448 W. LeConte Stevens—The Stereoscope, 
rected upon an object, A, in the foreground of a scene. Let 
E be the midpoint between C and C’; then EA is a median ; 
on this prolonged let B be an object in the background. Par- 
allel to ©’ C and to the vertical plane of the sensitized plates 
a 
hy 
? 
B, 
me 
need 
ab ba 
behind the lenses, let two planes be passed through A and B 
respectively. Let P and.Q be any points on these planes, so 
O 
the distances E A,, EB,, EP, and EQ, are determined. In 
binocular vision the direction of the object seen is always esti- 
mated from the position of the combined binocular eye, EK, and 
is coincident with that of the median between the two visual 
