182 



Prof. J. LeConte on some Phenomena 



perpendicular, looking straight forward at a point exactly at 

 the same height as the eyes, the visual plane therefore in the 

 primary position, and then slowly increasing or decreasing the 

 convergence of the optic axes so that the vertical lines of the two 

 images passed slowly over one another, it was plainly seen that 

 the verticals of the two images were not parallel, but crossed 

 each other at a small angle. 



This, my original diagram, however, is not well adapted to 

 experiments on this subject, for two reasons : (1) it is difficult to 

 distinguish the image of one eye from that of the other ; (2) it 

 is difficult to control perfectly the convergence of the eyes. When 

 the vertical lines approach each other, they, as it were, leap and 

 cling together as a single line, even though they really cross at 

 a considerable angle ; the really crossing lines, by a well-known 

 law of stereoscopic combination, being seen as a single line in- 

 clined to the visual plane. I therefore constructed a similar 

 diagram, one-half of which consisted of black lines on a white 

 ground, and the other half of white lines on a black ground. It 

 is convenient also to have two small circles, one on each half and 

 similarly situated (fig. 6). If I place such a diagram perfectly 



Fig. 6. 

































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perpendicularly before me, with the head perfectly erect and the 

 eyes at precisely the same height as the small circles, and then 

 stereoscopically combine the circles by crossing the eyes, I dis- 

 tinctly see the white and black lines, both vertical and hori- 

 zontal, crossing one another at small angle, as if the images 

 of both eyes had rotated on the visual line in opposite direc- 

 tions. This angle of crossing increases as the plane of the 

 diagram is brought nearer, and decreases as the diagram is 

 carried farther from the eyes. Or these different angles of cross- 

 ing may be obtained without moving the diagram or the head, 

 by converging the eyes more and more and causing the white 



