Answer:
Without a bridle, or a saddle, across a thing I ride a-straddle. And those I ride, by help of me, though almost blind, are made to see.
Two little holes in the side of a hill. Just as you come to the cherry-red mill.
There are millions of me. I am very small but when moving fast I am deadly.
I have an eye but cannot see, You'll head inside when you see me.
What comes once in a minute, twice in a moment, but never in a thousand years?
I’m grown from darkness but shine with a pale light. Very round I am and always a lady's delight.