and Vision by Optic Divergence. 447 
difference whether, at a given moment, the visual axes are con- 
vergent, parallel, or divergent. The combined external image 
e is made to appear nearer me convergence and farther 
by divergence, but this has no perceptible effect upon the ratio 
between the distances of its different parts. Though the dis- 
tinctness in separation between foreground and background is 
greatly enhanced by the slight variation in axial relations that 
is necessitated, the estimation of absolute distance is determined 
results in a judgment not wholly in obedience to eit 
ally the former prevails, but the weakness of the sendon effect 
1s 8 Gti Drs by contrasting it with that obtained by squinting 
thus reversing the sense of the physiological element. The 
Sodigment may be regarded as a compromise rather than an 
independent selection. In vision by divergence, and in vision 
through the stereoscope generally, se? binocular relief is largely 
due to the variable relation between the optic axes, as different 
parts of the stereograph are ane while the "judgment of 
absolute distance is mainly due to physical perspective and com- 
parison with remembered realities ; it is modified by focal adjust- 
ment, and is in practice nearly, but not quite, zrdependent of the 
optic ‘angle. This remark would not apply if the optic angle 
were very large. 
o diagrams can ever pei spac: with, pores accuracy the 
sco 
results. 
In fig. 2, let C and C’ be the centers of two camera ienses 
whose healed axes are as usual parallel, and a pair of sec- 
ondary axes forming an angle, @, in a horizontal ee, are di- 
