How is it that someone first discovered that they could fix their poor eyesight by looking through a very specific thickness of glass?  Maybe it was an accident.  Maybe they just happened to be wandering around, seeing nothing but a blur, until they walked past this single window pane that was just the perfect thickness.  Maybe the people on the other side of the window didn't bother shutting their curtains because they knew that the person was practically Blind.  Maybe the (otherwise) blind person got an eyeful through the window but didn't tell anyone for a while.

There's something nice about a secret.  Knowing something, but not sharing it with everyone, creates a little slice of the world that is for you.  There are no opinions to deal with.  Everyone always has something to say about everything.  I'll bet the blind guy kept the window thing to himself for ages, up until the point he realised it might be better to see the rest of the world as well.