SECT. XVIII. REFLECTION. 157 



of light, those arising from total reflection must be by far the 

 most vivid. The delusive appearance of water, so well known 

 to African travellers and to the Arab of the desert as the Lake 

 of the Gazelles, is ascribed to the reflection which takes place 

 between strata of air of different densities, owing to radiation of 

 heat from the arid sandy plains. The mirage described by Cap- 

 tain Mundy in his Journal of a Tour in India probably arises 

 from this cause. " A deep precipitous valley below us, at the 

 bottom of which I had seen one or two miserable villages in the 

 morning, bore in the evening a complete resemblance to a 

 beautiful lake ; the vapour which played the part of water as- 

 cending nearly half way up the sides of the vale, and on its 

 bright surface trees and rocks being distinctly reflected. I had 

 not been long contemplating this phenomenon, before a sudden 

 storm came on and dropped a curtain of clouds over the scene." 

 An occurrence which happened on the 18th of November, 

 1804, was probably produced by reflection. Dr. Buchan, while 

 watching the rising sun from the cliff about a mile to the east of 

 Brighton, at the instant the solar disc emerged from the surface 

 of the ocean, saw the clitf on which he was standing, a windmill, 

 his own figure and that of a friend, depicted immediately oppo- 

 site to him on the sea. This appearance lasted about ten 

 minutes, till the sun had risen nearly his own diameter above 

 the surface of the waves. The whole then seemed to be elevated 

 into the air, and successively vanished, The rays of the sun 

 fell upon the cliff at an incidence of 73 from the perpendicular, 

 and the sea was covered with a dense 'fog many yards in height, 

 which gradually receded before the rising sun. When extra- 

 ordinary refraction takes place laterally, the strata of variable 

 density are perpendicular to the horizon, and, if combined with 

 vertical refraction, the objects are magnified as when seen through 

 a telescope. From this cause, on the 26th of July, 1798, the 

 cliffs of France, fifty miles off, were seen as distinctly from Hast- 

 ings as if they had been close at hand ; and even Dieppe was 

 said to have been visible in the afternoon. 



The stratum of air in the horizon is so much thicker and more 

 dense than the stratum in the vertical, that the sun's light is 

 diminished 1300 times in passing through it, which enables us 

 to look at him when setting without being dazzled. The loss of 

 light, and consequently of heat, by the absorbing power of the 



