: 
88 W. B. Rogers on Binocular Vision. 
tial to the full perspective effect. In fig. 62 the 6% 
horizontal lines of course cannot be so combined, 
and yet when the oblique ones are thrown into re- 
lief by their union severally with the opposite parts 
of a these horizontal lines are seen to form parts 
of the resulting perspective figure. As might be 
expected in this case the appearance of relief ceases 
when the eyes are directed to the vertical line 
which forms the base of the three perspective triangles, that i is 
when we maintain fixedly the convergence uniting @ with 6. 
But as soon as the optic axes are allowed to vibrate towards the 
points of the triangles the relief is resumed. a 
The particular case of combination just described is interest- | 
ing from its ene egagey to Bok ee ee referred to by Sir D 
Brewster (Phil. Mag., 1844, 24, p. 442), and in regard to 
which he states a quite different sonalt. Using a figure like the 
above, excepting that the three oblique lines are drawn at a.much 
greater angle with the vertical, he remarks that the line a (A B in 
his figure). will not coalesce with the three obligtie lines at once 
(marked C D in his figure) ; but each separate portion of A will, 
when the two other portions are concealed or removed, coalesce 
with the corresponding portion of C D. On repeating the ex- 
periment with the same figure I find that the binocular combina- 
tion of all three of the oblique lines with the vertical is as readily 
ected as that of any one of them separately. On unitingone = 
of them with the opposite segment of the vertical A, I always : 
observe that the two others at the same moment assume a like 
perspective attitude. Owing to the great horizontal breadth of 
the figure in Brewster’s experiment, the union in either case is 
imperfect, and unless the axes are kept in quicix vibration from 
side to side, the relief disappears. But when, as in fig. 62, the 
oblique lines are less inclined to the vertical, ‘and the breadth of 
the figure much reduced, the perspective resultant for the whole | : 
is at once obtained. It is proper to add that precisely the same 
effects present themselves, whether 1 employ Brewster’s, Wheat- _ 
stone’s, or my own stereoscope, or effect the combination without 
any instrument. 
In the simple experiments above described, where the right line 
is used as one of the objects, there is some thing surprising even — 
to the practised observer, in the transformation wrought upon the 
complex figure as soon as the right line coincides with it. We — 
see it approach ane it touches the figure, then it instantly van- 
ishes as a separate object, and at the same Mae as if by 
magic, the amaale resultant stands before u 
It is well to remember that all these: retites A effects can be 
readily obtained from the- figures (57, &e., to 62) as they stand 
on the page, by first — with a uy ee white paper all the — 
& 
