au moyen de quoi les équations (16) se changent dans les suivantes :
![{\displaystyle {\begin{aligned}&-{\frac {\operatorname {d} ^{2}t}{\operatorname {d} x^{2}}}=2k^{2}P\left({\frac {\operatorname {d} t}{\operatorname {d} x}}\right)^{3},\\\\&{\frac {\operatorname {d} ^{2}y}{\operatorname {d} x^{2}}}{\frac {\operatorname {d} t}{\operatorname {d} x}}-{\frac {\operatorname {d} y}{\operatorname {d} x}}{\frac {\operatorname {d} ^{2}t}{\operatorname {d} x^{2}}}=2k^{2}Q\left({\frac {\operatorname {d} t}{\operatorname {d} x}}\right)^{3},\\\\&{\frac {\operatorname {d} ^{2}z}{\operatorname {d} x^{2}}}{\frac {\operatorname {d} t}{\operatorname {d} x}}-{\frac {\operatorname {d} z}{\operatorname {d} x}}{\frac {\operatorname {d} ^{2}t}{\operatorname {d} x^{2}}}=2k^{2}R\left({\frac {\operatorname {d} t}{\operatorname {d} x}}\right)^{3}.\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d93648869ae0371dce4567e28e34282838cb5345)
Éliminant
des deux dernières, au moyen de la première elles deviennent, en divisant par \frac{\operatorname{d}t}{\operatorname{d}x},
![{\displaystyle {\begin{aligned}&{\frac {\operatorname {d} ^{2}y}{\operatorname {d} x^{2}}}=2k^{2}\left(Q-P{\frac {\operatorname {d} y}{\operatorname {d} x}}\right)\left({\frac {\operatorname {d} t}{\operatorname {d} x}}\right)^{2}\\\\&{\frac {\operatorname {d} ^{2}z}{\operatorname {d} x^{2}}}=2k^{2}\left(R-P{\frac {\operatorname {d} z}{\operatorname {d} x}}\right)\left({\frac {\operatorname {d} t}{\operatorname {d} x}}\right)^{2}\,;\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/b3c876f02f188e342c3c61114fa3fc2bddba0fb9)
mais, dans l’hypothèse actuelle, l’équation (13) devient
![{\displaystyle 1+\left({\frac {\operatorname {d} y}{\operatorname {d} x}}\right)^{2}+\left({\frac {\operatorname {d} z}{\operatorname {d} x}}\right)^{2}=\left(w^{2}+4k^{2}u\right)\left({\frac {\operatorname {d} t}{\operatorname {d} x}}\right)^{2}\,;}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d74a394578b3d3c62349f1a696600b8ac50283f3)
éliminant donc
des deux précédentes, au moyen de cette dernière, on obtiendra, pour les deux équations différentielles de la trajectoire décrite,
![{\displaystyle \left.{\begin{aligned}&\left(w^{2}+4k^{2}u\right){\frac {\operatorname {d} ^{2}y}{\operatorname {d} x^{2}}}=2k^{2}\left(Q-P{\frac {\operatorname {d} y}{\operatorname {d} x}}\right)\left\{1+\left({\frac {\operatorname {d} y}{\operatorname {d} x}}\right)^{2}+\left({\frac {\operatorname {d} z}{\operatorname {d} x}}\right)^{2}\right\},\\\\&\left(w^{2}+4k^{2}u\right){\frac {\operatorname {d} ^{2}z}{\operatorname {d} x^{2}}}=2k^{2}\left(R-P{\frac {\operatorname {d} z}{\operatorname {d} x}}\right)\left\{1+\left({\frac {\operatorname {d} y}{\operatorname {d} x}}\right)^{2}+\left({\frac {\operatorname {d} z}{\operatorname {d} x}}\right)^{2}\right\}\,;\end{aligned}}\right\}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/61d03135415c136931da0af03496219cc3aebc55)
(27)
mais il sera communément plus simple de recourir aux équations (16).
Dans un prochain article, nous nous occuperons proprement du phénomène du mirage.