92 J. LeConte—Phenomena of Binocular Vision. 
at of convergent motion, would completely explain all the 
he com- 
plete contrast between the two laws. We would thus formulate 
the contrast : 
1. When the eyes move in the same direction parallel to each 
other, in the primary plane, there is no torsion or rotation on 
the optic axis; but when they move in the primary plane in 
opposite directions as in convergence, they rotate outward, i. e., 
toward the temples, thus . # 
. When the plane of sight is elevated, and the eyes move 
together parallel to each other, then if the lateral motion is to 
the right, the rotation is to the right, if ‘to the left, the rota- 
tion is to the left; but when in the same position of the visual 
plane, the eyes move in opposite directions, as in convergence, 
then as the right eye moves to the left (toward the nose) it 
rotates to the right, and as the left eye moves to the right (i. ¢., 
toward the nose), it rotates to the left. If Listing’s law operated 
at all in convergence, it would tend to neutralize the contrary 
effect of convergence; but such is not the fact. 
3. When the visual plane is depressed, the direction of rota- 
tion is the same for parallel motion and convergent motion; in 
both cases the rotation is contrary to the direction of motion. 
But there is this great difference between the two; by the law 
of parallel motion, the rotation increases with the angle of depres- 
sion, while by the law of convergent motion, it decreases to zero 
45°. If Listing’s law operated at all in convergence, it would 
in this case codperate and zncrease the motion, but the reverse 
is the fact, the rotation decreases, 
writers and to appearance, I have called this change a rotation 
on the optic axis, yet it seems to me it cannot be properly so 
called. or all parallel motions of the eyes are rotations on 
equatorial axes, and therefore on axes in a plane perpendicular 
to the polar or optic axis, and therefore cannot be resolved into 
rotations on the latter. In parallel rotation, therefore, the so- 
called torsion is only apparent and the result of position, or in 
other words the result of reference to a new spatial meridian. 
Turning the eye from side to side in the primary plane pro- 
duces no torsion because all the spatial meridians are there 
parallel, but turning from side to side in an elevated plane pro- 
