in the "treatise on the conic sections" wallis popularised the symbol ∞ for infinity. he wrote, "i suppose any plane (following the "geometry of indivisibles" of cavalieri) to be made up of an infinite number of parallel lines, or as i would prefer, of an infinite number of parallelograms of the same altitude; (let the altitude of each one of these be an infinitely small part 1/∞ of the whole altitude, and let the symbol ∞ denote infinity) and the altitude of all to make up the altitude of the figure."