white-eye, common name for warblerlike, arboreal birds, including 85 species in the family Zosteropidae, and for certain species of ducks. The members of Zosteropidae, with the exception of a few species, are marked by a ring of tiny, white feathers surrounding the eye and are also known as silvereyes and spectacle birds. They are predominantly olive to yellow-green above, with whitish or grayish abdomens. With the exception of a few species of the genus Zosterops, such as Z. erythropleura of NE China, white-eyes are tropical, dwelling in wooded habitats from sea level to timberline and ranging from sub-Saharan Africa to Asia and Australia. They are typically small, except for several giant species such as two in the South Pacific genus Woodfordia, which also lack the white eye ring. Aided by their short, pointed, slightly decurved bills and brushlike tongues, white-eyes feed on a varied diet consisting of insects, fruits, berries, and nectar. They are much disliked by farmers because of their habit of piercing fruit with their bills. White-eyes are highly gregarious birds, given to constantly calling in a soft, warbling song. They build their deep, cup-shaped nests in tree forks, in which the female deposits two or three white or pale blue eggs. Incubation periods may be as short as 101/2 days, among the shortest known of any bird. The common name white-eye is also given to certain of the unrelated pochards (ducklike birds) of the family Anatidae and especially to the white-eyed pochard (Nyroca ferina). These are probably called white-eyes for their tiny irises set in a large white sclera. Although these white-eyes are worldwide in distribution, they are found primarily in the Northern Hemisphere. N. ferina is found throughout Great Britain and Europe. In its breeding plumage, it has a chestnut-red head with a light gray body bordered in black on the breast and tail. With their webbed feet set far back on their bodies, pochards are poor walkers, but they are among the best of the diving birds. They feed on a variety of aquatic animals, using their muscular tongues as pistons to pump water through their bills. The water is strained through bony plates lining the inner edges of the bill. Their young are especially well developed at birth and rapidly take to water. White-eyes are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Passeriformes, family Zosteropidae; or order Anseriformes, family Anatidae.

seeing-eye dog: see guide dog.

eye fly, common name for a group of small flies of the family Chloropidae. Eye flies are common in parts of California and in the S United States. They inhabit grasslands and meadows and breed mainly on decaying matter and excrement. Eye flies are attracted to animal secretions such as are found in the eyes, mouth, and open wounds. Some are thought to carry diseases such as conjunctivitis; West Indian species may transmit yaws, a disease caused by a spirochete. Eye flies are classified in the phylum Arthropoda, class Insecta, order Diptera, family Chloropidae.

eye bank, site for the collection, processing, and assignment of donated eyes. A donor's eyes are removed as soon as possible after death, sealed in a sterile container, and sent to the eye bank. There they are microscopically examined for corneal damage and then shipped to surgeons who have requested them. The intact eyes, if kept at a temperature of 4°C;, may be preserved up to 48 hours. Subsequently, the corneas (the clear coverings of the eye) can be removed, preserved in glycerin, and stored at room temperature for six to eight months. Corneal transplants may restore vision to persons whose own corneas have become scarred through illness or injury. If free of bacteria, the vitreous humor, the fluid filling at the back of the eye, can be refrigerated and kept up to six months; it is used in the treatment of detached retina. The first eye bank in the United States, Eye-Bank for Sight Restoration, Inc., was founded in New York City in 1945.

eye, organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other arthropods. In an alternate pathway to the one that transmits visual images, the eye perceives sunlight. This information stimulates the hypothalamus, which passes the information on to the pineal gland. The pineal gland then regulates its production of the sleep-inducing chemical, melatonin, essentially setting the body's circadian clock (see biorhythm).

The Human Eye

Anatomy and Function

The human eye is a spheroid structure that rests in a bony cavity (socket, or orbit) on the frontal surface of the skull. The thick wall of the eyeball contains three covering layers: the sclera, the choroid, and the retina. The sclera is the outermost layer of eye tissue; part of it is visible as the "white" of the eye. In the center of the visible sclera and projecting slightly, in the manner of a crystal raised above the surface of a watch, is the cornea, a transparent membrane that acts as the window of the eye. A delicate membrane, the conjunctiva, covers the visible portion of the sclera.

Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (that is, to focus). The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The pupil is the round opening in the center of the iris; it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue.

The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. Between the cornea and iris and between the iris and lens are small spaces filled with aqueous humor, a thin, watery fluid. The large spheroid space in back of the lens (the center of the eyeball) is filled with vitreous humor, a jellylike substance.

Accessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears, keeping the cornea moist, clean, and brilliant, and drainage ducts that carry the excess moisture to the interior of the nose. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions.

Eye Disorders

In addition to errors of refraction (astigmatism, farsightedness, and nearsightedness), the human eye is subject to various types of injury, infection, and changes due to systemic disease. Strabismus is a condition in which the eye turns in or out because of an imbalance in the eye musculature. A cornea damaged by accident or illness can sometimes be corrected by excimer laser or surgically replaced with a healthy one from a deceased person. Experimental retinal implants, consisting of electrode arrays that receive visual data from an external camera, have been used to partially restore sight to persons with damaged retinas, enabling some recognition of shapes, light and dark areas, and motion. Eyes that are used in various ways for surgical repairs are supplied by eye banks. People can arrange to have their eyes donated to such organizations after their death.

