in the strictly scientific sense, the permanent and irreversible loss of vision in both eyes, the inability to distinguish light from darkness. In cases of total blindness visual acuity is equal to 0.

In the practical and social sense, blindness has a much broader definition. A person is considered to be blind when he cannot see well enough to orient himself in his surroundings or get around outside his home without help. A blind person may be photoreceptive, that is, able to distinguish light and the direction it is coming from, and may be able to see the outlines of large objects. The legally blind include persons who are practically sightless, possessing extremely low visual acuity (to 0.02). An individual who does not see well enough to work even when wearing glasses is said to be industrially blind. A person whose vision has deteriorated to the point that he cannot do his customary work is considered occupationally blind. For example, individuals engaged in intellectual work are said to be occupationally blind when they can no longer see well enough to read.

According to the World Health Organization (WHO), there were 10 to 15 million blind persons in the world in 1972. The main causes of blindness are glaucoma, trachoma, and traumatism of the eyes. The causes of blindness and the number of blind persons vary from country to country. For example, pox diseases caused many cases of blindness in prerevolutionary Russia, whereas no such incidences have been reported in the USSR. In economically developed countries blindness is primarily caused by progressive myopia, traumatism, or vascular lesions of the retina, including those produced by diabetes mellitus. In developing countries infectious and parasitic diseases are the main causes of blindness. Of the approximately 500 million persons with trachoma, approximately 2 million are blind. The disease is widespread in Asia, Africa, and Latin America. Prior to the October Revolution of 1917,21.5 percent of all incidences of blindness were caused by trachoma. By 1968 trachoma had been eradicated as a major disease in the USSR. In industrial countries, where occupational injuries are fairly common, most of the blind are men.

Blindness can be controlled by preventing certain eye injuries and diseases. In the USSR, for example, glaucoma is diagnosed early, and those suffering from the disease have regular checkups. Recent advances in eye surgery have restored some degree of vision to many blind persons.

The All-Russian Society for the Blind was organized in 1925; similar societies have been organized in other Union republics. A network of specialized secondary and eight-year schools has been established for the education of children who are blind or have poor eyesight. Textbooks, literary works, and magazines are published in Braille. Special sound-recording studios and record and tape libraries have been established.

Blind adolescents and adults receive occupational training at special industrial enterprises organized by societies for the blind. After finishing their studies they may work at these enterprises or at enterprises in other localities.

In capitalist countries philanthropic societies are the chief organizations involved in finding work for the blind. The World Council for the Welfare of the Blind, which works in cooperation with UNESCO and other agencies of the UN, was established in Paris in 1951.