W. B. Rogers on Binocular Vision. 211 
point of convergence of similar lines, extended through d and d. 
Hence the lower end of the resultant will be at a greater distance 
than the upper end ; and for a similar reason, the intermediate paits 
of points in the compouent lines will be united at various inter- 
mediate distances, and thus the resultant will appear as a perspec- 
tive line receding as it extends downwards. A like explanation 
obviously applies to the other cases above described. 
Such is a general account of the formation of the perspective 
resultant by the union of inclined lines. But there are features 
in this phenomenon which from their bearing on the theory of 
binocular combination are deserving of a more minute and varied 
examination. Asa first step towards this the following experi- 
ments are of interest as showing the chief conditions in the for- 
mation of the perspective resultant and illustrating the different 
degrees of completeness of the effect. 
12. Comparative results with lines differently inclined. 
Diagrams well suited for this comparison are represented in 
figs. 20, 21, and 22, in each of which the lines are separated be- 
j\ fA 
low by an interval of {4 inch and have a vertical height of the 
same amonnt. The inclination of the lines in fig. 20 is 40°, and 
in the others 30°, 20°, and 10° respectively. Confining my re- 
marks in the first place to the effects observed when the upper 
stage of the stereoscope is used and the binocular union Js pe 
duced beyond the diagram, I begin with fig. 20, the lines of whic 
make an angle of 40°. ° 
Having placed the figure properly on the stage, J first ae 
the visual convergence so as to bring together the upper ends 0 
the lines, and maintaining this convergence, steadily I see the ae 
lines combined into the figure of an inverted \/.” It en bya 
change of convergence unite the two lower ends and retaining 
the adjustment I observe them to form a \/ in the erect alte 
In the same way by intermediate degrees of convergence 
