190 Prof. J. LeConte on some Phenomena 



the optic centres, or about a quarter of an inch from the root of 

 the nose. 



I attribute these phenomena to a slight distortion of the 

 ocular globe under the action of the oblique muscles — a distor- 

 tion which increases with the degree of optic convergence. We 

 will refer to this again. 



In all the experiments described above, the greatest care was 

 taken that the visual plane should be in the primary direction, 

 i. e. at right angles to the line of the face, and especially that 

 the median line of sight should be at right angles to the 

 plane of the diagram. I now wished to try the effect of turn- 

 ing the visual plane upward and downward. Meissuer, from 

 his experiments on the position of the horopter, had arrived 

 at the conclusion that the rotation of the eye was zero, what- 

 ever the degree of convergence, when the visual plane was in- 

 clined downward 45° from its primary position, and that the 

 rotation increased as the plane was elevated toward the eye- 

 brows. I was anxious to test this result. The plane of the 

 diagram still remaining vertical, T now turned the face upward 

 (taking care, however, that the eyes should still be on an exact 

 level with the circles of the diagram) until the eyes looked in 

 the direction of the point of the nose. In this position, on ste- 

 reoscopically combining the small circles, the lines, both vertical 

 and horizontal, in all cases maintained their true position : i. e. 

 in the diagram with parallel lines (fig. 6), the coincidence of all 

 the lines was perfect; in the diagram with inclined verticals 

 (fig. 7), the horizontals coalesced perfectly and the verticals 

 crossed at their true angle of inclination ; while in the diagram 

 with the verticals and horizontals both inclined (fig. 8), both 

 the verticals and horizontals crossed at their true angle of in- 

 clination. I tried the same experiment for various distances, 

 and therefore various degrees of optic convergence, but always 

 with the same result. There is, therefore, no rotation of my 

 eyes when the plane of vision is inclined 45° downward. In con- 

 tinuing the inclination still further downward, I observed a de- 

 cided rotation of the eyes in the contrary direction, i. e. in- 

 ward. As the eyes are raised from the position 45° downward, 

 the rotation increases until the visual plane is again in its pri- 

 mary direction. When the visual plane is raised above this, 

 however, I do not find the rotation to increase as stated by 

 Meissner, except in cases of extreme convergence, but rather to 

 decrease again, although it does not again become zero*. In 



* More recent experiments, just concluded, have convinced me that in 

 my own eyes, if the convergence is very slight, the outward rotation does 

 reach zero and may even be converted into an inward rotation. The reason 



