208 W. B. Rogers on Binocular Vision. 
presented to one eye and ¢ and d to the other, it is easy bya 
proper convergence of the axes to unite a with ¢ and 6 with d. 
The two resultant lines will of course be vertical and will be in 
a plane parallel to that of the diagram, and they will be situated 
behind or in front of it according as the union has been effected 
by converging the eyes beyond the diagram or before it. 
hen the intervals of the vertical lines forming the respective 
pairs are unequal (fig. 17), the union of a with 6 and of ¢ with d 
16. 17. 
i 
a r c d a 6 ec 3 
forms two resultant verticals which lie at different distances be- 
fore or behind the plane of the drawing. Supposing the union in 
the present case to be effected behind that plane, the resultant of 
ac will be more remote than that of bd. If the combination is 
* produced in front of that plane, the resultant of a c will be nearer 
to us than that of 6 d. 
When the intervals between the verticals are nearly equal, the 
change of convergency requisite in passing from one resultant to 
the other is so quickly brought about that we can scarcely pe!- 
ceive that the two resultants are not seen each in perfect single- 
ness at the same moment. If however the difference be great, 
as when a 6 is one inch and cd half an inch, although we can 
readily unite a with ¢ and 6 with d, a considerable effort is ne- 
cessary in passing from the one convergency to the other. Hence 
as soon as we form a distinct single resultant of ac, that of bd 
becomes double, and when again 6d are united into one line 
the resultant of ae becomes double. The same effect occurs 
when the inequidistant vertical lines form parts of any of the 
twin drawings of a more complex kind which are usually pro- 
i“ for stereoscopes, as will be pointed out under a subsequent 
ead. 
is proved by the fact that, however intently we fix the view 08 
one of them, the other appears clearly visible as a single line, a0 
when the attention is directed to a point between the two, the 
singleness of each is perfectly preserved. 
_Sir D. Brewster, in one of his valuable papers on binocular 
vision, maintains that in order to see an object distinctly the optic - 
