W. B. Rogers on Binocular Vision. 89 
drawings but the vertical line and the figure to be combined with 
it, and then effecting the union of these two, by cross vision, in 
front of the paper. 
25. Union of plane figures of unequal horizontal dimensions, 
We have already seen that two rectangles of the same height 
but of different breadth may be united into a single quadrilateral 
figure (20—fig. 44) having a perspective position and of which 
the near vertical side is shorter than the remote one. - With the 
triangles, fig. 63 and fig. 64, the coincidence and the perspective 
effect are very beautifully exhibited. 
63, 
_ In this case the resultant triangle turns the left extremity of 
its base towards the observer and averts the other, when the com- 
bination is effected by convergence beyond the plane of the paper ; 
and it takes the reverse attitude of relief when the eyes are con- 
verged to a point nearer than the plane of the paper. In fig. 
4 the horizontal inequality or difference of bases is th of an inch, 
t in fig. 64 it is 4th inch; and as might be expected in the latter 
case, a very slight pause of the view at either angle of the base 
causes the components of the side opposite to separate as two 
4 Vertical height, that is, should lie between the same horizontal 
> 
_ and_D, intended to be 
United with it, differ 
ely from one another. - 
Supposing in each case |/ 
that the union iseffected © 
Sconp Sznres, Vol. XXL, No. 61.—Jan., 1856. Pee 
oie 
