and Experiments on Vision. 389 
remains sound. These facts are generally, but in my opinion 
erroneously, attributed to an immediate sympathy between 
the pupils. For when the pupil of one eye becomes dilated 
from the application of Belladonna, the pupil of the other, so 
far from dilating, becomes smaller. It follows, therefore, that 
the size of the pupil is dependant, not only on the impression 
of light on the retina of its own eye, but on that also which is 
made on the retina of the other, and that the moving of the 
two together, which for the most part takes place, is only an 
accidental consequence of the fact which I have mentioned. 
2. As the action of the external muscles of the eye has been 
frequently resorted to, for an explanation of its capacity to 
see objects perfectly at different distances, I requested Dr. 
Cutting to attend to this matter. He accordingly ascertained, 
while his eye was in its natural state, the distance from his 
face of the nearest point, at which he could make the two 
optic axes meet, this being the greatest trial of strength, to 
which those muscles can be exposed. Shortly after, he re- 
peated the experiments, while, in consequence of the applica- 
tion of Belladonna, he was without the power of adapting his 
eye to different distances, and found, that the strength of 
those muscles was not diminished. It follows, therefore, not 
only that the external muscles have little or no concern in 
fitting the eye to see distinctly at different distances, but that 
the same is true with respect to the cornea, as we cannot sup- 
pose, that its mechanical properties were altered by the Bella- 
donna, or at least, that it became more inflexible from the 
application to it of the juice of that herb. I had before made 
a similar experiment on myself, by comparing what had been 
the strength of the external muscles of my eyes twenty years 
