66 COSMOS. 



The visibility of distant objects h modiiieL' by the absoip* 

 tion of the rays passing from the terrestriai object to th«« 

 naked eye at unequal distances, and through strata of an 

 more or less rarefied and more or less saturated with moist- 

 ure ; by the degree of intensity of the fight difiused by the 

 radiation of the particles of air ; and by numerous meteoro 

 logical processes not yet fully explained. It appears from 

 the old experiments of the accurate observer Bouguer that 

 a difierence of -^Qth. in the intensity of the light is necessary 

 to render objects visible. To use his own expression, we 

 only negxtivcly see mountain-tops from which but little light 

 is radiated, and which stand out from the vault of heaven in 

 the form of dark masses ; their visibility is solely owing to 

 the difierence in the thickness of the atmospheric strata ex- 

 tending respectively to the object and to the horizon. Strong 

 ly-illumined objects, such as snow-clad mountains, white 

 chalk clifis, and conical rocks of pumice-stone, are seen 'pos- 

 itively. 



The distance at which high mountain summits may be 

 recognized from the sea is not devoid of interest in relation 

 to practical navigation, where exact astronomical determina- 

 tions are wanting to indicate the ship's place. I have treat- 

 ed this subject more at length in another work,* where I 

 considered the distance at which the Peak of Tenerifie might 

 be seen. 



The question whether stars can be seen by dayfight with 

 the naked eye through the shafts of mines, and on very high 

 mountains, has been with me a subject of inquiry since my 

 early youth. I was aware that Aristotle had maintainedf 



* Humboldt, "Relation Hist, du Voyage anx Rigions Eqtiinox., torn, 

 i., p. 92-97; and Bouguer, Traite d'Optique, p. 3G0 and 365. (Com- 

 pare, also, Captain Beechey, in the Manual of Scieniijic Inquiry for the 

 Use of the Royal Navy, 1849, p. 71.) 



t The passage in Aristotle refeiTed to by BufFon occurs in a work 

 where we should have least expected to find it — De Generat. Animal., 

 V. i., p. 780, Bekker. Literally translated, it runs as follows : " Keen- 

 ness of sight is as much the power of seeing far as of accurately distin 

 guishing the differences presented by the objects viewed. These two 

 properties are not met with in tlie same individuals. For he who holds 

 his hand over his eyes, or looks through a tube, is not, on that account, 

 more or less able to distinguish differences of color, although he will see 

 objects at a greater distance. Hence it arises that persons in caverm 

 or cisterns are occasionally enabled to see stars." The Grecian 'Oovyna- 

 ra, and more especially (^piara, are, as an eye-v^^itness. Professor Franz, 

 observes, subterranean cisterns or reservoirs which communicate witJi 

 the light and air by means of a vertical shaft, and widen toward the bot* 

 torn, like the neok of a bottle. Pliny (lib. ii., cap. 14) says, " Altitude 



