IN NATURE, vol. xlvii. p. 303, you mention that “a beautiful optical phenomenon, which has not yet been satisfactorily explained, is described by M. F. Folie in the Bulletin of the Belgian Academy.” From what follows, it is evidently the same as that described in Tyndall's “Glaciers of the Alps” (Murray, 1860, p. 177 et seq. Tyndall gives a description of it in a letter from Prof. Necker to Sir David Brewster, from which I quote the following:—“You must conceive the observer placed at the foot of a bill between him and the place where the sun is rising, and thus entirely in the shade; the upper margin of the mountain is covered with woods, or detached trees and shrubs, which are projected as dark objects on a very bright and clear sky, except at the very place where the sun is just going to rise; for there all the trees and shrubs bordering the margin are of a pure and brilliant white, appearing extremely bright and luminous, although projected on a most brilliant and luminous sky. You would fancy you saw these trees made of the purest silver.”