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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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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