With the French physicist Augustin Jean Fresnel (1788–1827), he established the wave theory of light and studied its polarization. In 1811 he invented the polariscope for measuring the degree of polarization of light. This instrument allows light from solid and fluid surfaces to be distinguished, and Arago used it in several astronomical studies. One was an extensive study of the total solar eclipse of 1842, in which he determined that the Sun’s limb is gaseous, and examined polarized light from the chromosphere and corona.