of Binocular Vision. 199 



with the conviction that his theory was untenable ; but, contrary 

 to my expectations, his views have been in a great measure con- 

 firmed. The difficulty with Meissner's method, and, in fact, with 

 all previous experimental methods, as already stated, is the in- 

 distinctness of objects at any considerable distance from the 

 point of sight in any direction. In Meissner's experiment with 

 the three points B', A, and B (fig. 11), in lowering B' or eleva- 

 ting B the indistinctness was so great that I could not tell with 

 certainty whether the images approached each other or not ; and 

 in his second experiment with the thread, the obstinate disposi- 

 tion on the part of the eye to see single by stereoscopic combi- 

 nation, even when the images cross, interferes seriously with the 

 certainty of the result. But in my experiments, by virtue of 

 the complete dissociation of the axial and focal adjustments, the 

 lines are seen perfectly clearly ; and by making them pass each 

 other slowly, their relation to each other may be observed with 

 great exactness. 



I will now state my own results in regard to the horopter. 



It is evident that if, in convergence, the eyes rotate on the 

 optic axes, as my experiments prove, then in this state of the 

 eyes the horopter cannot be a surface, but a line ; and this line 

 cannot be vertical, but inclined to the visual plane. Perhaps 

 this requires further explanation. If the eyes in a state of con- 

 vergence be fixed on a vertical line, then if the eyes rotate the 

 line must be doubled except at the point of sight. This doubling 

 is the result of horizontal displacement of the two images in op- 

 posite directions ; and therefore the two images may be brought 

 together by bringing the doubled portion of the vertical line 

 nearer or carrying it further away. This is done in inclining 

 the line as in fig. 11. But all points to the right and left of the 

 horopteric line are also doubled by rotation ; but this doubling 

 is the result of vertical displacement of the images : now vertical 

 displacement cannot be remedied by increasing or decreasing the 

 distance, because the eyes are separated horizontally. Therefore 

 no form of surface can satisfy the conditions of single vision 

 right and left of the horopteric line. The restriction of the 

 horopter to a straight line and the inclination of that line to 

 the visual plane are therefore necessary results of rotation on 

 the optic axes. But I have also proved this by direct experi- 

 ment: 



If two lines, one white on black and the other black on white 

 (fig. 14), be drawn at an angle of l\° with the vertical, and 

 therefore 2J° with each other, then by bringing my eyes so 

 near to them at any point a a (taking care that the median 

 line of sight shall be perpendicular to the plane of the lines) 

 that the visual lines without crossing shall meet beyond the 



