50 BRITISH LAND BIRDS. 



the sight to the selection of the best supplied 

 fishing places.* 



One of the most surprising facts connected with 

 birds of prey is that wonderful acuteness of vision 

 which enables the eagle, for example, when soar- 

 ing aloft amid the clouds, to discern so small a 

 bird as a partridge upon the ground. When 

 looking for its prey the eagle sails in large circles, 

 with its tail spread out, and its wings scarcely 

 moving. Thus it soars aloft in its course, its 

 gyrations becoming less and less perceptible until 

 it dwindles to a mere speck, and is at length lost 

 to view, when it suddenly reappears, rushing down 

 like lightning, and carries off in its talons some 

 unhappy prey. 



The raptorial birds are endowed with a very 

 beautiful modification of the eye in relation to 

 this power of vision. The globe is surrounded 

 with a circle of bony plates, slightly moving on 

 each other, whereby its form is maintained, and 

 the muscles at the back of the eye are so arranged, 

 that, by their pressure, the front of the organ can 

 be rendered very prominent, or they can be quite 

 relaxed, and the front of the eye rendered nearly 

 flat. The first condition fits it for discerning near 

 objects; the second endows it with telescopic 



* Thompson. 



