W. B. Rogers on Binocular Vision. 215 
be understood as assuming that all the points of the component 
lines must be thus successively combined. It will hereafter, I 
think, be shown that this is neither necessary nor of usual occur- 
rence. But I wish to give prominence to the fact that such a un- 
ion of corresponding poiuts through some portion perhaps or very 
small part of the two lines is requisite to the formation of a per- 
Spective resultant. This view differing almost equally from that 
of Sir D. Brewster and Prof. Wheatstone, will be considered in 
more detail under a future head. ees 
The involuntary and rapid changes of axial convergence within _ 
certain small limits, which as we have seen, forms an interesting 
feature in such observations, does not seem hitherto to 
tracted special attention ; and the fact that the resultant has a 
perspective position even when the lines forming it cross ‘one an- 
other at a considerable angle, has not I believe been referred to 
by previous observers, although as will hereafter appear, it has 
an important bearing on the theory of binocular perpective. 
13. Review of some experiments of Prof. Wheatstone bearing 
on the doctrine of successive vision. : 
n commenting upon the doctrine of successive vision by 
points, Prof. Wheatstone remarks, that were this entirely true 
‘““no appearance of relief should present itself when the eyes re- 
main intently fixed on one point of a binocular image in the ster- 
eoscope. But in performing the experiment carefully, it will be 
found that provided the pictures do not extend too far beyond the 
relief when this condition is fulfilled,” (Phil. Trans.: 1838). 
The result here described is obviously at variance with the state- 
ments above made in regard to the resultant of two inclined lines 
viewed under a fixed convergence of the axes. But without in 
the slightest degree doubting the general accuracy and the great 
Ingenuity of Prof. Wheatstone’s observations, | am compelled 
in the present instance to dissent from his description of the phe- - 
nomena. In multiplied trials with his reflecting stereoscope and 
the refracting one of Sir D. Brewster, as well as with my own 
apparatus, in the course of which 1 have submitted to examina- 
tion most of the linear diagrams commonly used, I have invaria- 
bly found that by converging the axes steadily on one point of 
the image the lines situated laterally near but behind or before it 
In the perspective, are made to appear double, and that by contin- 
uing the same fixed convergence these lines are made to /ose their 
relief aud to take their places in a plane at the same distance as 
€ point to which the eyes are directed. 
hese effects are most striking in the case of figures having 
Considerable depth of perspective and in which the lines and an- 
gles of the posterior surface are nearly covered by those of the 
anterior, as in binocular drawings of polyhedral crystals. Con- 
