J. LeConte on Binocular Vision. 159 
principle of the difference between real and apparent verticality. 
The ignorance of this principle he believes has vitiated the 
results of all previous observers. He illustrates this principle 
thus: ‘“ When 
8. 
vertical line incline by an angle of about 1} degrees for it to 
are not fatigued 
On this principle Prof, Helmholtz builds his whole theory 
ofthe Horopter. But that this principle cannot account for 
aside: in the second place, it 
Whether the two halves be combined by a “stereoscope or by 
squinting.” If they are combined by a stereoscope as stereo- 
Scopes are usually constructed, the right half is looked at by 
ng 
gram ; coincidence in this case, tore, : , 
illustration of Prof. Helmholtz’s principle. But if they are 
combined by squinting, the eyes are crossed, and theref 
