The instrument consists of an L-shaped wooden board carrying a rotating brass circle fitted with indexes and viewers. Between the wood and the circle is a brass plate with a ring to suspend the instrument. To the back of the board is attached a brass arm with a support for a mirror or prism that reflected the light. By projecting a light ray through the diaphragm onto the mirror, one could observe that the ray was reflected by an angle equal to the angle of incidence.