J. LeConte on Binocular Vision. 155 
there be any rotation of the eye on the optic axis, in turning 
the eye in various diréctions the linear spectrum ought to in- 
cline to one side or the other. Suppose, then, the object be a 
bright red vertical line on a gray wall at the exact height of 
the eye: Helmholtz finds that on gazing at the bright line 
with one eye, taking care that the eye shall have its primary 
direction, and then turning the eye in a horizontal plane to 
the right or left, the spectrum retains perfectly its verticality. 
“T found,” he says, “ the results of these experiments in com- 
plete agreement with the law of Listing.” For the ingenious 
device of Helmholtz for getting the primary position of the 
eye we must refer the reader to his lecture. I have tried 
Helmholtz’s experiments with similar results. Nevertheless, I 
believe it may be demonstrated that though rotation of the eye 
does not take place under the circumstances of these experi- 
ments, yet it does so under other circumstances not touched b 
them ; and that in a manner which deeply affects the question 
of the Horopter. The law of Listing is doubtless true or 
hearly true when the eyes move together parallel to each other, 
but is far from being true in strong convergence. The experi- 
ments which follow prove beyond a doubt that in my own case 
and in most other cases tried, the eyes in convergence rotate 
on the optic axes outward, and that the amount of rotation 
mereases with the degree of convergence. Meissner* has 
attempted to determine experimentally the position of the 
Horopter, and from the position thus determined he infers the 
rotation of the eyes ; my experiments prove directly the rota- 
tion of the eyes, and irom this as well as from direct experi- 
ment I hope to establish the position of the Horopter. 
Helmholtz, it is true, admits some degree of rotation of the 
‘ye on the optic axis, particularly when the eye makes wide 
€xcursions in the field of view; but that he does not regard 
this as sufficient to interfere seriously with the law of Listing 
18 evident from the form of the Horopter which he deduces. 
creover, according to Helmholtz, these slight rotations are 
controlled by the law of Donders, viz: “that the eye returns 
“ways into the same position when the visual line is brought 
2 have already stated (p. 73) that when the squares of 
diagram (fig. 5) are combined by converging the 
* Bib. Un, Archiv. des Scien., II, vol. iii, p. 160. | 
a 
