Translations:The eikonal equation - book/2/en

${\begin{aligned}{\frac {dt}{ds}}&{\rm {=}}{\frac {\partial t}{\partial x}}{\frac {dx}{ds}}{\rm {+}}{\frac {\partial t}{\partial y}}{\frac {dy}{ds}}{\ =}\ \mathrm {grad} \ {\textit {t}}\cdot {\frac {d\mathbf {r} }{ds}}.\end{aligned}}$

(13)