night-blooming cereus: see cactus.

night school: see vocational education.

night crawler: see earthworm.

night blindness, inability to see normally in subdued light. It is usually a result of vitamin A deficiency. The rod cells, one of two light-sensitive areas of the retina of the eye, are impaired in their capacity to produce a chemical compound called rhodopsin, or visual purple, that is necessary for the perception of objects in dim light. Consequently, the visual threshold, or the minimum intensity of light necessary for sight, is greatly increased. Folk medicine has long recognized the role of the ingestion of liver in alleviating the condition, but it was not until the first quarter of the 20th cent. that vitamin A was identified as the crucial element. Treatment of night blindness consists of the oral or intravenous administration of vitamin A.

midsummer day and midsummer night, names given to the feast of the nativity of St. John the Baptist (June 24) and the preceding night (St. John's Eve, June 23). Because midsummer is about the time of the solstice, it has been associated with solar ceremonies since long before Christianity. Relics of such ceremonies are the bright bonfires and the merrymaking of midsummer night. Formerly it was considered the one night of the year when supernatural beings were about. The importance of this night to love and lovers is undoubtedly a survival of fertility rites. Shakespeare and Mendelssohn are among the masters who have used the festival as a subject for a major work.

Twelfth Night, Jan. 5, the vigil or eve of Epiphany, so called because it is the 12th night from Christmas, counting Christmas as the first. In England, Twelfth Night has been a great festival marking the end of the Christmas season, and popular masquerading parties are typical entertainment.