I was hanging a mirror in the living room over the fireplace. I had set my level down when I noticed a back light illuminating the bubble window. These types of levels are known as spirit or bubble levels. The term spirit comes from the fact that the most common liquid used is a yellow colored Ethanol. Spirit levels are employed to indicate how horizontal (level) or how vertical (plumb) a surface is. Canon 7D, 1/60 sec @ f/4.6, ISO 800, 50mm macro lens.