Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/141

 Rh as well as of Turtles, Tortoises, and Lizards; and in a less degree in Crocodiles. (Pl. 10, Figs. 4, 5, 6.)

In living animals these bony plates are fixed in the exterior or sclerotic coat of the eye, and vary its scope of action, by altering the convexity of the cornea: by their retraction they press forward the front of the eye and convert it into a microscope; in resuming their position, when the eye is at rest, they convert it into a telescope. The soft parts of the eyes of the Ichthyosauri have of course entirely perished; but the preservation of this curiously constructed hoop of bony plates, shows that the enormous eye, of which they formed the front, was an optical instrument of varied and prodigious power, enabling the Ichthyosaurus to descry its prey at great or little distances, in the obscurity of night, and in the depths of the sea; it also tends to associate the animal, in which it existed, with the family of Lizards, and exclude it from that of fishes.

A further advantage resulting from this, curious apparatus

assist to maintain the prominent position of the front of the eye, which is so remarkable in birds. In Owls, whose nocturnal habits render distant vision impossible, Mr. Yarrel observes, that the bony circle (Pl. 10, Fig. 4,) is concave, and elongated forwards, so that the front of the eye is placed at the end of a long tube, and thus projects beyond the loose and downy feathers of the head; he adds; "The extent of vision enjoyed by the Falcons is probably denied to the Owls, but their more spherical lens and corresponding cornea give them an intensity better suited to the opacity of the medium in which they are required to exercise this power. They may be compared to a person near-sighted, who sees objects with superior magnitude and brilliancy when within the prescribed limits of his natural powers of vision, from the increased angle these objects subtend." Yarrel on the Anatomy of Birds of Prey, Zool. Journal, v. 3, p. 188.