Editing Augustin-Louis Cauchy (section)

===Wave theory, mechanics, elasticity===
In the theory of light he worked on [[Augustin-Jean Fresnel|Fresnel's]] wave theory and on the [[dispersion (optics)|dispersion]] and [[polarization (waves)|polarization]] of light. <!-- In [[optics]], he developed the wave theory, and his name is associated with the simple dispersion formula. Exactly which formula? --> He also contributed research in [[mechanics]], substituting the notion of the continuity of geometrical displacements for the principle of the continuity of matter.<ref>{{cite book |last=Kurrer |author-link=Karl-Eugen Kurrer |first=K.-E. |date=2018 |title=The History of the Theory of Structures. Searching for Equilibrium |location=Berlin |publisher=[[John Wiley & Sons|Wiley]]|pages=978–979 |isbn=978-3-433-03229-9}}</ref> He wrote on the equilibrium of rods and elastic membranes and on waves in elastic media. He introduced a 3 × 3 symmetric [[matrix (mathematics)|matrix]] of numbers that is now known as the [[Cauchy stress tensor]].{{sfn|Cauchy|1827|p=42|loc="''De la pression ou tension dans un corps solide''" [On pressure or tension in a solid body]}} In [[Elasticity (physics)|elasticity]], he originated the theory of [[stress (physics)|stress]], and his results are nearly as valuable as those of [[Siméon Poisson]].{{sfn|Chisholm|1911}}