Same as E+25+30, using Hall's prism.

In the Hall's Prism Apparatus, the hypotenuse of a small 90 deg prism A that has a slight positive curvature is sandwiched against the flat side of a larger 90 deg prism B. The interference fringe pattern produced in the air film between the glass sides is concentric rings. Transmission Case: Collimated white light from the carbon arc passes through both prisms and is focused on a projection screen by lens 1. The center of the fringe ring pattern is bright. Reflection Case: Collimated white light from the carbon arc reflects off the abutted glass prism sides and reflects again from a front surface mirror, then is focused on a projection screen by lens 2. The central spot (where the two glass surfaces touch) is dark. There, light reflected from the flat surface changes phase by 180 deg , whereas light internally reflected from the curved surface does not. Same as E+25+30, using Hall's prism. Ref.: Modern College Physics by Harvey White,6th Ed., p.546-548