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The pupils play relatively little part in the adaptation of vision to darkness. Normally, both dilate (in darkness) or constrict (in bright light) together. Try looking in a mirror and shining a bright light into only one eye. Both pupils will normally constrict.

Adaptation to darkness occurs in the cone cells of the retina and most of it takes place in the first ten minutes or so, with further adaptation taking place over the next twenty minutes when for most people adaptation will be complete. The cone cells are most sensitive in the green part of the spectrum and are relatively insensitive to red light, which is why we use red light to view things when we don't want to lose our dark adaptation. Incidentally, this doesn't work well with people who have red-green colour blindness (6 to 10 % of European males) because of their insensitivity to red light.

The retinal adaptation can be lost very rapidly, depending on the light intensity. Many amateur astronomers will be familiar with the person on "open nights" at the observatory who decides to take a flash photo of the sky through the telescope, instantly destroying everyone's dark adaptation. Astronomers have long routinely closed their observing eye when moving into sufficient light intensity to take notes.

Bill Morris

Pukenui

New Zealand