W. LeConte Stevens—Notes on Physiological Optics. 301 
equal and opposite in the two eyes, are perfectly corrected in 
the binocular combination of each pair; the resultant curves 
are hence perfect ellipses. 
If a pair of small circles whose vertical diameters are ad and 
a’b’ be drawn above the large circles, the visual lines directed 
to their centers are similarly oblique to their vertical diameters 
but oppositely oblique to their horizontal diameters. The 
external projections of their retinal pictures are hence slightly 
distorted ellipses, of which the upper vertices are farther apart 
and the lower vertices nearer together than their centers. The 
binocular combination is hence an ellipse whose plane is ob- 
lique, the upper vertex being farther, and the lower vertex 
hearer to the observer. A pair of small circles below the large 
ones are binocularly combined with opposite obliquity. : 
No explanation is now needed to show that if the planes of 
the cards be revolved into the positions P”’Q” and P’’Q”’ 
(fig. 4), the combination of the concentric circles must present 
# Concave surface and the obliquity of the plane of each pair of 
conjugate small circles, when binocularly viewed, must be re- 
ally advances slowly to its new position” ” is now easily under- 
s re . . 
en 
EO exceeds E’O’, there must be dissociation between the two 
focal adjustments which are generally adapted to the same dis- 
tance. To this must be added the necessary dissociation be- 
face appears convex or concave. Despite this inconvenience, if 
the experiment be performed with axial parallelism, the image 
soon becomes clearly defined. With strong axial convergence, 
a8 in Brewster’s method, the dissociation is far more difficult on 
account of the extreme muscular strain that is necessary. _ 
By holding the cards, fig. 4, with their planes coincident 
and then drawing them apart in this plane, so that 6° or 7° of 
divergence of visual lines is necessitated in retaining retinal 
usion, the image changes very perceptibly from that of a flat 
plate to that of a shallow coneavity. By cross vision the oppo- 
site is obtained; but not so strikingly, for the muscular strain 
of 7° of divergence I find to be as great as that of 60° or 70° of 
Convergence, : ; : 
If vertical lines, an, a’n’, fig. 5, be combined binocularly 
