AN ae 
hs 
oe 
pupil is apt to 
76 J. LeConte on Binocular Vision. 
| 
the image must be indistinct, for the rays diverge from an en- | 
tirely different distance. But gradually the lenses adjust them- | 
selves to the actual divergence, i. e., for rays diverging from the — 
real object, while the optic axes remain adjusted for the dis- 
tance of the image. The difficulty experienced in dissociating 
these two adjustments causes the interval of indistinctness. | 
The perception of the difference between the image and a real 
object is the sense of this dissociation. Consensual movements — 
have been, perhaps, brought about by the necessities of single 
and distinct vision ; Helmholtz has shown* that other consen- 7 
sual movements may be dissociated when the necessities of — 
single vision require it ; these experiments show that the con-— 
sensual adjustments of the eye may be dissociated when the 
necessities of distinct vision require it, : 
was now anxious to determine what part was taken by the | 
pupil. Is the contraction of the pupil more intimately asso- 
ciated with the axial or the focal adjustment ? This question — 
has been discussed by E. H. Weber, Cramer and Dondersf | 
Weber believes it is directly iated withthe axial adjustment, | 
Cramer and Donders with the focal adjustment, To test this | 
question, while I was obtaining the image and making it clear, © 
an assistant standing behind and a little to one side observed tr 
my pupil reflected in a small mirror conveniently placed. Af | 
ter gazing intently at the real object until the pupil was steady, | 
as soon as I converged the optic axes so as to obtain the image [| 
No. 1, the pupil was observed to contract decidedly, but a8 | 
the image became clear it again expanded to its original siz. 
Again at the moment of obtaining the 2d image the pupil com _ 
tracted still more strongly, but as soon as the image became [| 
clear it again expanded, nearly, if not entirely to its original | 
size, The same phenomena were observed for each of the im- | 
oe only that in the nearest images when the convergence of | 
at the distance of twelve inches until the pupil became steady. | 
So soon as I gazed beyond the object the pupil of course ex | 
panded ; but as soon as the image became clear it again com 
tracted to eee ees original size. In this last experiment the 
: be unsteady. This might have been expected; 
for as we have already said, it is much more difficult to obtail: 
this image clear or to retain it when obtained. 
There is no doubt of the fact, therefore, that the contractit 
* Proc. Roy. Soc. April, 1864.” | : 
+ Donders, accommodation and refraction of the eye, Trans., p. 574. 
