W. B. Rogers on Binocular Vision. 207 
positions shown in (fig. 15), where the line presented to the left 
eye appears in a higher position than that presented to the right, 
although when directly looked at they are seen to be on the same 
level. By inclining the diagram so that a (fig. 14) shall be lower 
4. 15. 
D 
a 
_ than 6, the resultant parallels (fig. 15), approach one another, and 
_ when the proper inclination is attained, (in this particular case 
about eight degrees, ) the two lines unite to form a single inclined 
resultant. 
The following experiment, very easily repeated is one of many 
of the same kind which I have made to test the peeuliar adjust- 
ment of my eyes when strongly converged. Placing myself in 
a seat about 15 feet from a window, with my eyes ona level with 
one of the horizontal bars, I direct my view to the end of a pen- 
cil or other small object held before me at the distance of distinct 
vision, I then observe whether the two images of the horizontal 
bar form one line, and if they do not, I adjust the position of my* 
head until 1 find them entirely free from vertical displacement. 
Without moving my head [ now converge the optic axes to some 
point much nearer than the pencil, and I see the two images of 
the horizontal bar at different levels, the right image being lower 
than the other. 
_ In all these cases of convergence toa point nearer than the 
limit of distinct vision the effect is obviously such as would arise 
from a rotation of the right eye in an upward and of the left eye 
in a downward direction. It thus appears that in converging my 
eyes towards a point beyond the limit of distinct vision, there oc- 
curs a very slight displacement of the two images in opposite verti- 
cal directions, indicating a motion of the left eye upwards and o 
the right eye downwards, and that in converging them to a point 
nearer than the same limit a much greater displacement occurs 
corresponding to a much more considerable rotation of the eyes 
in the reverse directions. 
movement seems to furnish a much simpler and more probable 
explanation. eae a 
8. Combination of pairs of vertical lines. 
If two pairs of equidistant and equal vertical lines (fig. 16) be 
_ Placed on either stage of the stereoscope so that a and 6 shall be 
