and Vision by Optic Divergence. 449 
axes, but always somewhere in front.* This is universally 
true for normal eyes, as may be abundantly learned by experi- 
ment, whether the axes be convergent, parallel or divergent, 
and whether the median be at right angles or oblique to the 
interocular line, LR. In fig. 4, 
if K,A, and E,P, represent these 
medians, we have both direction 
and distance determined for these 
foreground points. To the right __....._._/_4.\ 
eye, B, (fig. 3) appears beyond 
and to the right of A, at an /y 
angular distance wep te! by A 
to the left eye, beyond and to ae ey “Gpba- 
left at an equal angular distance ; fe ae 
to the binocular eye, E, (fig. 4), it is hence homonymously 
oA pe at b’,),. To secure single vision of it, the optic angle 
must be diminished, and oe the rectus muscles this at 
A,, as in fig. 4. Similar remarks ms to 
If a be less than @, as is often the case, shila fact will cause 
the observer to estimate A, to be more distant than it is repre- 
sented in the drawing, but ry no means neceeernyd so distant 
as the actual vertex of @ a be reduced to zero or become 
negative the sensation of still further change of ae ten- 
sion makes the ery position of A, recede still more, and 
“but I find that an excellent effect is produ uced when the i 
are foe parallel by pictures rie at an spoliaene not 7: 
8°, and even a difference of 16° or 17° has no decidedly bad 
effect.” His preconception that optic ober arenes even 
though slight, is ene prevented his apprehension of 
more than part of the truth. He states, as a remarkable pecu- 
liarity, that “ Nee the optic axes are parallel, or nearly so, 
the image does not appear to be refe ane to the distance we 
should, from thie circumstance, sup to be, but it is per- 
ceived to be much nearer.” Such ae angles as 17° are sel- 
* This Journal, III, vol. i, p. 33 et seq. 
+ Wheatstone, Physiology of Vision, Phil, Mag., 1852, pp. 513-514. 
Am. Jour. Sci. eee Series, VoL. XXII, No. 182.—DEcEMBER, 1881. 
