EDITORS INTRODUCTION. 



of its aim. The eye of these birds must therefore be constructed after the plan of a telescope, and 

 its focus adapted to long-sightedness. Its axis must be lengthened to an extent greater than is 

 compatible with a spherical form of the eye-ball. To meet this requirement a circlet of bony plates, 

 constituting a firm but at the same time somewhat flexible ring or hoop, is introduced into the 

 composition of the outer coat of the eye, whereby the requisite elongation is effected, and the organ is 

 thus adapted for perfect vision at a great distance. (See Fig. 4.) 



The above beautiful arrangement, however, constitutes but a part of the mechanism required. 

 A telescope adjusted for distant vision 

 is quite useless when brought to bear 

 upon an object close at hand, and its 

 focus must necessarily be altered in 

 accordance with the changed con- 

 ditions. In the case of the tele- 

 scope, the needful adjustment would 

 be effected by shortening or length- 

 ening the sliding tube ; but in the 

 bird some other plan is evidently- 

 indispensable, and few contrivances 

 in animal mechanics are more admi- 

 rable than that which is adopted. 

 Embedded in the transparent vitreous 

 humour of the eye is a peculiar appa- 

 ratus called the " marsupium," the 

 texture of which resembles that of 

 the human iris. Now the iris, as we 

 all know, being eminently sensitive 

 to the intensity of light, by its spon- 

 taneous contractions and dilatations 

 is enabled to alter the diameter of 

 the pupil of the eye, and thus exactly 

 control the quantity of light admitted. 

 The marsupium, equally sensitive, 

 and equally spontaneous in its action, 

 swells or contracts its dimensions, fill- 

 ing or emptying itself like a sponge, 

 and thus adjusting the lenses of the eye so as to secure perfect vision at whatever distance the object 

 to be seen may be placed. The quickness of sight with which birds are gifted is equally remarkable. 

 The swallow is proverbially one of the swiftest flyers in the feathered creation, and yet in the full 

 career of its flight it is looking on the right hand and on the left, upwards and downwards, for its 

 food. The insects upon which it preys are often exceedingly minute, sometimes flying above and 

 sometimes below the level of the swallow's course, and yet they are seen and captured without any 

 diminution of the prodigious rate at which the bird is flying. Nay, more, any one who attentively 

 watches one of these birds skimming over a meadow, may perceive that it will capture two or even 

 three insects in such quick succession as to convince him that the swallow must have " had an eye 

 upon them " all at once, and yet they are caught, as it were, in a moment. 



Another admirable contrivance peculiar to the feathered race, is the existence of a thin, 



Fig 



-SECTION OF THE HEAD OF AN EAGLE, SHOWING THE 

 STRUCTURE OF THE EYE. 



