216 LETTERS ON NATURAL MAGIC. 



ignorant beholder regards the modern magician as but an 

 ordinary man, who borrows from the sciences the best 

 working implements of his art. But when, in the midst 

 of solitude, and in situations where the mind is undis- 

 turbed by sublunary cares, we see our own image de- 

 lineated in the air, and mimicking in gigantic perspective 

 the tiny movements of humanity ; when we see troops in 

 military array performing their evolutions on the very 

 face of an almost inaccessible precipice; when in the 

 eye of day a mountain seems to become transparent, and 

 exhibits on one side of it a castle which we know to exist 

 only on the other; when distant objects, concealed by 

 the roundness of the earth, and beyond the cognizance of 

 the telescope, are actually transferred over the intervening 

 convexity and presented in distinct and magnified outline 

 to our accurate examination; when such varied and 

 striking phantasms are seen also by all around us, and 

 therefore appear in the character of real phenomena of 

 nature, our impressions of supernatural agency can only 

 be removed by a distinct and satisfactory knowledge of 

 the causes which gave them birth. 



It is only within the last forty years that science has 

 brought these atmospherical spectres within the circle of 

 her dominion ; and not only are all their phenomena 

 susceptible of distinct explanation, but we can even re- 

 produce them on a small scale with the simplest elements 

 of our optical apparatus. 



In order to convey a general idea of the causes of these 

 phenomena, lot A B C D, Fig. 35, be a glass trough 

 filled with water, and let a small ship be placed at S. 

 An eye situated about E, will see the topmast of the ship 

 S directly through the plate of glass B D. Fix a convex 

 lens a of short focus upon the plate of glass B I), and a 

 little above a straight line S E joining the ship and the eye ; 

 and immediately above the convex lens a place a concave 



