68 J. LeVonie on Binocular Vision. 
Art. VI.—On some phenomena of Binocular Vision ; by JoSEPH 
ed ae Prof. Chem. and Geol. in University of South 
Caro 
L Aggucnnts of the eye. 
therefore produce a perfect image, on the retina. The 
or binocular adjustment is necessary for single vision : the sec- 
ond or focal adjustment is necessary for distinct vision. The 
first is distinctly sensible for all distances within 100 yards and 
erhaps for much greater distances : the second is scarcely if 
at all ‘eotnible for distances beyond two yards 
To the two adjustments mentioned above ‘may be added a 
third, viz: contraction of the pupil. The design of the con- 
traction of the pupil is probably to increase the clearness of 
definition of the retinal image by cutting off the most diver- . 
gent rays from very near objects, and thus to decrease the spher- 
ical aberration which is not entirely corrected in the eye | 
by the form of the lens. The pupil, however, also contracts | 
involuntarily under the stimulus of strong light, without re- | 
gard to distance. This must be carefully distinguished from 
= fis gas contraction, which is to some extent at least vol- 
pte three adjustments of the eye, viz: binocular or axial | 
adjustment, focal adjustment and contraction of the pupil are | 
associated in every voluntary act of sight. They are accom- | — 
plished by one act of volition. They are so intimately asso- 
d that they cannot be voluntarily —— It is usu- 
ally impossible to converge the optic axes on any point, with- 
prs at the same time adjusting the lens and contracting the pu- 
@ manner saitable for perfect vision at that distance. 
Buch oe associated movements are called consensual 
= : ccna bites ges easly a a this D te 
transference of attention _ near to agate 
