2,66 Mr. Smith's Observations on the 
rendering the eye more convex, enlarge the angle under which 
the object is seen. How necessary is this structure to these 
animals in particular ; for without it a bird would be conti- 
nually exposed to have its head dashed against a tree when 
flying in a thick forest, its motions being too rapid for the 
common structure of the eye. The eagle, when soaring high 
in the air, observes small objects on the earth below him, in- 
conceivable to us, and darts upon them instantaneously. Here 
we must allow that there must be an extraordinary alteration 
in the focus in this eye, in almost an instant of time. How 
could this be performed unless the animal had this apparatus? 
The eyes of quadrupeds, as I shall afterwards shew, can per- 
form this alteration, though not in the same degree, as it is 
not necessary, their modes of life being different. A swallow 
sailing through the air pursues a gnat or small fly to almost 
certain destruction. This apparatus is very distinct in all these 
birds. Wherever we find the subsistence or safety of an ani- 
mal intrusted to, or depending more particularly upon one 
sense than the rest, we are sure to find that sense proportion- 
ably perfect ; as in quadrupeds the organ of smelling is re- 
markably perfect, and leads them to their prey, so the eyes of 
birds are proportionably perfect, f>eing 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 coats, 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 it approaches 
the cornea becoming suddenly flat. The cornea, though small 
when compared with the size of the whole eye, is more convex. 
