650 



SCIENTIFIC RECREATIONS. 



ference in temperature in air strata, which cause it to act like a concave lens 



when the density of the water 

 diminishes towards the centre, 

 and like 



a convex lens when it 

 is increased. 



This can be proved by 

 heating the air (by hot irons) 

 above the glass vessel filled 



Fig. ^.-Explanation of Mirage. w ifa Q^ an( J f^e effects will 



be just the same as through the lenses. Dr. Wollaston obtained the 

 mirage by using a clear syrup, about one- third of the vessel .full, and 

 filling it with water. The gra- 

 dual mingling of these fluids will 

 produce the phenomenon. The 

 illustration in the margin (fig. 

 73 1 ) shows us the rays proceeding 

 from the ship's hull, and re- 

 fracted into the line reaching the 

 eye, above the line proceeding 

 from the mast, so the ship ap- 

 pears hull uppermost ; the rays 

 cross at x. But if they did not 

 cross before they reach the eye, the 

 erect position. 



The Spectre of the Brocken arises from a different cause. Such appear- 

 ances are only shadows, projected on thin clouds or dense vapours at 

 sunrise, or when the sun's rays are directed horizontally, for of course 

 vertical rays will throw the shadow on the ground on to the zenith. Bal- 

 loons are also reflected thus, and much interest has been caused by the 

 appearance of a twin balloon, until the aerial voyagers hav.e discovered the 

 cheat by seeing the shadowy aeronaut imitating their actions, and the second 

 balloon has been discovered to be an airy nothing. 



image would 



Fig. 731. The Mirage. 



appear as at s' /' in an 



