Five years after the publication of his Horologium Oscillatorium, Huygens described his wave theory of light. Though proposed in 1678, it was not published until 1690 in his Traité de la Lumière. His mathematical theory of light was initially rejected in favour of Newton's corpuscular theory of light, until Augustin-Jean Fresnel adopted Huygens' principle to give a complete explanation of the rectilinear propagation and diffraction effects of light in 1821. Today this principle is known as the Huygens–Fresnel principle. As an astronomer, Huygens began grinding lenses with his brother Constantijn jr. to build telescopes for astronomical research. He was the first to identify the rings of Saturn as "a thin, flat ring, nowhere touching, and inclined to the ecliptic," and discovered the first of Saturn's moons, Titan, using a refracting telescope.