182 W. B. Rogers on Binocular Vision. 
ing of the head sufficed to unite the upper extremities and that 
this movement when very small was made almost unconsciously. 
But while this adjustment explains in certain cases the optical 
coincidence of points and lines situated at unequal distances 
above or below the horizontal direction, it does not apply to the 
cases in which the head is kept perfectly fixed during the aet of 
combination. In these conditions the coincidence would seem 
to result from a vertical rotation of one or both eyes, or perhaps 
an equivalent change in the direction of the transmitted pencil 
due to some alteration of the form of one or more of the re- 
fracting surfaces. As such an adjustment has not, I believe, 
been suggested by preceding writers on vision, the following de- 
tails founded on personal observation may, it is hoped, throw light 
upon the subject. 
Recurring to the experiment mentioned under a former head 
6) and using a similar diagram (fig 83), I place it beyond the 
83. 
b 
n 
G 
c 
limit of distinct vision, so that the optic axes directed crosswise 
stated, that a and 6 readily coalesce. I then turn the paper so as 
slightly to depress a, and repeating the effort I observe a taking a 
position just beneath and close to 6, but quickly after uniting with 
it. Turning the paper still further in the same direction, until 
by the usual convergence I bring a below } and midway between 
this line and c I find that with some effort I can still cause the 
two to unite. Lastly, I depress @ a very little more, so that the 
_ converging action may carry it below the middle of the interval 
between the other lines, and now I see it quickly coalesce with ¢. 
Usually this union does not take place instantly unless a is brought 
to the same level with c—that is, to such a position that the cor- 
esponding ends of the two lines are in the same horizontal di- 
rection. The effect in this case, is a little different from that of 
_ the experiment formerly described (6) where the axes were made 
to intersect much nearer than the limit of distinct vision, as 12 
_ the present conditions the action of the two eyes is more nearly 
equal. It should be remarked also that the limit of vertical sep- 
aration compatible with a union of a with 5 or ¢ is liable to con- 
erable variation, especially when the eyes have become fatigued 
: periment. . ; 
vould appear from these results that the eyes possess an 
usting power which enables them to unite two lines or points 
