Answer:
My second is performed by my first, and it is thought a thief by the marks of my whole might be caught.
It's equally comfortable in an orchestra and a geometry textbook. What is it?
A shimmering field that reaches far. Yet it has no tracks, And is crossed without paths.
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.
Hoofed frolicking.
What sphinxes employ and players enjoy.