W. B. Rogers on Binocular Vision. 95 
& perspective plane, but in this case under the same conditions of 
combination with the preceding, it is inclined the opposite way. 
A like effect is produced by removing the circle and putting 6 in 
_ its place, and then combining } and a. In all three resultants 
the longer axis of the ellipse is horizontal. Its length is great- 
est in the ellipse formed by the binocular union of a@ with 8 
If now both ellipses, placed as in the figure, be used at the 
Same time to combine with the circle, we obtain by rapid alter- 
nation of combinations a resultant consisting of two perspective 
ellipses intersecting in their shorter or vertical diameter. It is to 
be remarked that in this experiment it sometimes happens that we 
cannot pass from one combination to the other rapidly enough to 
obtain the effect of an apparently simultaneous view of the whole 
of the two resultant ellipses. In this case the two near halves 
or the two remote halves of the intersecting ellipses, or either el- 
lipse entire, is all that we distinctly see, but it is only necessary 
to pass quickly from one to the other to perceive the whole of 
_ the curves together. : 
e have thus seen that the resultant formed by the union of 
two elliptic ares or of an elliptic and circular one, under the given 
conditions, lies wholly in one plane, while that formed from arcs 
of unequal circles lies in a curved surface. This remarkable dif- 
ference of effect is readily explained when we compare the geo- 
metrical conditions demonstrated under a previous head (26) with 
the properties of these associated curves. ; 
In the case of the ellipse and circle or the two ellipses, (fig- 
eee 
rvature, as in fig. 71, the corresponding horizontal dimen- 
Cease to have a constant proportion. Bringing them to- 
ther as in fig. 73, we perceive that the relative ine- 73. 
cessive point, in other words, that the resultant must lie in a 
curved or warped “i. ne ued) 
