- 
and Vision by Optic Divergence. 861 
ing itn vision ms must be eliminated as far as possible ; 
and all e t the first may be nearly eliminated by using only 
skeleton rohausee In ordinary stereographs their combined 
effect is usually greater than that due to binocular en xen 
If for convenience we apply the term physical perspectiv 
to the combined effect of the elements enumerated, then that 
of focal and of axial adjustment may be called physiological 
perspective. The latter might be regarded as mathematical if 
the theory set forth at the beginning of this paper were strictly 
applicable in all cases. It is well known roan focal adjustment 
does not vary sensibly for distances of more than 6”, and that 
its effect is greatest just beyond the near limit of “dtetitved 
vision, which is also about the average distance at which a 
stereoscope wey is held when regarded. It is also well known 
that in normal binocular vision, the convergence “of axes does 
or contraction of the exterior rectus, or y both, while the 
focal mine is still adapted to the distance of the object in 
fro as near-as convenient. The judgment of distance 
which results pie the conflict of elements produced by this 
unusual codrdination of muscular actions is necessarily by no 
means mathematical in accurac 
ectly apprehended.” combining stereoscope sicphide ty 
axial divergence, ite with or without the instrument, I secur 
vision so ¢l no defect is appreciable at any point how- 
ever carefully acces: it does not seem necessary then to 
assume that any parts of the sensation are neglected. The 
on & Phyticlogicat Optics nes x —- we compare a ‘stereoscopic 
* This Journal, III, vol. ix 
+ Popular Lectures on Scienti a iboats Ist series, p. 307. 
t Optic Physiologique, p. 828, edition 1867. 
