Nyctalopia (Gr., night, a privative, and p eye), night blindness. The disease varies in intensity; in mild and recent cases there being only a greater or less indistinctness of vision after sunset, while in others the patient is entirely unable to distinguish objects by the light of the moon or by artificial light, or even to see a lighted candle placed directly before the eyes. During the day the pupils move naturally, but after nightfall they remain usually dilated and sluggish or motionless. In old cases they are occasionally observed to be contracted. The disease is said to be sometimes congenital and hereditary; more commonly it is produced by continued exposure to the bright light of the sun during the day, particularly when the strength is impaired by over-fatigue, watching, and a faulty diet. It is of common occurrence in warm and tropical climates, particularly among strangers from a more temperate region. It sometimes seems to be produced by the reflection from snow. Avoidance of exposure to excessive light is in general all that is necessary to obtain a cure. If the disease is accompanied by any gastric or other derangement, this should of course be attended to.

When the complaint has proved obstinate, a succession of blisters to the temples has been found beneficial. - The term heme-ralopia, day blindness, has been applied to a defect of vision the opposite to nyctalopia. Beyond the photophobia common, to those who have been long habituated to darkness, to albinos, and to children laboring under strumous ophthalmia, this has no real existence.