into the cornea the choroid becomes continuous with the iris, a round curtain, the structure seen through the cornea, differently coloured in different individuals. In its centre is a round hole, the pupil, which appears as if it were a black spot. The iris forms a diaphragm suspended in the space behind the cornea which is filled with clear fluid, the aqueous humour. The iris consists of a framework of connective tissue, and its posterior surface is lined by cells containing pigment which gives the colour to the eye. In its substance are bundles of involuntary muscular fibres, arranged in a ring round the margin of the pupil. In a bright light the circular fibres contract and the pupil is made smaller; but in the dark these fibres relax and allow the pupil to dilate more or less widely, thus allowing more luminous rays to enter the eye. Just behind the pupil is the crystalline lens, resembling a small, very strongly magnifying glass, convex on each side, though more so behind. Minute bands of muscle in the choroid can alter the convexity of the lens, and thus adapt its focus to the position of objects at different distances. The large space in the globe of the eye that intervenes between the retina lining its inner surface and the lens in front is occupied by a transparent jelly-like mass called the vitreous body. The internal surface of the choroid, or rather the pigmentary layer which covers it, is lined by the retina or nervous tunic upon which the images of the objects that we see are focussed. The retina is a very complex mechanism consisting of certain elements which are stimulated by light, and others that transmit the effects of such stimulation as a nervous current to the optic nerve which carries it to the brain. It is composed of cones and cylinders or rods, joined together like the stakes of a palisade, perpendicular to the plane of the membrane, and forming by their free extremities a mosaic, each microscopic division of which is about 0.0008 of a line in diameter according to Helmholtz, and represents a section of a rod. These rods and cones are believed to be the agents by whose aid the waves of light become transformed into the stimulus of a sensation. The ocular globe is put in motion in the orbit by six muscles, grouped two by two, which raise or lower the eye, turn it inward or outward, or on its antero-posterior axis. In these movements the centre of the globe is immovable, and the eye moves round its transverse and vertical diameters. These three orders of movements are independent of each other, and may be made singly or in combination, in such a manner as to direct the pupil towards all points of the circumference of the orbit. Each eye is furnished with two eyelids, moved by muscles, which shield it from too much light and keep it from being injured. They are fringed with short, fine hairs called eyelashes; and along the edge of the lids is a row of glands similar to the sebaceous glands of the skin. The eyebrows, ridges of thickened integument and muscle, situated on the upper circumference of the orbit and covered with short hairs, also regulate to some extent the admission of light by muscular contraction. In reptiles, some fishes (sharks, &c.), in birds, and in some mammals a third eyelid or nictitating membrane is present, and can be drawn over the surface of the eye so as to clear it of foreign matters, and also to modify the light. The lachrymal apparatus is composed of, firstly, the lachrymal gland, which lies in a depression of the orbital arch; secondly, the lachrymal canals, by which the tears are poured out upon the conjunctiva a little above the border of the upper lid; thirdly, the lachrymal ducts, which are destined to receive the tears after they have bathed the eye, and of which the orifices or lachrymal points are seen near the internal ends of the lids; fourthly, the lachrymal sac, in which the lachrymal ducts terminate, that empties the tears into the nasal canal. The tears, by running over the surface of the conjunctiva, render it supple and facilitate the movements of the globe and eyelids by lessening the friction. The influence of moral or physical causes increases their secretion, and when the lachrymal ducts do not suffice to carry them off they run over the lids.
