The author began by recapitulating the observations already made known on this subject.

The facts generally admitted (principally on the authority of M. Arago) appear to be, (1.) That a clear sky reflects light polarized in planes passing through the sun, the eye of the observer, and the point of the sky observed, (2.) That this polarization is a maximum in a zone 90° from the sun. (3.) That in the parts of the sky nearly opposite to the sun, this description ceases to be accurate; for the polarization, in a vertical plane passing through the sun and the observer, vanishes at an angle with the sun considerably less than 180°,—perhaps 150° or 160° (varying according to circumstances),—and gradually reappears in a plane perpendicular to the former, at greater angles than this. (4.) That the polarization is more intense in the neighbourhood of the horizon than of the zenith. (5.) M. Babinet has recently remarked that, under certain circumstances, there is a second neutral point in the neighbourhood of the sun.