156 EXTRAORDINARY REFRACTION. SECT. XVIII. 



the upper edges of the sun and moon being less refracted than 

 the lower, they often appear to be oval when near the horizon. 

 The looming also or elevation of coasts, mountains, and ships, 

 when viewed across the sea, arises from unusual refraction. A 

 friend of the author's, while standing on the plains of Hindostan, 

 saw the whole upper chain of the Himalaya Mountains start into 

 view, from a sudden change in the density of the air, occasioned 

 by a heavy shower after a very long course of dry and hot 

 weather. Single and double images of objects at sea, arising 

 from sudden changes of temperature which are not so soon com- 

 municated to the water on account of its density as to the air, 

 occur more rarely and are of shorter duration than similar 

 appearances on land. In 1818 Captain Scoresby, whose obser- 

 vations on the phenomena of the polar seas are so valuable, re- 

 cognised his father's ship by its inverted image in the air, although 

 the vessel itself was below the horizon. He afterwards found 

 that she was seventeen 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 top- 

 masts or their hulls meeting, according as the inverted image is 

 above or below the direct image (N. 393). Dr. Wollaston has 

 proved that these appearances are owing to the refraction of the 

 rays through media of different densities, 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 con- 

 sequence 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. 194). 



Many of the phenomena that have been ascribed to extra- 

 ordinary refraction seem to be occasioned by a partial or total 

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

 densities (N. 189). It is well known that, when light falls 

 obliquely upon the external surface of a transparent medium, as 

 on a plate of glass or a 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 



