Abstract: A device is disclosed for sensing information relating to the orientation of a selected direction or plane with respect to the horizontal. In one embodiment, the invention comprises a housing (40), a vial (70) mounting in the housing, a source of electromagnetic radiation (62, 64), sensors (66, 68) and associated electronic circuitry, and a rail (12) for positioning the housing. The vial is elongated along a central longitudinal axis and is partially filled with liquid (72) to form a bubble (74) in the vial. The source of radiation is positioned in a housing plane to one side of the vial such that radiation passes through the vial and forms a shadow of the bubble on the other side of the vial. The shadow is detected by the sensors, and the associated electronic circuitry produces an output signal having a characteristic related to the bubble position along the longitudinal axis.