bikd's-eye views. 21 



moderate exhibitions of a bird's visual powers. Watch a 

 Huinming-l)ird : 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 oil' 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 we a Sparrow-hawk's power of flight we could not 

 follow him for want of his powers of vision. 



Observe an eao^le circlino- in the air. He is soarinc: 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, eufolds 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 ; 

 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 ; as 

 trusty sailors to their j^osts 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; 



