172 J. LeConte on Binocular Vision. 
middle plane of the head as in the primary visual plane, the 
Horopter is the circle just described [Prévost’s horopterie 
circle] and a straight line going through that point [direction 
not stated].” 
“There is but one case in which the Horopter is reallya > 
plane, viz: when the point of convergence is in the middle | 
plane of the head and at an infinite distance. Then the Ho fF 
ropter is a plane parallel to the visual plane and beneath it, at | 
a certain distance which depends upon the angle between the | 
really and apparently vertical meridians, but which is nearl 
as great as the distance of the feet of the observer from his 
eyes when he is standing. Therefore, when we look at a point 
on the horizon, the Horopter is the ground on which we stand. 
2 When we look at the ground on which we stand at any po 
equally distant from both eyes, the Horopter is not a plane, 
but the straight line which ‘is one of its. parts coincides com- 
pletely with the horizontal plane on which we stand.” 
point of sight is at considerable distance and in which, there 
fore, the rotation of the eye is very small. I am not able? 
test all of Prof. Helmholtz’s conclusions by calculations based 
of 23° with each other or of 1}° with the vertical, they be 
intersect each other at the distance of about five fect below 7) 
eyes or about the feet of the observer standing erect. Now} 
: these two lines be placed thus \/ before the observer whos? 
scopic combination would be a line lying al he ground . 
_ the horopteric plane would be greater. We have already sh 
