Abstract: A rotatable memory medium having a memory surface of ellipsoid surface defining two foci. A laser beam is directed to one focus and then reflected to the memory surface, and from the latter to the other focus where a sensor is positioned. The memory surface has a memory track of spiral shape, and the laser beam reflected to the memory surface is so reflected by a T-mirror, and the source of the laser beam remains stationary. The ellipsoid memory surface assumes a plurality of different shapes, such as one-half of an ellipsis on one side of a plane containing both foci, or generated about either an internal axis or an external axis.