At length a device described as a prismatic ring was developed, a new
contribution to optical science. In use it is comparable to a solid
glass prism which changes the angle between its sides, giving to a beam
of light passing therethrough a hinged or oscillating action on one side
of the prism while maintaining a fixed axis of the beam on the other
side of the prism.
As a convenience in fabrication this prismatic ring is ground into the
face of a glass disc of suitable size, of selected mirror plate, which
gives the ring its own support on the rotating shaft upon which it is
mounted.