NATURAL 
great or Small fly to almost certain 
destruction. This apparatus is very 
distinct inallthese birds. Wher- 
ever we find the subsistence or safety 
of an animal entrusted to, or de- 
pending more particularly upon, 
one sense than the rest, we are sure 
to find that sense proportionably 
perfect 5 as inquadrupeds, the organ 
of smelling is remarkably perfect, 
and leads them to their prey ; so the 
eyes of birds are proportionably 
perfect, being the means, not only 
of their support, but from them 
they receive the first intimation of 
approaching danger. 
The eyes of birds, like those of 
other animals, consist of three coasts, 
the sclerotica, choroides, and retina. 
The human eye, as well as those of 
quadrupeds, is nearly spherical ; in 
birds the sphere is more oblate, the 
sclerotica, as itapproaches the cor- 
nea, becoming suddenly flat. The 
cornea, though small, when com- 
pared with the size of the whole eye 
is more convex, as it forms the seg~ 
ment of a smaller circle added to the 
larger formed by the sclerotica. The 
reason or advantage of this flatness 
is not very evident. Iv prevents 
them, perhaps, from projecting so 
far aS to expose them to danger 
from the trees and grass amongst 
which these animals live. 
After having examined the eyes 
of birds, and seeing this curious ap- 
paratus, | was next led to the exa- 
mination of the eyes of quadrupeds, 
‘that I might see in what manner 
they resemble the eyes of birds, and 
if Tcouid account for their being 
able to accommodate their eyes to 
objects at different distances. 
This was a subject I found in- 
yolved in much difficulty, as the 
eyes of quadrupeds appeared on 
examination not to have these im. 
HISTORY. [*6 
bricated scales which are so obvious 
in birds, but all this difficulty vanish- 
ed on taking hold of one of the four 
recti muscles of the eya of a sheep; 
and by tearing and dissecting, I. 
found that it terminated in, and 
with the other parts composed, the 
cornea ; so that on the first volition 
of the mind, the recti muscles on 
contracting will have the power of 
fixing the eye, and keeping it steady, 
and at the same time by contracting 
more or less, will adapt the focus 
of the eye to the distance of the 
object, but in a less degree than in 
birds. On these muscles giving 
over acting, the eye will be restored 
to its former state by the elasticity 
of the sclerotic coat. 
From a knowledge of these cir- 
cumstances, we may, from rational 
principles, explain, why people, by 
being long aecustomed to view smail 
objects, obtain, in time, a sort of 
microscopic power, if it may be so 
called ; that is, the muscles which 
contract the cornea,‘will, by cus- 
tom, increase their power of action, 
and grow stronger, like the othag 
muscles of the body ; other pheno-~ 
mena of vision on these principles 
may be explained, 
Observations on the Influence which 
incites the Muscles of Animals te 
contract, in Mr. Galvani’s Ex- 
periments, by W. C. Wells, M. D. 
F. R. S. from the same. 
R. Volta, in his letters to Mr. 
Cavallo, which have been 
read to this society, not only has 
shewn that the conclusions, whick 
Mr. Galvani drew from his experi- 
ments on the application of me- 
tals to the nerves and museles of 
animals, are in various respects 
erroneous, 
