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 conditions. In the case of the telescope, the needful adjustment would be effected by shortening or lengthening the sliding tube; but in the bird some other plan is evidently indispensable, and few contrivances in animal mechanics are more admirable than that which is adopted. Embedded in the transparent vitreous humour of the eye is a peculiar apparatus 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 spontaneous 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, filling 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. [Illustration: Fig. 5.--SECTION OF THE HEAD OF AN EAGLE, SHOWING THE STRUCTURE OF THE EYE.]