Help wanted!

Looking for something else this morning in my father's old writing bureau, I came across this in one of the small drawers. I have no idea what it is, and never saw him using it, but I suspect it may have something to do with surveying possibly. Is there anyone out there who has a clue what it is? T is made of brass, is about two inches in diameter, has three legs with fine screws to adjust its positioning, and a very reflective black mirrored surface...