W. B. Rogers on Binocular Vision. 179 
the effect which will result when a right line is combined with 
a figure composed of two or more curves. When the breadth 
of the latter is so small as to allow of a very rapid alternation of 
union, the resultant presents itself in perfect relief, at the same 
time apparently in every part. This is well shown when the 
vertical line, fig. 78, is combined with either of the compound 
~ 
78. 
ec 
curve figures a, b, c,d. When combined with a it gives a re- 
sultant consisting of two curves (conic sections) in relief with 
their vertices touching, with 6} the effect is that of two curves 
like the former uniting to enclose a perspective plane, with c it 
ptesents a sigmoid line turned somewhat edgewise towards the 
view and with d an undulating line in a similar position. _ 
A remarkable example of this kind of combination is furnished 
y fg. 79 which from its bearing on an ob- 79. 
hereafter is deserving of more particular 
Mention. Placing ab before the left and ¢ 
before the right eye, we may readily unite 
either of these lines with the sigmoid e and 
With a slight ehange of optical convergence 
pass from the one combination to the other. : 
In each case as might be expected the resultant is a deeply in- 
flected doubly-curved line in a perspective attitude. ; 
_ Even when the bent line has the rapid flexure represented Pos 
j 80. i Z 
¢ 
4 ? 
Setved that while we readily suc- “ " = * | 
ceed in combining } with the whole length of ¢, excepting a 
short distance at each end.gve cannot at any one time produce 
“1 entire union of a with ce. We may however by se 
efforts combine the two upper or the two lower halves of these 
lines successively. This difference is explained by the fact that 
the distances between the corresponding points of 6 and c do not 
