Ask anybody why a pirate wears an eye patch and they'll tell you "because his eye got poked out by a sword fight".
But, the real reason lies with nighttime vision versus daylight vision.
Have you ever gone to the movie theater on a very bright sunny day. As you walk into the theater, you realize you can't see. Everything is so dark. Your eyes were adjusted to the daylight and now have to go through a period of adjusting to the dark.
This adjustment is because the receptors in the eyes, the rods and cones, are switching on and off. Daylight vision uses cones. Nighttime vision uses rods. So, there is a period of time where one type of receptor has to get warmed up while the other shuts down. (This is a main reason why vision at dusk is difficult.)
It's believed Pirates wore eye patches so that they could adjust their vision from the bright sun on the top deck, to the vision needed in the dark lower levels of the ship...in particular...where ammunition and cannon's were housed.
The patch would be used to protect the vision in the eye that the pirate would want to use in the darkness of lower levels of the boat. He would merely need to flip up the patch that was over the eye and that eye would be ready to see in the darkness of the underneath.
People who have their eyes examined at the eye doctors notice this phonemenon to some extend. The eye doctor usually shuts off the left eye while testing the right eye. After testing the right eye, the doctor will open up the block over the left eye and block the right. Many times, patients notice that there is an orangish glow or hue when the left eye is opened up. This occurs because the left eye has begun to dark adapt and the cones have to readjust.
So, the next time you see a pirate wearing a patch...just realize they want to be ready to see in the dark!!
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