There are no very special peculiarities in the eyes of reptiles, and we therefore pro ceed to notice the most remarkable points presented by the eye in fishes. From the comparatively great density of the medium (water) through which the rays of light pass before they impinge upon the transparent structure of the eye of the fish, it is obvious that this organ must act as a very powerful refractive apparatus. The main peculiarity in the eye of the fish is the size, extreme density, and spherical shape of the lens, which give it such an extraordinary magnifying power that it has been employed as a simple microscope. See Brewster's Treatise on the Microscope,"p. 31. But its focus being shortened in proportion as its power is increased, it is necessary that the retina should be brought near its posterior surface. For this purpose, the eyeball is Battened by diminishing the quantity of vitreous humor, which being of nearly the same density as the external water, exerts no perceptible power in bringing the rays of light towards a focus; and this flattened form is maintained by the existence of two cartilaginous plates in the tissue of the sclerotic, which in sonic of the larger fishes is actually converted into a bony cup. The aqueous humor having here no refractive power, is barely sufficient to allow the free suspension of the iris. The pupil is very large, so as to take in as much light as possible, but is generally motionless. Their eyes being constantly washed by the water in which they live, no lachrymal apparatus is necessary, nor does any exist; and the same remark applies to the cetacea amongst the mammals. We thus see that throughout the sub-kingdom of the vertebrata the eye is constructed according to one general scheme, with modifications to suit the mode of life of 'individual classes.
In all the above cases, the structure of the eye is essentially the same; that is to say, we have certain dioptric media for collecting the divergent rays to their proper focus on the retina, and we have the means of adjusting the eye for different distances. But if we examine the eyes of insects, we find that they are constructed on different principles.
In these animals, we have simple and compound eyes usually associated in the same individual. The simple eyes resemble in many respects the corresponding organs in higher animals, but the compound eyes are extremely elaborate and complex in their structure. They are two in number, appearing as hemispherical masses on the sides of the head. When examined with the microscope, their surface is seen to be divided into an enormous number of hexagonal facets, which are in fact cornere. In the ant, there are only 50 of these facets in each eye; in the common house-fly, 4,000; in butterflies, upwards of 17,000; and in some of the beetles more than 25,000. Each cornea is found to belong to a distinct eye, provided with a nervous apparatus, and exhibiting a lens, iris, and pupil. Strauss Durckheim, who has carefully studied these structures in the cockchafer, sugmests that, the eyes of insects being fixed, nature has made up for their want of mobility by their number, and by turning them in all directions; so that it might be said that these animals have a distinct eye for every object.
Compound eyes of similar structure occur in many of the crustaceans.
3. Having now described the anatomical structure Jf the eye in man and certain of the lower animals, we are able to proceed to the consideration of the uses of the various parts of this organ. Assuming a general knowledge of the ordinary laws of geometrical optics (see DIOPTRIES, LENS, etc.), we will trace the course of the rays of light proceed
ing from any luminous body through the different media on which they impinge. If a luminous object, as, for example, a lighted candle, be placed at about the ordinary dis tance of distinct vision (about 10 in.) from the front of the eye, some rays fall on the sclerotic, and being reflected, take no part in vision; the more central ones fall upon the cornea, and of these some also are reflected, giving to the surface of the eye its beau tiful glistening appearance; while others pass through it, are converged by it, and enter the aqueous humor, which probably exerts no perceptible effect on their direction. Those which fall on and pass through the outer or circumferential part of the cornea are stopped by the iris, and are either reflected or absorbed by it; while those which fall upon its more central part pass through the pupil, and are concerned in vision. In consequence of its refractive power, the rays passing through a comparatively large surface of the cornea are converged so as to pass through the relatively small pupil and impinge upon the lens, which, by the convexity of its surface, and by its greater density towards the center, very much increases the convergence of the rays passing through it. They then traverse the vitreous humor, whose principal use appears to be to afford support to the expanded retina, and are brought to a focus upon that tunic, there an exact but inverted image of the object.
This inversion of the image may be easily exhibited in the eye of a white rabbit or other albino animal, after removing the muscles, etc., from the hack part of the globe. The flame of a candle held befdre the cornea may be seen inverted at the back of the eye, increasing in size as the candle is brought near, diminishing as it retires, and always moving in a direction opposite to that of the flame.
The adaptation of the eye to distinct vision at every distance beyond that of a few inches, is extremely remarkable, and numerous attempts have been made to explain the , mechanism by which its focal length admits of alteration under the influence of the will. One view that has met with much support is,.that the focal length is modified by a slight movement of the lens. In the eye of the bird 'there is a structure termed the ciliary muscle, which obviously approximates the lens to the cornea when a short field of view is required, and although the corresponding structure is only slightly developed in man and mammals, it is probably sufficiently strong to produce the slight action required; while for the vision of distinct objects the lens is carried back towards the retina by the elasticity of the connecting tissues. It would appear, however, from the recent researches of Cramer, Helmholtz, Allen Thomson, and others, that the accom modation is effected rather by a change in the form than in the position of the lens. It has been experimentally proved, that when the eye is turned from a distant to a near object, the antero-posterior diameter of the lens becomes elongated, and the anterior surface becomes More convex, while the opposite changes take place in turning the eye from a near to a distant object. According to Helmholtz, the radius of curvature of the anterior surface of the lens diminishes on turning the eye to a near object from ten to six millimeters (from about 0.4 to 0.24 of an in.), while the most projecting point of the same surface is brought forward about 0.2 of an inch.