Since the theory of relativity it has been orthodox to treat time alongside the other three spatial dimensions, as the fourth dimension of a unified spacetime. In the late 19th century, physicists such as Helmholtz popularized work of Riemann that suggested that there might be a fourth spatial dimension, into which things might disappear, only to reappear elsewhere (see incongruent counterparts). The idea was taken up by theosophists and workers on parapsychology. It is analysed in the 1884 classic Flatland by Edwin Abbot, and mentioned in work by H. G. Wells and Oscar Wilde. It reappeared in orthodox physics in the work of Kaluza, who showed in 1919 that when the Riemann tensor metric (see relativity) is rewritten in five dimensions, a unified theory of gravity and electromagnetism can be produced. Currently one favoured number of dimensions is ten. See also geometry.