J. LeConte— Phenomena of Binocular Vision. 91 
ape eyes in various pasitinge is seen 
But again, and finally, Helmholtz’s cmiergals in regard to 
the direction of torsion are, it seems to me, in direct contradic- 
tion to his own general formulation “of Listing’ s law, taken from 
Listing himself. This general formula is as follows: ‘“ When 
the line of regard passes from the primary position to any other 
position, the angle of torsion of the eye in its second position, is the 
same as uf the eye had come to this second position by turning geek 
a fixed axis perpendicular both to the first and to the second 
tion of the line of regard.”* Now an axis which satisfies these 
conditions can be none other than an equatorial axis, 1. €., an 
axis at right angles to the polar or optic axis, In turning from 
side to side horizontally, it is a vertical equatorial axis. In 
turning up and down, it is a horizontal equatorial axis. In 
turning obliquely, as in the experiments on torsion, it is an 
oblique equatorial axis, Now let any one take a elobe, and 
placing the dauater ie in a vertical plane, make a distinct ds 
and horizontal mark across the pole. If now the glo 
turned on an oblique equatorial axis so that the pole shall hae 
upward and to the right, it will be seen that the polar cross is 
no longer vertical and horizontal, but has rotated to the Pe 
not the left, as Helmholtz’s statements would indicate. Turn 
ing of the elobe on a fixed axis so that the pole looks sawied 
and to the left, will cause the cross to rotate to the left. So 
turning downward and to the right, produces rotation to the 
left, and to the left rotation to the right. If the globe turns 
thus on an axis inclined 45° to the vertical, and through an are 
of 90°, the rotation of the polar cross will be exactly 45°, 
Thus Listing’ s law, as understood by himself, is in exact accord- 
ance with my resu 
I have now, I believe, established on the firm basis of exper- 
iment, the true law of torsion of the eyes, when moving paral- 
lel to each other. 4 have also shown that it is identical with 
Listing’s law, properly understood, and I shall peice con- 
tinue to call it by that name. Misled by Helmholtz’s very pos- 
itive statements, I commenced this investigation with the expec- 
* Op. cit., p. 606 
