Page "Franz Ernst Neumann" Paragraph 3

Later, Neumann attacked the problem of giving mathematical expression to the conditions holding for a surface separating two crystalline media, and worked out from theory the laws of double refraction in strained crystalline bodies.

He also made important contributions to the mathematical theory of electrodynamics, and in papers published in 1845 and 1847 established mathematically the laws of the induction of electric currents.

His last publication, which appeared in 1878, was on spherical harmonics ( Beitrage zur Theorie der Kugelfunctionen ).