Hopefully this isn't too far off Tolkien, if it is, I'll delete it, but it's pretty fascinating info. (Guess where I saw it?...Mythbusters
)
Pirates possibly wore eyepatches so they could protect one eye from the brutal sun, and when they go underdeck/or at night they switch the eye patch over and can see at night. I tried it and it actually works, it's like natural human night vision, very cool. Just get an eye patch, and at night, turn on the lights in your house, make it nice and bright for about half an hour, then make it pitch black, switch the patch over...bingo nightvision in one eye. You can see everything! It's actually more like wight-vision. It all has something to do with rods and cones in your eyes.
So, don't give those fancy elves credit for everything, we can see very well at night too!
hmm...wonder if Tolkien as aware of this bit of info.