactly at the same height as the eyes, the visual plane there 
fore in the primary position, and then slowly increasing % 
decreasing the convergence of the optic axes, so that the ver 
tical lines of the two images passed slowly over one another, ™ 
was plainly seen that the verticals of the two images were n0t 
parallel, but crossed each other at small angle. 
This my original diagram, however, is not well adapted t 
experiments on this subject for two reasons: 1. It is difficult 
to distinguish the image of one eye from that of the othe! 
2, It is difficult to control perfectly the convergence of the 
eyes. When the vertical lines approach each other, they, 8 it 
single line inclined to the visual plane. I therefore co 
structed a similar diagram, one-half of which consisted ot 
black lines on a white ground and the other half of white lines 
ona black ground. It is convenient also to have two sma 
circles, one on each half and similarly situated (fig. 6). if : 
place such a diagram perfectly perpendicularly before me, W! 
the head perfectly erect and the eyes at precisely the game 
