196 Prof. J. LeConte on some Phenomena 



with the vertical meridian) and is therefore seen single. But if 

 the eyes rotate on the optic axes outward, then the image of a 

 vertical line still falling on the vertical meridian must cross the 

 line of demarcation in opposite directions in the two eyes, and 

 therefore cannot be seen single except at the point of sight, the 

 image of which corresponds to the central point of the retina 

 of each eye. In order that the image of a line shall fall on the 

 line of demarcation in both eyes and thus be seen single, it must 

 be inclined at a certain angle with the vertical, the lower end 

 being nearer and the upper end further away. It is moreover 

 evident, upon a little reflection, that when the eye rotates, the 

 horopter cannot be a plane or a surface of any kind ; for objects 

 right and left of the horopteric line must all be doubled by dis- 

 placement of the horizontal line of demarcation GH (fig. 12), 

 which therefore no longer coincides with the horizontal meri- 

 dian, E F. 



From various experiments made at different distances and 

 with different degrees of inclination of the visual plane upward 

 and downward, Meissner concludes : — (1) That, looking straight 

 forward at an infinite distance, the horopter is a plane at right 

 angles to the visual lines. (2) That for all other distances, the 

 visual plane remaining the same, the horopter is a straight line 

 passing through the point of sight and increasing in inclination 

 to the visual plane as the convergence of the optic axes increases. 

 (3) That in turning the visual plane downward, the inclination 

 of the horopteric line with that plane becomes less and less, 

 until at 45° downward it becomes perpendicular, and therefore 

 the horopter again expands into a plane at right angles to the 

 median line of sight. (4) That in raising the visual plane up- 

 ward toward the eyebrows, the inclination of the horopter to the 

 visual plane increases. 



We have given Meissner' s investigations more in detail, be- 

 cause by entirely different methods we have confirmed almost all 

 of them. 



Claparede by similar experiments fails to confirm the conclu- 

 sions of Meissner, and therefore rejects them. He concludes, 

 partly from his own experiments and partly from calculation, 

 that te the horopter is a surface of such a form that it contains 

 a straight line perpendicular to the plane of vision and passing- 

 through the point of sight, and that every plane passing through 

 the optic centres makes, by intersection of this surface, the 

 circumference of a circle." In other words, he believes that 

 the horopter is a surface which contains the horopteric vertical 

 BAB' (fig. 13) and the horopteric circle A of Prevost, 

 and that in addition the surface is further characterized by the 

 fact that, while the point of sight remains at A, the intersection 



