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.