302 



SCIENTIFIC AMUSEMENTS. 



gradually less and less refractive power the nearer the ground ; 

 hence the ray, ABC, becomes continually refracted to a direction 

 more nearly horizontal, until at a point, B, it becomes totally in- 

 ternally reflected (Expt. 337) ; it then passes upwards, suffering 

 successive refractions in the inverse order as it gradually traverses 

 more and more refractive layers ; and finally, it meets the eye of 

 the observer at C, producing the same effect as though it had been 

 emitted from the point D. An inverted image is thus seen, just 

 as though reflected from water, the appearance of water itself 

 being caused by the similar reflection of the sky. 



It is difficult to reproduce this effect on a small scale, but it 

 may sometimes be seen when a large plate of hot metal is allowed 

 to cool, e.g., when an armour plate for a warship is being rolled in a 

 factory for the preparation of such articles, or when a blast furnace 

 for smelting iron is " tapped," so that the molten metal runs out into 

 a series of moulds in a large bed of sand, and there solidifies to "pigs" 

 of cast iron ; at a certain stage of the cooling of the whole " pig 

 bed," the phenomenon of mirage is sometimes clearly discernible. 



The contrary kind of refraction and reflection is sometimes 

 observable at sea, inverted images of ships being formed in the air 

 above the real object. In the case of the mirage of the desert, the 

 lowest layers of air are the hottest, and therefore the least refrac- 

 tive, convection (Chapter XX.) not taking place sufficiently rapidly 

 to interfere with this general disposition ; hence the inversion 

 takes place as represented in fig. 143 ; but in the sea mirage (fig. 

 144), the lower layers of air are the coolest, and, consequently, 

 the bending of the rays takes place in the opposite direction, an 

 ascending ray, ABC, being gradually bent towards the horizontal di- 

 rection, then re- 

 flected at B, and 

 passing down- 

 wards again with 

 a gradually in- 

 creasing slope, 

 so that an ob- 

 server at C, sees 

 Fig. 144. Ship seen inverted by mirage. the image ag 



though it were situated at D. Sometimes the coast line, or a 

 vessel, &c., below the actual horizon becomes visible in an inverted 

 position above the sea level owing to this action. 



In making astronomical observations, as also in the trigono- 

 metrical surveying of countries and similar operations, the effects 

 of refraction, owing to the passage of light through layers of air of 

 different densities, have to be allowed for. 



