Scr. XVm. PHENOMENA FROM REFLECTION. 151 



temperature which are not so soon communicated to the 

 water on account of its density as to the air, occur more 

 rarely and are of shorter duration than similar appear- 

 ances on land. In 1818, Captain Scoresby, whose ob- 

 servations on the phenomena of the polar seas are so 

 valuable, recognized his father's ship by its inverted 

 image in -the air, although the vessel itself was below 

 the horizon. He afterward found that she was seven- 

 teen miles beyond the horizon, and thirty miles distant. 

 Two images are sometimes seen suspended in the air 

 over a ship, one direct and the other inverted, with their 

 topmasts or their hulls meeting, according as the in- 

 verted 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 densities, by the veiy 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 

 extraordinary refraction seem to be occasioned J>y a 

 partial or total reflection of the rays of light at the sur- 

 faces of strata of different densities (N. 184). It is well 

 known that when light falls obliquely uponjhe external 

 surface of a transparent medium, as on a plate i 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 an^es made by 

 the incident and reflected rays with a perpendicular to 

 the surface being equal, as the brightness of the re- 

 flected 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 travelers and to the Arab of the des- 

 ert as the Lake of the Gazelles, is ascribed to the re- 

 flection which takes place between strata of air of dif- 

 ferent densities, owing to radiation of heat from the 

 arid sandy plains. The 'mirage described by Captain 



