96 COSMOS. 



tations have occasionally been observed by daylight, with 

 the aid of powerful telescopes, as in 1 792, by Flaugergues, and 

 in 1820, by Struve. Argelander (on the 7th of December, 

 1849, at Bonn) distinctly saw three of the satellites of 

 Jupiter, a quarter of an hour after sunrise, with one of Fraun- 

 hofer's five-feet telescopes. He was unable to distinguish 

 the fourth ; but, subsequently, this and the other satellites 

 were observed emerging from the dark margin of the moon, 

 by the assistant-astronomer, Schmidt, with the eight-feet helio- 

 meter. The determination of the limits of the telescopic 

 visibility of small stars by daylight, in different climates, 

 and at different elevations above the sea's level, is alike 

 interesting in an optical and a meteorological point of 

 view. 



Among the remarkable phenomena whose causes have been 

 much contested, in natural as w r ell as in telescopic vision, we 

 must reckon the nocturnal scintillation of the stars. Ac- 

 cording to Arago's investigations, two points must be spe- 

 cially distinguished in reference to this phenomenon 40 



40 The earliest explanations given by Arago of scintillation 

 occur in the appendix to the 4th book of my Voyage aux 

 Regions equinoxiales, torn, i, p. 623. I rejoice that I am able 

 to enrich this section on natural and telescopic vision, with 

 the following explanations, which, for the reasons already as- 

 signed, I subjoin in the original text. 



Des causes de la scintillation des etoiles. 



" Ce qu'il y a de plus remarquable dans le phenomene de 

 la scintillation, c'est le changement de couleur. Ce change- 

 ment est beaucoup plus frequent que 1'observation ordinaire 

 I'indique. En effet, en agitant la lunette, on transforme 

 1' image dans une ligne ou un cercle, et tous les points de cette 

 ligne ou de ce cercle paraissent de couleurs differentes. C'est 

 la resultante de la superposition de toutes ces images que Ton 

 voit, lorsqu'on laisse la lunette immobile. Les rayons qui se 

 reunissent au foyer d'une lentille, vibrent d' accord ou en 

 disaccord, s'ajoutent ou se detruisent, suivant que les couches 



