56 COSMOS. 



The visibility of distant objects ii, modifier' by the absorp- 

 tion of the rays passing from the terrestrial object to thf 

 naked eye at unequal distances, and through strata of air 

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

 ure ; by the degree of intensity of the light diffused 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 difference of -^-^1^ in the intensity of the light is necessary 

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

 only negatively see mountain-tops from wliich 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 difference 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, Avhite 

 chalk cliffs, 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 Teneriffe might 

 be seen. 



The question whether stars can be seen by daylight 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 avx Regions Equinox., torn, 

 i., p. 92-97; and Bouguer, Traite d'OpUque, p. 360 and 365. (Cora- 

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

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



t The passage hi Aristotle referred 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 the 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 distinguisli differences of color, although he will see 

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

 or cisterns are occasionally enabled to see stars J'^ The Grecian 'Oovyua- 

 ra, and more especially (ppeara, are, as an eye-witness, Professor Franz, 

 observes, subterranean cisterns or reservoirs which communicate with 

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

 tom, like the neck of a bottle. Pliny (lib. ii., cap. 14) says, " AltitudD 