Eyes in Other Animals

The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods (such as the squid and octopus), and in some spiders. In each of those groups the camera type of eye evolved independently. In some species, e.g., kestrels, the eye can perceive ultraviolet light, an aid to tracking prey.

Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms (such as the earthworm), mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects. They form no image, or a very poor one.

The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1,000 tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens. The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects (e.g., bees) it can detect the polarization of light. Because arthropods are so numerous, the compound eye is the commonest type of animal eye.

evil eye, principally Sicilian and Mesoamerican superstition, although it is known in other cultures. According to the Native American version, a person who stares fixedly at a pregnant woman or a child or who is too admiring or physically affectionate with children may produce a malicious effect on their lives, whether or not by intent. In rural Sicily any person or animal was considered vulnerable to the evil eye, and many individuals wore protective amulets or charms to nullify its effects.

electric eye: see photoelectric cell.

cross-eye: see strabismus.

compound eye: see eye.

cat's-eye, gemstone that displays a thin band of reflected light on its surface when cut as a cabochon. Its name is derived from its supposed resemblance to the eye of a cat. The optical effect, known as chatoyancy, is caused by the reflection of light from very thin, closely spaced filaments in parallel arrangement within the stone. True cat's-eye, a variety of chrysoberyl from Sri Lanka and Brazil, is the most valuable, but some quartz, tourmaline, and a few other minerals that display chatoyancy are also used as gems. A golden-yellow species called tiger's-eye is a form of quartz that contains crocidolite asbestos.

or tiger's-eye

Semiprecious quartz gem displaying chatoyancy, a vertical luminescent band like that of a cat's eye. Bands of parallel, blue asbestos (crocidolite) fibres are first altered to iron oxides and then replaced by silica. The gem has a rich yellow to yellow-brown or brown colour and a fine golden lustre when it is polished. The best stones come from South Africa. Seealso cat's-eye.

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Organ that receives light and visual images. Non-image forming, or direction, eyes are found among worms, mollusks, cnidarians, echinoderms, and other invertebrates; image-forming eyes are found in certain mollusks, most arthropods, and nearly all vertebrates. Arthropods are unique in possessing a compound eye, which results in their seeing a multiple image that is partially integrated in the brain. Lower vertebrates such as fish have eyes on either side of the head, allowing a maximum view of the surroundings but producing two separate fields of vision. In predatory birds and mammals, binocular vision became more important. Evolutionary changes in the placement of the eyes permitted a larger overlap of the two visual fields, resulting in the higher mammals in a parallel line of direct sight. The human eye is roughly spherical. Light passes through its transparent front and stimulates receptor cells on the retina (cones for colour vision, rods for black-and-white vision in faint light), which in turn send impulses through the optic nerve to the brain. Vision disorders include near- and farsightedness and astigmatism (correctable with eyeglasses or contact lenses), colour blindness, and night blindness. Other eye disorders (including detached retina and glaucoma) can cause visual-field defects or blindness. Seealso ophthalmology; photoreception.

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Superstition holding that a glance can cause injury or death to those on whom it falls. The belief was found in ancient Greece and Rome as well as in folk cultures around the world, and it has persisted into modern times. Children and animals are believed to be particularly vulnerable. The evil eye is often thought to stem from envy and malice toward prosperity and beauty, and thus in many cultures unguarded praise of one's possessions or children is thought to invite misfortune. Safeguards include amulets, charms, and sacred texts; in Asia children may have their faces blackened for protection.

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or photoelectric cell or electric eye

Solid-state device with a photosensitive cathode that emits electrons when illuminated and an anode for collecting the emitted electrons. Illumination excites electrons, which are attracted to the anode, producing current proportional to the intensity of the illumination. In a photovoltaic cell, light is used to produce voltage. In a photoconductive cell, light is used to regulate the flow of current. Photocells are used in control systems, where interrupting a beam of light opens a circuit, actuating a relay that supplies power to a mechanism to bring about a desired operation, such as opening a door or setting off a burglar alarm. Photocells are also used in photometry and spectroscopy.

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Any of several gemstones that display a luminous band reminiscent of the eye of a cat. Grayish green or greenish quartz cat's-eye is the most common type; although it comes from the Orient, it is often called occidental cat's-eye to differentiate it from the rarer, and more valuable, precious or oriental cat's-eye, which is a greenish variety of chrysoberyl. Crocidolite cat's-eye (African cat's-eye) is more commonly known as tigereye. Corundum cat's-eye is an imperfect star sapphire or ruby in which the star is reduced to a luminous zone.

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Painting of a large eye or eyes used to ward off evil. The symbol appears most commonly on Greek drinking vessels from the 6th century BC and was perhaps thought to keep dangerous spirits from entering the mouth with the wine. It is also used in Turkish and Egyptian art.

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