- 
Fd , ; P te Fl 
328 W. B. Rogers on Binocular Vision. 
* 
b while ¢ appears in the same plane and at the same inclina- 
tion asin the diagram. But the moment the convergence In- — 
creases, a and c unite and the perspective resultant rises from the 
plane of the paper. aks 
ow in effecting the bocular union while looking towards the. 
lower end of the lines, it is evident that for a moment a@ an 
both form their images centrally on the retinal surface of the two 
eyes, and therefore according to the received law of single vision 
by corresponding points they might be expected to appear as a 
_single line, that is to say, in the same place. That this coinci- 
‘dence does not occur is explained by the fact that a is seen in the 
binocular direction and that this direction being compounded of 
the impression made by a and ¢ cannot be the same as that due 
to 6 alone or to a and 6 combined. Referring more directly to 
the visual actions concerned, we would say that in virtue of chang- 
ing convergence the perspective idea becomes in this case asso- 
ciated with a and c, but no such idea is attached to 6. Henceeven 
when the images of a and 6 fall on corresponding points of the 
retinas, the conditions under which they are perceived are differ- 
ent and the two lines should not be expected to appear in 
same place and direction. 
he fact here demonstrated, that the images: formed on cor- 
places, is certainly at variance with the law of corresponding 
points, as usually expressed. he contradiction however disap- 
pears, when, as is obviously proper, we restrict the application of 
the law to the cases in which the images are neither of them 
disturbed by combination with a third image. 
Another feature of the experiment worthy of especial notice 
is the fact that although the images of a and 6 are always formed 
on the retinas in parallel positions, no matter to what point of the 
perspective resultant we may for the moment direct our eyes, yet 
we never perceive them as parallel. The explanation is obvious. 
We perceive a in the binocular direction, that is, in the position 
of the resultant formed by the union of a and c, and therefore in 
an oblique plane, while we see } only in its natural vertical po- 
sition. 
19. Experiment of Prof. Wheatstone to disprove the theory 
of corresponding points. 
Prof. Wheatstone adduces the following experiment to show 
“that similar pictures falling on corresponding points of the two 
retinze may appear double and in different places.” 
“ Present in the stereoscope to the right eye a vertical line and 
to the left eye a line inclined some degrees from the perpendicular ; 
the observer will then perceive, as formerly explained, a line the 
extremities of which appear at different distances before the eyes: 
raw on the left hand figure a faint vertical line exactly corres- 
