フレネルの方程式は19世紀の産物です。18世紀にはありません。
en.wikipedia.org/wiki/Fresnel_equations#History
(抜粋)
In 1821, however, Augustin-Jean Fresnel derived results equivalent to his sine and tangent laws (above), by modeling light waves as transverse elastic waves with vibrations perpendicular to what had previously been called the plane of polarization.