154 J. LeConte on Binocular Vision. 
the subjects of the motions of the eye and of the Horopter. 
b ; 
seems to be an abstract of more extended researches whi 
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 Helmholtz — 
results or in describing my own experiments, I find it neces- 
sary to define the terms I 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 wit 
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 plant 
of the eye, and the intersection of such a plane with the retia 
i a meridian of the eye. The vertical line of de 
the optic axis. Again he states that ‘vertical and horizontal 
lines ne their vertical or horizontal position in the field 
en the eye is moved from its primary direction vert 
cally or horizontally.” This law had been previously stated by 
——s but without proof; Helmholtz claims to have é to 
and then turn the eye upon a comparatively obscure field, 
spectrum having the form of the object will be seen. As st” 
spectra are the result of a temporary modification of the scan : E 
itself, they must follow the motions of the eye with the gr 
est exactness. If therefore the bright object be a line, then f ae 
