1869. Cayley, Math. Papers, VI. 334. If there is at each point of the line one and the same tangent plane, then the section of the surface by the tangent plane contains the line at least twice; if it contain it twice only, the line is torsal; if three times the line is oscular, and the tangent plane containing the torsal or oscular line may in like manner be termed a torsal, or an oscular tangent plane.
