32 DISTANCE AT WHICH MOUNTAINS 



vested with perpetual snow. The Sugar-loaf which 

 constitutes the summit of the former no doubt re- 

 flects a great degree of light, on account of the white 

 colour of the pumice with which it is covered ; but 

 its height does not form a twentieth part of the total 

 elevation, and the sides of the volcano are coated 

 with blocks of dark-coloured lava, or with luxuriant 

 vegetation, the masses of which reflect little light, 

 the leaves of the trees being separated by shadows 

 of greater extent than the illuminated parts. 



Hence the Peak of Teneriffe is to be referred to 

 the class of mountains which are seen at great dis- 

 tances only in what Bouguer calls a negative man- 

 ner, or because they intercept the light transmitted 

 from the extreme limits of the atmosphere ; and we 

 perceive 4heir existence only by means of the dif- 

 ference of intensity that subsists between the light 

 which surrounds them, and that reflected by the par- 

 ticles of air placed between the object of vision and 

 the observer. In receding from Teneriffe, the Sugar- 

 loaf is long seen in a positive manner, as it reflects 

 a whitish light, and detaches itself clearly from the 

 sky ; but as this terminal cone is only 512 feet high, 

 by 256 in breadth at its summit, it has been ques- 

 tioned whether it can be visible beyond the distance 

 of 138 miles. If it be admitted that the mean breadth 

 of the Sugar-loaf is 639J feet, it will still subtend, at 

 the distance now named^ an angle of more than three 

 minutes, which is enough to render it visible ; and 

 were the height of the cone greatly to exceed its 

 basis, the angle might be still less, and the mass yet 

 make an impression on our organs ; for it has been 

 proved by micrometrical observations, that the limit 

 of vision is one minute only when the dimensions 

 of objects are the same in all directions. 



As the visibility of an object, which detaches it- 

 self from the sky of a brown colour, depends on the 

 quantities of light the eye meets in two lines, of 

 which one ends at the mountain and the other is 



