W. B. Rogers on Binocular Vision. 205 
lower stage may be combined in a position nearer than their true 
place, and in either case the resultant is a single line parallel to 
the components and including them co-extensively throughout 
its whole length. 
9. 11. 
Unequal parallels (fig. 11) may also be united into a paral- 
lel resultant, but in this case the component lines are not co-ex- 
tensive throughout the resultant. By marking their extremities 
with dots or short lines we can observe the limits of each, and 
we then perceive that as long as the eyes and the diagram pre- 
serve their relative position unchanged, the two lines are not co- 
incident, even when they are very nearly of the same length. 
But by turning the head or the diagram so as to rotate the plane 
of the optic axes in relation to the figure, we may cause either 
the upper or the lower ends of the parallels to coincide according 
to the direction of the movement. When the upper ends are 
thus brought together the lower ones are seen to recede from one 
another; when the lower ends are made to coincide the upper 
ones separate to their greatest distance—the interval in both cases 
being marked by the dots on the resultant. The dotted line (fig. 
11), shews the direction parallel to which the line joining the 
two eyes must be placed in order by a due conyergence of the 
Optic axes to unite the points a and 8, and the angle made by this 
line with the horizontal measures the angular movement of the 
head or the diagram necessary to bring about the coincidence. 
When the parallels are nearly coincident, the adjusting motion 
of the head is so slight as to be made almost unconsciously. Thi 
movement is, I think, one of the causes of the apparent union of 
unequal figures described by Prof. Wheatstone, and in that con- 
hection will be referred to in a subsequent page. 
7. Vertical rotation of the eyes associated with their converging 
movement. : 
My experiments with unequal parallel lines have led me to ob- 
Serve effects accompanying the convergence of the axes to points 
either beyond or within the limits of distinct vision which indi- 
cate a rotation of the eyes in opposite vertical directions. As sim- 
ilar phenomena are noticed by a friend whom I have induced to 
repeat the experiments, I infer that they do not arise from any ab- 
