1864.] 



Human Eye in relation to Binocular Vision. 



197 



time looking down at a near object, as in reading or writing, I found some- 

 times that the horizontal lines of fig. 2 crossed each other ; but they be- 

 came parallel again when I had looked for some time at distant objects. 



In order to define the position of the corresponding points in both fields 

 of vision, let us suppose the observer looking to the centres of the two sides 

 of fig. 2, and uniting both pictures stereoscopically. Then planes may be 

 laid through the horizontal and vertical lines of each picture and the 

 centre of the corresponding eye. The planes laid through the dif- 

 ferent horizontal lines will include angles between them, which we may call 

 angles of altitude ; and we ma}" consider as their zero the plane going 

 through the fixed point and the horizontal meridian. The planes going 

 through the vertical lines include other angles, which may be' called angles 

 of longitude, their zero coinciding also with the fixed point and with the 

 apparently vertical meridian. Then the stereoscopic combination of those 

 diagrams shows that those points correspond which have the same angles 

 of altitude and the same angles of longitude; and we can use this result 

 of the experiment as a definition of corresponding points. 



We are accustomed to call Horopter the aggregate of all those points of 

 the space which are projected on corresponding points of the retinae. 

 After having settled how to define the position of corresponding points, the 

 question, what is the form and situation of the Horopter, is only a geome- 

 trical question. With reference to the results I had obtained in regard 

 to the positions of the eye belonging to different directions of the visual 

 lines, I have calculated the form of the Horopter, and found that gene- 

 rally the Horopter is a line of double curvature produced by the inter- 

 section of two hyperboloids, and that in some exceptional cases this line of 

 double curvature can be changed into a combination of two plane curves. 



That is to say, when the point of convergence is situated in the middle 

 plane of the head, the Horopter is composed of a straight line drawn 

 through the point of convergence, and of a conic section going through 

 the centre of both eyes and intersecting the straight line. 



When the point of convergence is situated in the plane which contains 

 the primary directions of both the visual lines, the Horopter is a circle 

 going through that point and through the centres of both eyes and a 

 straight line intersecting the circle. 



When the point of convergence is situated as well in the middle plane of 

 the head as in the plane of the primary directions of the visual lines, the 

 Horopter is composed of the circle I have just described, and a straight 

 line going through that point. 



There is only one case in which the Horopter is really a plane, as it was 

 supposed to be in every instance by Aguilonius, the inventor of that name, — 

 namely, when the point of convergence is situated in the middle plane of 

 the head and at an infinite distance. Then the Horopter is a plane 

 parallel to the visual hues, and situated beneath them, at a certain distance 

 which depends upon the angle between the really and apparently vertical 



