\Bibitem{Rum53} \by Yu.~B.~Rumer \paper The optical-mechanical analogy \jour Russian Math. Surveys \yr 1953 \vol 8 \issue 6 \mathnet{http://mi.mathnet.ru//eng/rm8264} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=62633} \zmath{https://zbmath.org/?q=an:0053.47006}