The rhomb has outside angles of 54° and 126°. The light coming into the top of the rhomb was polarized at an angle of 45° to the plane ABCD. Upon reflection at R at Brewster's angle, one of the components of the linearly polarized light is retarded by 45°, and the same thing occurs at F. Thus, one of the components of the light is retarded by 90°, the condition for circularly polarized light.
The double rhomb at the left. below is at Glasgow University and was made by Soliel of Paris before 1850. The one at the right is at the United States Military Academy.
