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W. B. Rogers on Binocular Vision. 83 
In this experiment the coexistence of the two perspective re- 
sultants is so nearly perfect that it is only by fixing the gaze most 
A intently on one of the extremities that I can modify the per- 
spective figure. In looking towards the intersection, it is im- 
possible by any steadiness of view to change the relief of the 
two resultants. The small range of axial movement through 
which the eyes vibrate involuntarily even in our effort to keep 
them steady, is sufficient to develop both the resultants, so that 
ey appear to be quite simultaneous, 
s in this combination we do not first form one entire per- 
spective line and then the other, but begin to form both at the 
same moment and complete the combination for both in the same 
insensibly short interval, it follows that the two intersecting re- 
sultants are produced by the same gradation of convergence and 
in the same time as would be required to combine @ with b alone 
or with c alone. Hence the effect is as perfect and as nearly in- 
stantaneous as when a single resultant is developed from two in- 
clined lines. 
Que of the most simple and striking illustrations of an alternating 
combination is furnished by fi in whi * 
the lines 6cd are made to unite successively or al- 
ternately with a. The diagram being adjusted on 
the upper stage, so as to bring a opposite the left 
and 6 ¢ d opposite the right eye, if we begin by 
’ 
‘Uniting ab at the lower extremity, and then 
glance upwards, we find a to be wholly united ts Bcd 
with 0, forming a perspective line rising from 
the plane of ed. Glancing at the upper angle we observe ¢ also 
iM perspective but in an opposite direction, and when we pass to 
the lower angle we have d as well as ¢ in relief, while 6 tends to 
subside from its perspectiveness. ‘I'he passage from one of these 
Combinations to the other is so easy and rapid that it is quite dif- 
ficult to maintain the figure in any one of these phases for more 
than one or two seconds; and as we carry the eyes generally 
of the perspective resultant of these lines in 
& position appro hi: ae ae 2 A tan 
Over the lines we have a clear image of ali three resnitants united 
__ Ito a zigzag or N in a perspective position, in which the succes- 
_ Slveé combinations seem to be almost simultaneous. 
23. Alternations of more complex figures. 
ery curious alternations of combination a sd ‘ 
are afforded by fig. 51, when placed on the 
Stage of the stereoscope so that a 6 and cd 
may be opposite the left and right eye re- 
Spectively. Beginning by effecting the un- 
ion of 6 and ewe have a figure consisting 
Se Ge Me be Sage ¥ 
