NATURAL PHILOSOPHY. 155 



of the interference of light, the twinkling of the stars in telescopes, 

 M. Arago finds no difficulty in applying the same explanation to ob- 

 servations with the naked eye. He then proceeds to show how the 

 twinkling of the planets may be accounted for by the same principle, 

 and he concludes the exposition of his views on this interesting sub- 

 ject by suggesting three different modes of measuring the scintillation 

 of a star. We shall confine ourselves to a brief notice of the first of 

 these scintillometres, as he terms them, which is unquestionably the 

 most ingenious in its conception, and the most conclusive as regards 

 the character of its results. It depends on an experiment, which is 

 originally due to M. Arago himself, and which he first gave an account 

 of in the year 1824. If a diaphram be placed before the object-glass 

 of a telescope, so as to allow the light to pass through a circular aper- 

 ture, and if a star be observed with the telescope when the eye-piece 

 is in the position of distinct vision, the image of the star will resemble 

 a vascillating disc of light surrounded by alternate dark and bright 

 rings. If the eye-piece be now gradually pushed in, there is a second 

 position, in which the luminous disk in the center will be replaced by 

 a black hole surrounded by alternate bright and dark rings. By con- 

 tinuing to push the eye-piece towards the focus a third position will 

 be found, in which the image will resemble that observed in the first 

 instance ; and thus a constant recurrence will take place as the eye- 

 piece is pushed forward, the image alternating between a luminous 

 center surrounded by dark and bright rings, and a central dark hole 

 surrounded by bright and dark rings. Now, to determine the second 

 position of the eye-piece, viz : that in which the image of the star ex- 

 hibits a black hole in its center, instead of observing the star directly, 

 the eye-piece may be placed exactly midway between the first and 

 third positions. If the telescope be now directed to a star which 

 twinkles, the phenomenon will manifest itself in a succession of acci- 

 dental reappearances of a luminous point in the dark hole, and these 

 reappearances will be more numerous as the twinkling is stronger. 



M. Arago states, that although he feels convinced that in connecting 

 the phenomena of scintillation with the principle of interferences, he 

 has viewed the subject in its proper aspect, still he is far from being 

 of the opinion that nothing further remains to be done. " For exam- 

 ple," says he, " no one of nay acquaintance has connected in an en- 

 tirely satisfactory manner the planetary disks which the stars acquire, 

 and the rings which surround them with the theory of interferences." 

 He adds, that he had been informed that M. Schwerd, a geometer of 

 Germany, had succeeded in this research ; but he was told at the same 

 time that according to the calculations, which were applied exclusively 

 to simple lenses not achromatic, the diameters of the planets when 

 seen in a telescope with a reduced aperture ought to appear enlarged 

 like the stars, a result entirely contradicted by observation. 