Vision.—The retina renders the eye sensible of light, and we may therefore consider it as the essential organ of vision. The function of the other portions is to focus the luminous rays on the surface of the retina, a condition necessary for distinct vision and the clear perception of objects. The visual impressions are transmitted from the retina to the brain by means of the optic nerve. The two optic nerves converge from the eyes toward the centre of the base of the brain, where there is a partial
interlacement of their fibres in such a manner that a portion of the right nerve goes to the left side of the brain, and a part of the left nerve to the right side; this is called the chiasma or commissure of the optic nerves. The principal advantage of having two eyes is in the estimation of distance and the perception of relief. In order to see a point as single by two eyes we must make its two images fall on corresponding points of the retinas; and this implies a greater or less convergence of the optic axes according as the object is nearer or more remote. To accommodate the eye to different distances the lens is capable of altering itself with great precision and rapidity. When we look at a near object, the anterior surface of the lens bulges forward, becoming more convex the nearer the object; the more distant the object the more the lens is flattened. When the transparency of the cornea, the crystalline lens, or any of the contents of the globe of the eye is destroyed, either partially or entirely, then will partial or total blindness follow, since no image can be formed upon the retina; but although all the media and the cornea be perfectly transparent, and retain their proper forms, which likewise is necessary to distinct vision, yet injury or inactivity of the optic nerve, or injury of the parts of the brain with which it is connected, may produce disturbance of vision or total blindness. Defective vision may also arise from the crystalline lens being so convex as to form an image before the rays reach the retina (a defect known as short sight or myopia), in which case distinct vision will be procured by interposing a concave lens between the eye and the object of such a curvature as shall cause the rays that pass through the crystalline lens to meet on the retina; or the lens may be too flat, a defect which is corrected by convex lenses. In old age, and in fact in most people after about forty-five years of age, the elasticity of the lens becomes reduced, and convex lenses become necessary to make it possible to focus near objects. This condition is known as presbyopia. In the lower organisms the organs of sight appear as mere pigment spots. In higher forms, simple lenses or refracting bodies occur. Insects, crustaceans, &c., have large masses of simple eyes or ocelli aggregated together to form compound eyes—the separate facets or lenses being optically distinct, and sometimes numbering many thousands. In the cephalopods well-developed eyes presenting a distant analogy in structure to those of the highest animals are found; and in all vertebrate animals the organ of vision corresponds generally to what has been described, though they vary much in structure and adaptation to the surroundings of the animal.—Cf. J. Herbert Parsons, Diseases of the Eye.
Eye (ā), a municipal borough, England, county Suffolk, 19 miles north of Ipswich. Up till 1885 it sent a member to Parliament, and it still gives its name to a parliamentary division of East Suffolk. Pop. 1781.
Eyebright (Euphrasia officinālis), a small plant belonging to the nat. ord. Scrophulariaceæ, which is common in Britain and most parts of Europe, in North Asia, &c. It is an annual, half-parasitic on grass-roots, from 3 to 8 inches high, often much branched. The whole plant has a bitter taste. Under the name of euphrasy it formerly enjoyed a great reputation in diseases of the eyes.
Eyemouth, a fishing-town of Berwickshire, Scotland, at the mouth of the Eye, an important place in the thirteenth century. Pop. 2477.
Eye-piece, in a telescope, microscope, or other optical instrument, the lens, or combination of lenses to which the eye is applied. As the use of a single lens limits the field of view, eye-pieces are generally formed from two lenses or lens systems, called the eye-lens and the field-lens, placed at a distance apart. The Ramsden or positive eye-piece consists of two lenses of equal focal length placed at a distance apart equal to two-thirds of the focal length of either lens. The object looked at requires to be a short distance in front of the field-lens. In the Huyghens or negative eye-piece the focal lengths of the field- and eye-lenses are in the ratio 3 to 1, and the distance between the lenses is twice the focal length of the eye-lens. As a result the image formed by, say, the objective of a telescope is in focus when between the two lenses.
Eylau (ī´lou), a small town, about 28 miles distant from Königsberg, in Prussia, famous for a bloody battle fought between Napoleon and the allied Russians and Prussians, on the 7th and 8th of Feb., 1807. Both sides claimed the victory. The loss of the Allies was about 20,000 men, while that of the French must have been considerably greater.
Eyre (ār), Edward John, Australian explorer and colonial governor, born in Yorkshire 1815, died in 1901. He went to Australia in 1833, in 1839 discovered Lake Torrens, and in 1840 explored its eastern shores and the adjacent Flinders Range. He then commenced his perilous journey along the shores of the Great Australian Bight, and reached King George's Sound, in Western Australia, a distance of 1200 miles, with a single native boy, having left Adelaide more than a year before. In 1845 he published Discoveries in Central Australia. After filling several governorships, he was appointed Governor of Jamaica in 1862. In 1865 he was confronted with a negro rebellion, which he crushed with some severity, and was recalled. On his return to England John Stuart Mill