L 179 ] 



XX. On some Phenomena of Binocular Vision. By Joseph 

 LeConte, Professor of Chemistry and Geology in the Univer- 

 sity of South Carolina*. 



[Continued from vol. xxxvii. p. 140.] 

 II. Rotation of the Eye on the Optic Axis. 



NEARLY all the experiments described in this paper had 

 already been made and the results obtained, when my 

 attention was called to Helmholtz's Croonian Lecture " On the 

 Normal Motions of the Eye in relation to Binocular Vision "f. 

 From this lecture I received some useful hints as to the best 

 method of experimenting on this subject, which have been of 

 great service to me, and have made my results much more satis- 

 factory, without, however, materially modifying them. As these 

 results differ very greatly and fundamentally from those of 

 tlelmholtz, I repeated the experiments daily for many weeks, 

 modifying them in every conceivable way to avoid the possibility 

 of error. I am perfectly sure, therefore, that the results are 

 true for my own eyes ; and as far as I have been able to have 

 them verified, they are true also for most other normal eyes. 

 Unfortunately, however, the difficulty of verification for other 

 eyes is very great. Many of these experiments, which I find 

 perfectly easy, are almost impossible for most persons. 



Helmholtz's lecture, I suppose, is the most authoritative state- 

 ment which we have of the present condition of science on the 

 subjects of the motions of the eye and of the horopter. It 

 seems to be an abstract of more extended researches which I have 

 not seen. On this account it is obscure in some parts; yet I 

 think I cannot be mistaken in his general results. In order to 

 make myself clear, whether in discussing HelmhohVs results or 

 in describing my own experiments, I find it necessary to define 

 the terms 1 shall most frequently use. The position of the eye 

 when the optic axes are parallel and at right angles to the 

 vertical line of the face, as when with head erect we look at a 

 point on a distant horizon, is called by Helmholtz the primary 

 direction of the eye, and the visual line in this case is the primary 

 direction of the visual line. All other directions are called 

 secondary directions. A plane which passes through the visual 

 line is called a meridian plane of the eye, and the intersec- 

 tion of such a plane with the retina we will call a meridian of the 

 eye. The vertical line of demarcatian is that meridian of the eye 

 upon which the image of an apparently vertical line falls when 

 we look directly at the line, and which therefore divides the 

 retina into two equal halves containing corresponding points 



* From Silliman's American Journal for March 1869. 

 t Proc. Roy. Soc. April 1864, vol. xiii. p. 186. 



