86 W. B. Rogers on Binocular Vision. 
a much greater disparity of measurement is compatible with ap- 
parent coalescence in the former case than in the latter. As this 
essential distinction hen not been pigs sei to by preceding en- 
quirers, it will be proper to consider the two effects separately 
before treating of the union of figures ek in both directions. 
First.—Of the union of figures having the same height but 
differing in horizontal measurement. 
Most of the illustrations given under the last two heads are 
properly referable to this class. ‘Thus the apparently simultane- 
ous union of pairs of verticals of which the intervals are unequal 
(fig. 17), and the combination of a vertical with two er 
w 
although the combination is really successive and alternating, ‘it 
is eflected so rapidly and unconsciously as to make __ eee 
sion of a figure developed simultaneously in all its parts. — 
The union of pairs of points at unequal dikes on a hori- 
zoutal line or what amounts to the same thing, the combination 
of the wea horizontal lines connecting such points, gives, as 
already show , @ perspective resultant in the horizontal 
plane; and this, when the difference of the length is not too 
great, is as truly a case of ehiabidotiee as the combination of two 
piesa inclined lines into a perspective resultant. In neither 
case is the union absolutely simultaneous throughout. When 
the yes dwell upon either end of the resultant, the components 
separate at the other end, in the case of the inclined lines bya 
divergence of their remote extremities, in the case of unequal 
horizontal lines, by the sliding of one upon the other. As long 
however as the eyes are suffered to glance from end to end of 
either perspective line the union of the extremities is perfect and — 
apparently coexistent. : 
24. Union of a right line with a system of right lin 
Examples of this ‘class of combinations have oueaptelt under @ 4 
the preceding heads, but the following instances will serve to — 
illustrate more strikingly the union of dissimilar and unequal 
figures. 
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