ILLUSIONS 



133 



instead of up or down, as following the perpen- 

 dicular, may or may not be of frequent occur- 

 rence. I do not even know if the butte appear- 

 ance is to be attributed to that. The opportunity 

 to see it came to me but once, and I had not 

 then the time to observe whether the buttes in 

 the mirage had sides the reverse of the originals. 

 Besides, it is certain that mirage is caused in 

 other ways than by the bending of light-rays. 

 The most common illusion of the desert is the 

 water-mirage and that is caused by reflection, 

 not refraction. Its usual appearance is that of 

 a lake or sea of water with what looks at a dis- 

 tance to be small islands in it. There are those 

 with somewhat more lively imagination than 

 their fellows who can see cows drinking in the 

 water, trees along the margin of the shore 

 (palms usually), and occasionally a farm-house, 

 a ship, or a whale. I have never seen any of 

 these wonderful things, but the water and 

 island part of the illusion is to be seen almost 

 anywhere in the desert basins during hot weath- 

 er. In the lower portions of the Colorado it 

 sometimes spreads over thousands of acres, and 

 appears not to move for hours at a stretch. At 

 other times the wavering of the heat or the 

 swaying of the air strata, or a change in the 



Other 



causes for 

 mirage. 



Water- 

 mirage. 



The lake 

 appearance. 



