OPTICAL ILLUSION OF ME. W. G. S. 397 



this object by the sinking of Artesian wells and the 

 dissemination of streams of water along the surface of the 

 ground by means of steam pumps, which would soon be 

 sheltered by vegetation if water were steadily supplied, 

 for it is astonishing how rapidly the desert blooms and 

 becomes covered with verdure whenever plants find 

 moisture to support them. And there appears little 

 reason to doubt that these deserts are great basins full of 

 water, but only fuller still of sand. Dean Stanley, in 

 his " Sinai and Palestine," speaking of the general charac- 

 teristics of the wadys or valleys, says : " There is nearly 

 everywhere a thin, it might almost be said a transparent 

 coating of vegetation." 



Allied to the class of optical illusions mentioned in Sir 

 David's third Letter, the following, communicated to the 

 editor by the gentleman by whom it was experienced, is 

 sufficiently distinctive to deserve notice. One day, not 

 many years since, Mr. W. G. S., the gentleman in question, 

 was after an interval of years walking along a picturesque 

 country road in the neighbourhood of Edinburgh, when 

 reaching a certain point he was suddenly, and without any 

 previous preparation for such a spectacle, surprised by the 

 appearance of a magnificent cavalcade of knights and 

 dames on horseback arrayed in the costume of the Middle 

 Ages, accompanied with a suitable following of squires 

 and other attendants, with hounds and hawks : many 

 of the ladies with falcons on their wrists, as if the party 

 were bent on hunting. The cavalcade consisted of a 

 numerous company, and defiled with gay but graceful 

 dignity, passed him and disappeared in a woodland path. 

 The gentleman stood riveted in surprise for some moments 

 after the spectacle had disappeared ; for, convinced it was 

 only an illusion, he was astonished at its vivid magnificence 

 and complete details. At length, overcoming his wonder, 

 he proceeded on his walk, still ruminating on the pageant 



