206 W. B. Rogers on Binocular Vision. 
normal adjustment of my own eyes, and I therefore deem them 
worthy of mention in this place. 
In attempting to unite unequal parallel lines by converging the 
axes beyond the plane of the diagram, I find it easier to bring 
about the terminal coincidence above described, when the lower 
of the dots is presented to the right eye than when opposite the 
other. The effect is very obvious when fig. 12 is adjusted on the 
a r 8 b 
upper stage of the stereoscope, so that a and the right half of m 
are presented to the left eye, while 6 and the other half of m are 
opposite the right eye. Fixing the long line in a horizontal po- 
sition in the plane of the optic axes, I cause a and s to approach, 
and in doing so I find that they either unite at once, or are readily 
brought together without shifting the head or paper. But when 
I make the same effort with b and r, 1 can only bring the latter 
below the former, and find it requisite to turn slightly the head 
or the diagram in order to unite them. It would therefore seem, 
-that when converged beyond the limit of distinct vision, my eyes 
tend to revolve vertically in special directions, the right eye turn- 
ing very slightly downwards and the left upwards. 
hen I direct my eyes to a point much nearer than the limit 
of distinct vision the evidence of vertical rotation in connexion 
with the converging movement is very remarkable. Too observe 
this I adjust the lower and upper stages of the apparatus to the 
distance of 12 and 7 inches respectively and placing (fig. 13) oa 
b 13 
- a 
c 
the lower stage in such position as by cross vision to present b 
and ¢ to the right eye and a to the left, I converge the axes a lit- 
tle below the aperture. As the two parts of the figure approach 
one another, I see a and b coming nearer to the same horizontal 
Jeve] and when the due convergence of the axes is attained I ob- 
serve them to coalesce completely. But retaining the figure 4 
the head in the same position, I find it impossible to bring about 
aunion of aande. In order to do this I must either turn mY 
head so as to depress the left eye below the level of the other, of 
I must turn the diagram in the opposite direction. When this 
latter motion is used to produce the coincidence of a and ¢, I find 
on subsequently inspecting the figure that I have been compelled 
to depress a considerably below the level of «¢. 
A like effect appears when I view (fig. 14) as in the preceding 
experiment, In this case a and b instead of combining takes the 
