PRISMATIC RING:
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.