As an optical phenomenon the twinkling, or to use the more scientific phrase, the scintillation, of the stars is a matter which has been strangely ignored by physicists. Indeed, the only investigators who seem to have dealt with it in any sort of detail are two Italians, Secchi and Respighi, Dufour, a Frenchman, Montigny, a Belgian, and the Rev. E. Ledger, an Englishman. Secchi has truly remarked that the twinkling of the stars is one of the most beautiful of the minor phenomena of the heavens. Light, sometimes bright, sometimes feeble, sometimes white, sometimes red, darts about in intermittent gleams, like the sparkling flashes of a well-cut diamond, and works upon the feelings of even the most stolid spectator. The theory of twinkling is still surrounded by many difficulties. One thing, however, is certain—it has nothing to do with recurrent changes in the intrinsic light or physical condition of the star itself, but arises during the passage of its rays through our atmosphere; it depends, therefore, in some way or other on the varying conditions of the atmosphere. On the summit of high mountains, according to the observations of all careful observers (notably Tacchini, who studied the subject on Mount Etna), the light of the stars is steady, like that of the planets; and it is so likewise during the hours of calm which often precede terrestrial storms. The vibrations are usually more frequent near the horizon, and diminish with the elevation of the star above the horizon; in other words, with the lessening of the thickness of the atmospheric strata which the rays of light have to traverse. Nevertheless, during windy weather, and specially with northerly wind, it may be noticed that the stars twinkle high up above the horizon, and even as far as the zenith. From these and other similar considerations we are justified in drawing the conclusion that twinkling largely depends on the condition and movements of the atmosphere.