* 216 W. B. Rogers on Binocular Vision. 
verging my eyes fixedly to the near apex of such a figure I see 
the remote one double and the oblique edges diverging from it 
also double ; and in the same manner I find it easy to double ev- 
ery one of the angles or edges by fixing the eyes on the angle or 
the figure. When the image is a truncated square pyramid with 
its plane of truncation nearest the eyes, by looking intently on 
one of the vertical sides of the base I see the corresponding side 
of the smaller square double, or by fixing my eyes upon the lat- 
ter I see the former double. In any of these cases a continuance 
of the same convergence causes the angles and lines in question 
to appear in the same plane. 
Among the figures suited to illustrate this effect I have found 
none to: present more striking results than the square pyramid or 
the truncated cone, with the addition in each case of a vertical 
line drawn through the centre of the base. The twin drawings 
adapted to produce these forms are represented in figs. 25 and 26. 
25. 25a, 
“CORDS seme mera 
if, after obtaining the perspective resultant of fig.-25, I look in- 
tently on the apex of the pyramid so as to see it at a single point, 
of the pyramid is converted into two lines diverging from an an- 
gle of the base. By continuing the same degree of convergence 
the image loses its relief and presents the appearance of fig. 
Her 
In the case of the truncated cone (fig. 26), on directing the 
axes to the vertical line in the base so as to see it distinctly single, 
the small end appears as two intersecting circles; fixing the 
View on the centre of the small end the vertical line in the base 
becomes double, and the large circle appears indistinctly as tw 
intersecting circles. In either case a fixed convergence destroys 
