Answer:
I am whole but incomplete. I have no eyes, yet I see. You can see, and see right through me. My largest part is one fourth of what I once was.
What looks back but cannot see?
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
A little pool with two layers of wall around it. One white and soft and the other dark and hard. Amidst a light brown grassy lawn with an outline of a green grass.
Often held but never touched. Always wet but never rusts. Often bits but seldom bit. To use it well you must have wit.
If lightning strikes an orchestra who is the one most likely to get hit?