SINGLE VISION. 49 



clination to the horizon in their progress to the 

 visual base, as the real single one. In this man- 

 ner, every thing may be shown to be true, with 

 respect to the single and double appearances of 

 objects without the plane of the optic axes, 

 which has already been done with regard to 

 those placed in it. But farther ; since any 

 point, taken at pleasure, in the common inter- 

 section of the three perpendicular planes, ap- 

 pears single, the whole of the line of intersec- 

 tion must appear so, and likewise every point 

 of a plane made to pass through it, parallel to 

 the visual base. Such a plane necessarily in- 

 cludes the horopter, and is the same as that, 

 which is called by Aguilonius the plane of the 

 horopter. 



To exemplify the principal property of this 

 plane, I shall mention an experiment, which at 

 first I did not understand, though the result 

 was a direct consequence of my own principles. 

 I suspended a fine chord at right angles to the 

 horizon, and retreating a step or two, I looked 

 steadily at a point in it, which was upon a level 

 with my eyes. The chord, in these circum- 

 vstances, appeared single ; but whenever I di- 

 rected my eyes to any other point of it, either 

 above or below the former, two chords would 

 appear, crossing each other at the part, to which 

 the eyes were directed. In the first case, the 



