GOLDEN EAGLE. 19 



was taken from a fine specimen at the garden of the Zoolo- 

 gical Society, where it had lived for eight years. 



The power of vision in birds is observed to be very extra- 

 ordinary ; and in none is it more consj)icuous than in the 

 Eagles, and the Falconida generally. It has been stated 

 that, probably in the whole range of anatomy, no more perfect 

 adaptation of structure to function could be adduced than is 

 to be found in the numerous and l)eautiful modifications in 

 the form of various parts of the eyes of different animals, 

 destined to exercise vision in media of various degrees of 

 transparency as well as density. The figure on the right 

 hand of the vignette below represents the circle, composed 

 of fifteen bony plates, by which the orb of the eye of the 

 Golden Eagle is supported. These bony plates are capable 

 of slight motion upon each other. The figure on the left 

 hand in the vignette below represents the crystalline lens of 

 the same bird ; the lens being subject to great variety of form 

 in different birds. In the Eagle, the proportion of the axis 

 to the diameter of the lens is as three and eight-tenths to 

 five and seven-tenths ; in the Eagle-Owl, which seeks its 

 prey at twilight, the relati\e proportions of the lens are as 

 six and seven-tenths to seven and eight-tenths ; and in 

 the Swan, which has to select its food under water, the pro- 

 portions of the lens are as three to three and eight-tenths. 

 Birds have also the power of altering the degree of the 

 convexity of the cornea. With numerous modifications of 

 form, aided by delicate muscular arrangement, birds appear 

 to have the power of obtaining such variable degrees of 

 extent or intensity of vision as are most in accordance with 

 their peculiar habits and necessities. 



