If you find yourself in the high mountains or in the snow in bright sunlight without sunglasses, your eyes will hurt within five minutes, and after a couple of hours, you might even experience “snow blindness.” But in Alaska, this problem was solved 2,000 years ago.
Glasses were typically made of wood, bone, or walrus tusk. The curved shape prevented the glasses from fogging up from your breath. The materials didn’t crack in the cold.
The narrow slit blocks ultraviolet light from the snow while improving distance vision. Previously, soot was applied inside to eliminate residual glare.
Source: alp.org.ua