Lastly wo come to the explanation of Arago himself. That keen observer was very much struck with the experiment of Nicholson, who observed Sirius through a telescope, and enured the image to vibrato by tapping on the tube, when it appeared like a coloured luminous ribbon. On applying this experiment to other stars the coloured ribbon was observed with stars up to the sixth magnitude, but no trace of colour was observed with a star of the seventh. Another method of studying scintillation is to direct an achromatic telescope towards a brilliant star and then to move the eyepiece out of focus. Tho image then becomes an Irregular disc approaching a circular form, and of a diameter greater or less according to the position of the eye piece. The disc oscillates as though a number of discs of different colours were in motion. A third method of observing scintillation through a telescope is to contract the aperture by means of an opaque screen with a bolo in the centre placed before the object glass. On viewing the imago of a star out of the focus, and consequently enlarged, its centre will appear to be pierced with a dark but regular hole. Tho opaque screen used by 3L Arago had a central opening of • only three or four centimetres in diameter ; the image of a star at the focus was round and precise, but surrounded with a series of light and dark rings perfectly well defined. The lustre of Guam rings con stantly varied in different parts of their contour, often disappearing, at dertaiu points Under these circumstances, if the eye-piece were gradually pushed inwards, the image of the star would gradually dilate, and a well-defined black spot would appear in the centre. A further motion of the eye-piece inwards caused the dark spot to dilate into a small luminous disc, occupying the centre, surrounded by a dark ring, and this by a larger luminous ring. Or in a third position of the eye-piece, still nearer to the object-glass, the centre of the image is obscure, but surrounded by a largo brilliant ling, succeeded by a dark ring, and this again surrounded by a luminous nng.
All these phenomena plainly refer the phenomena of scintillation to the interference of light. [INTERFERENCE.] Luminous rays proceed
ing from a point, such as a fixed star, and passing through our atmo sphere, subject as it is to great variations in heat, moisture, and density, must have vary unequal velocities ; if such rays be brought to a focus by means of a lens, as they are by the lens of the eye, the effect of their reaching us with different velocities will be seen by the alternate brightening and darkening of the imago in such focus, by tho known laws of interference. If, however, the rays proceed not from a point, but from a disc, such as a planet, the effects world so far dis turb each other as to prevent their beiug observed, or if observed, but imperfectly. 31. Arago has contrived three methods, or instruments (scintillonteterg, as he calls them), based upon the change of the central spot from dark to light, which occurs the more frequently in propor tion as the scintillation is strong ; for example, in preparing a tele scope as a seintillometer as above directed, with a pierced screen before the object-glasa, on directing it to the stars named in the first column of the following table, at the heights above the horizon indicated in the second oolumn, the central point became luminous, in the course of five minutes, the number of times mentioned in tho third column By observations of this kind 31. Arago thinks it possible to decide what are the climates, the seasons, the elevations, and other circum. stances, under which scintillation entirely disappears.
While writing this article, we have received from 31. Andres Poey, Director of the 1'hysice.31eteorological Observatory of Havannali, a Memoir entitled Loi do Is Coloration at do la D6coloration des Etoiles, du Soleil, et des Planetes,' reprinted from the ' Annuaire ' of the Meteorological Society of France, not viiL, 1861. Our space will not allow us to do more than point out one remarkable result obtained by 31. Poey, namely, that when, in viewing a fixed star through a telescope, the eye-piece is drawn out the series of rings obtained are comple mentary in colour to those seen when tho eye-piece is pushed iu towards the object-glass.