VISIBILITY OF STARS. 75 



admirable observers, the brothers Hermann and Adolph Schla- 

 gentweit, who have recently explored the eastern Alps, as far 

 as the summit of the Gross Glockner, (13016 feet,) were never 

 able to see stars by daylight, nor could they hear any report of 

 such a phenomenon having been observed amongst the goat- 

 herds and chamois hunters. Although I passed many years 

 in the Cordilleras of Mexico, Quito, and Peru, and frequently 

 in clear weather ascended, in company with Bonpland, to eleva- 

 tions of more than fifteen or sixteen thousand feet above the 

 level of the sea, I never could distinguish stars by day-light, 

 nor was my friend Boussingault more successful in his subse- 

 quent expeditions; yet the heavens were of an azure so 

 intensely deep, that a cyanometer (made by Paul of Geneva,) 

 which had stood at 39 when observed by Saussure on 

 Mont Blanc, indicated 46 in the zenith under the tropics at 

 elevations varying between 17000 and 19000 feet. 23 Under 

 the serene etherially-pure sky of Cumana, in the plains near 

 the sea-shore, I have frequently been able, after observing an 

 eclipse of Jupiter's satellites, to find the planet again with the 

 naked eye, and have most distinctly seen it when the sun's 

 disc was from 18 to 20 above the horizon. 



The present would seem a fitting place to notice, although 

 cursorily, another optical phenomenon, which I only observed 

 once during my numerous mountain ascents. Before sunrise, 

 on the 22nd of June, 1799, when at Malpays, on the declivity 

 of the Peak of Teneriffe, at an elevation of about 11400 feet 

 above the sea's level, I observed, with the naked eye, stars 

 near the horizon flickering with a singular oscillating motion. 

 Luminous points ascended, moved laterally, and fell back to 

 their former position. This phenomenon lasted only from 



23 Humboldt, Essai sur la Geographic des Plantes, p. 103. 

 Compare also my Voy. aux Regions equinox., torn. i. pp. 143, 



248. 



