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.
Graddy reports six cases, and many others are constantly
occurring.
Other forms of retinal injury from too great or too prolonged
exposure to light are "moon-blindness," due to sleeping with the
eyes exposed to bright moonlight, and that due to lightning--a
case, e.
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