582 BIRD’S-EYE VIEWS. 
moderate exhibitions of .a bird’s visual powers. Watch a 
Humming-bird : it darts away so swiftly that our eyes cannot 
follow it, and settles, light as a feather, upon a twig. We 
do not know how far off it discovered the twig; but, at 
whatever distance it was first brought into focus, the Hum- 
ming-bird’s eye adjusted itself during the fraction of a 
second that the bird was flying; and the twig was in focus 
at the instant the bird alighted upon it. Were we to move 
with the same velocity, our eyes would fail us; they could 
not accommodate themselves quick enough. See a Sparrow- 
hawk dash through a thick clump of bushes in headlong 
pursuit of its prey. Think you it rushes blindly, taking the 
chances of escaping the close-set obstacles in its way? It 
sees each stake and branch as it comes on, and avoids them 
all. Had wea Sparrow-hawk’s power of flight we could not 
follow him for want of his powers of vision. 
Observe an eagle circling in the air. He is soaring aloft 
higher and higher, till he becomes, to us, but a speck against 
the blue expanse. As he turns towards the sun, a signal is 
made, and quick as thought obedient servants obey the sum- 
mons. The nictitating membrane, asleep in its corner, starts 
up and spreads over the cornea in an instant: the quivering 
iris, ever on the alert, enfolds the crystalline lens in a close 
embrace ; and the tranquility of the retina is undisturbed. 
As he turns away, the enemy no longer harasses, and the 
guards retire. Now the great bird prepares to scan the 
ground below. His eye lies loosely in its socket; the mus- 
cles relax; the marsupium lies torpid; the lens falls back 5 
the cornea sinks ; the waters retire ; all are quiet. The retina 
alone glows and thrills with excitement. He is now far- 
sighted ; he descries an object on the earth smaller than him- 
self, even from this vast height ; and makes ready for the 
fearful plunge. He poises a moment; the word is given; 3 
trusty sailors to their posts to save the good ship in a storm, 
so rush the sentinels of the eye into action. Down he 
swoops; the muscles tauten, and the waters rush forward; 
