SECT, xviii.] PHENOMENA FROM REFLECTION. 175 



a ship, one direct and the other inverted, with their top- 

 masts or their hulls meeting, according as the inverted 

 image is above or below the direct image (N. 188). Dr. 

 Wollaston has proved that these appearances are owing to 

 the refraction of the rays through media of different den- 

 sities, by the very simple experiment of looking along a 

 red-hot poker at a distant object. Two images are seen, one 

 direct and another inverted, in consequence of the change 

 induced by the heat in the density .of the adjacent air. He 

 produced the same effect by a saline or saccharine solution 

 with water and spirit of wine floating upon it (N. 189). 



Many of the phenomena that have been ascribed to ex- 

 traordinary refraction seem to be occasioned by a partial or 

 total reflection of the rays of light at the surfaces of strata 

 of different densities (K 184). It is well known that, when 

 light falls obliquely upon the external surface of a trans- 

 parent medium, as on a plate of glass or stratum of air, one 

 portion is reflected and the other transmitted. But, when 

 light falls very obliquely upon the internal surface, the 

 whole is reflected, and not a ray is transmitted. In all cases 

 the angles made by the incident and reflected rays with a 

 perpendicular to the surface being equal, as the brightness 

 of the reflected image depends on the quantity 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 Captain Mundy in his Journal of a Tour in India pro- 

 bably 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 ascending nearly half way up the 

 sides of the vale, and on its bright surface trees and rocks 



