Hence the luminous appearance of the eyes of such animals in the dark.
Hemeralopia (etymologically day-blindness, but by common usage meaning day-vision or night-blindness) is a symptom of a peculiar degenerative disease of the retina, called retinitis pigmentosa.

It also occurs in some cases of extreme denutrition, numerous cases having been reported among those who make the prolonged fasts customary in the Russian church.

In retinitis pigmentosa the peripheral or extramacular portions of the retina are subject to a pigmentary degeneration that renders them insensitive to light, and patients so afflicted are consequently incapable of seeing at night as well as others.

They stumble and run against objects easily seen by the normal eye.
Snow-blindness occurs from prolonged exposure of the eyes to snow upon which the sun is shining.

Some years ago, some seventy laborers, who were clearing away snow-drifts in the Caucasus, were seized, and thirty of them could not find their way home, so great was the photophobia, conjunctivitis, and lacrimation.