J. LeConte—Phenomena of Binocular Vision. 85 
eyes horizontally to the left, the image inclines equally to 
the left, thus\. If, after renewing the spectrum, I now 
sight to travel 40° to the right, the image inclines to the /e/z, 
thus \ ; if the point of sight moves to the extreme left, the 
image turns to the right, thus /. 
In all cases the degree of inclination or torsion of the 
image increases with the degrec of elevation or depression of 
the visual plane and the amount of lateral excursion of the 
point of sight, right or left. Also the degree and direction 
of the torsion of the image will be the same for the same 
position of the line of sight, however that position may have 
been reached, whether by two motions along rectangular coér- 
dinates, as in the preceding experiments, or Gy oblique motion 
from the first or primary position. ; 
Experiment 2.—I next made similar experiments, using a 
horizontal, instead of a vertical image. Such an image may 
be made in the same way, by means of a horizontal slit in 
the window. When such an image is thrown on a perpendic- 
ular wall with the eyes in the primary position (i. e. with face 
perpendicular and the eyes akin horizontally) its position is 
of course horizontal. When the eyes move from side to side 
horizontally, or up and down vertically, it retains its perfect 
horizontality. But if the eyes be turned obliquely upward and 
to the right, the image inclines to the left, thus —; if upwa 
and to the left, the torsion is to the right, thus —. In depress- 
ing the plane of sight, movement of the eyes to the right makes 
the image incline to the right, thus —_, while movement to the 
left, makes it incline to the left, thus —. 
The Jact and the direction of the torsion of the images, both 
vertical and horizontal, are very easily established by the some- 
what rough method just described. But if we desire to meas- 
ure the amount of torsion of the image, the wall or other exper- 
imental plane must be covered with rectangular codrdinates 
vertical and horizontal. By this means I find that extreme 
oblique positions produce an inclination of the vertical image on 
the true vertical of the wall of about 15°, but of the horizontal 
image on the true horizontals of the wall of only about 5°. 
There is a reason for this difference, which we explain farther on. 
_ Putting now all these results together, the following diagram 
(Fig. 1) shows the direction and the degree of inclination of 
the image for all positions of the point of sight—the center 
representing the primary position, and the corners extreme 
oblique positions of the point of sight. 
Helmholtz’s results are exactly the same as my own, except 
that he makes the inclination of the vertical and the horizontal 
image exactly equal, while I find the inclination of the hori- 
