76 SCIENTIFIC ILLUSTRATIONS [Del 



Self and Circumstance Conspire to Delude. 



The effects of the mirage are extraordinary, but undoubtedly 

 they are heightened by the imagination of observers, generally 

 over-excited by fatigue, by privations, or sometimes by fever. 

 These causes contribute to vary the nature of the phenomenon 

 as seen by different eyes. Thus some gaze enraptured on 

 verdurous islands, bright as Armida's enchanted garden, with 

 feathery palms, and blooming flowers, and delicious sparkling 

 lakes ; other see in that dim far-off which is never reached 

 the laughing waves of the ocean, with ships calmly at anchor, or 



" Veering up and down, they know not why," 



and camels browsing quietly upon its shores ; others again see 

 before them the rolling river, its banks studded with groves and 

 palaces ; and all this while there is not a solitary real object on 

 the horizon whose presence might serve in some degree as a 

 foundation for their visions. It is the very phantasmagoria of 

 Nature her wildest, most wayward, and most fantastic sport. 

 The reflection of the sky, modified by the inequalities of, the 

 soil and the vibratory movements of the air, can alone account 

 for the singular deception. Imagination shows its victim, in 

 the reflected image of the cloudless sky, a sheet of water, which 

 is variously taken for a sea, a lake, or river; it invests the 

 slightest objects on the earth's surface with forms, colours, and 

 dimensions, which are easily metamorphosed into houses, ships, 

 men, animals ; and it seems certain that those which in Nubia 

 our fancy converts into camels, would in the Soudan be trans- 

 formed into elephants, and at Venice into gondolas. Imagina- 

 tion makes us its dupes, and gives to airy nothings 



" A local habitation and a name." 



It becomes absolutely necessary, therefore, to distinguish these 

 wholly personal illusions born of a heated brain from those which 

 are really due to a definite physical cause. The latter neces- 

 sarily suppose the existence of actual objects below or very little 

 above the horizon. Under such conditions, the most frequent 

 illusion is that which shows the sky or rocks reflected in the 

 expanse of rarefied air superincumbent on the earth's surface, 



