208 The Bird 
the surface of the retina to the optic centre, sees or knows 
anything about it, being simply the apparatus through 
which the bird looks, sees, and knows. In this Class of 
vertebrates the optic organs, both cerebral and ocular, 
are of great size, power, and effect; their vision far tran- 
scends that of man, unaided by artificial instruments, in 
scope and delicacy. The faculty of accommodation, that 
is of adjusting the focus of vision, 1s developed to a marvel- 
lous degree; rapid, almost instantaneous changes of the 
visual angle being required for distinct perception of 
objects that must rush into the focal field with the 
velocity at least of the bird’s flight. Observe an eagle 
soaring aloft until he seems to us but a speck in the blue 
sky expanse. He is far-sighted, and, scouring the earth 
below, descries an object much smaller than himself, 
which would be invisible to us at that distance. He 
prepares to pounce upon his quarry; in the moment re- 
quired for the deadly plunge he becomes at once near- 
sighted, seizes his victim with unerring aim, and sees well 
how to complete the bloody work begun. A humming- 
bird darts so quickly that our eyes cannot follow him, 
yet he instantaneously settles as lightly as a feather upon 
a tiny twig. How far off it was when first perceived 
we do not know; but in the intervening fraction of a 
second the twig has rushed into the focus of distinct 
vision, from many yards away. A  woodcock tears 
through the thickest cover as if it were clear space, avoid- 
ing every obstacle. The only things to the accurate per- 
ception of which birds’ eyes appear not to have accom- 
modated themselves are telegraph-wires and light-houses; 
