2o8 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, is developed to a marvel- 

 lous degree; rapid, almost instantaneous changes of the 

 visual angle being required for distinct perception of 

 ol)jects that must rush into the focal field with the 

 velocit}' 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 deadh' plunge he becomes at once near- 

 sighted, seizes his victim wuth unerring aim, and sees well 

 how to complete the bloody w-ork l)egun. A humming- 

 bird darts so quickly that our eyes cannot follow him, 

 yet he instantaneousl}^ settles as lightly as a feather upon 

 a tiny twag. 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; 



