and Vision by Optic Divergence. 445 
in extent it rarely exceeds 2° or 3°. I have attained 7°, 
and Helmholtz * gives 8° as his limit. Several persons of my 
own acquaintance have been found able to secure divergence 
without the stereoscope, and their estimates of the apparent 
distance, size and motion of the external image eae various 
conditions have not differed much from my own 
n the press of normal binocular vision, the expression 
“point of sight” may be applied theoretically to the intersection 
of optic axes. Its aiepate position, — not mathematically 
determined, may be estimated with more or less error, accordin; 
to the skill of the observer. But in oi disonray the stereoscope 
such a definition has to be totally abandoned. The point of 
sight then is only the point in space to which the observer 
mentally refers the binocular combination of images formed on 
corresponding retinal points, where the visual axes, whether 
tion of lines. In this estimation the relation between the ne 
axes is only one of a number of elements that are combine 
the formation of a judgment, whether vision be normal or ab 
normal, ven if stereographs are selected from which phys- 
ical perspective is in great measure eliminated, the optic angle 
e negative; and, when positive, its effect is still antago- 
nized by the disturbance of codrdination between focal and 
axial adjustments, or by the observer’s unconscious recognition 
of the circumstances under which he has been accustomed to 
he distance of the card remains constant, and tends to keep 
the focal adjustment so, while the eyeballs are rotating outwa 
are in the babit of associating diminution of convergence with 
increase of distance of the object of sight. As long as the eye- 
balls continue rotating outward, therefore, the object appears to 
recede and to enlarge correspondingly, the recession being 
_— Pras ty the change from marked convergence to parallel- 
t does not seem sonable to express this apparent rate 
in bisathiaatiedl terms. 
* Optique Physiologique, p. 616. 
