of Binocular Vision. 



183 



and black vertical lines to pass successively over each other. This 

 is more easily done if there are several small circles on each half, 

 similarly situated but at different distances from each other. In 

 this diagram, the lines being of different colours do not stereo- 

 scopically combine easily — they do not cling together as in the 

 other case. Their approach toward, or recession from, one an- 

 other, and the angle which they make with one another, may be 

 marked with the utmost exactness. Nor is there any danger of 

 confounding the two images ; for since the eyes are crossed, we 

 know that the white lines belong to the right eye and the black 

 lines to the left eye; we can therefore determine the direction in 

 which each image rotates. I find always that the black lines or 



the image of the left eye rotates to the right *►— *, and the white 



lines or the image of the right eye rotates to the left *-«*. Now, 



as the image always moves in a direction contrary to the motion 

 of the eye (differing in this respect from spectra), this indicates 



a rotation of both eyes on the optic axes outward ^ *^-*. 



To test this question still further, I constructed another dia- 

 gram, with the horizontal lines continuous across, but the verticals 

 not perfectly vertical, the upper ends of those of the right half 

 inclining to the left, and those of the left half to the right, by 

 about 1° 20' (fig. 7) . On bringing the circles together I found 

 that at a certain distance of the diagram (but only at a certain 



Fig. 7. 









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distance, depending upon the interval between the circles) thever- 

 ticals coalesced perfectly ; the horizontals, however, as might have 

 been expected, still crossed at a small angle, and in the same direc- 

 tion as before ; viz. the whites or right-eye image thus ^^ , 

 and the blacks or left-eye image thus "~"\^, indicating in this 

 case also rotation of each eye outward. Beyond the proper dis- 



