334 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
remain in sight for about a hundred yards of approach, then vanish. 
At 200 yards, with eyes 6 feet from the ground, the total change in 
direction of the light path would be about 1°, at 100 yards around 
2°, giving a tolerance of about 1° through which refraction operates. 
Two degrees is probably the maximum bending found in nature as 
produced by an air lens. It seems probable that the minimum bend- 
ing effective in mirage making is well under one-quarter of a degree. 
The bending potential of one of these small in-the-road lenses depends 
on its great density change within a shallow depth. It must have 
that to contain its direction-change are within its small diameter. 
The same bending capacity might be found in a much larger lens 
where the same density change is spread over many feet, instead of a 
few inches, and where the direction-change arc has a possible length 
of miles. 
For classic mirage effects, no other location can equal an old dry 
lake or sea bed. Bets have been won and lost over the reality or 
unreality of what seems to be a beautiful body of water often reflecting 
the nearby trees and hills. Sometimes, even in dry country, it is 
real water that complicates the betting. From a little distance it is 
indeed difficult to tell real water from a mirage. The seeming lakes 
vary greatly in size, from perhaps an acre to many square miles. The 
small mirages have a decided prejudice against operating over ground 
surfaces. The larger mirages are particular—just any ground will 
not do—but an old, level-fioored lake bed or sea bed fills the bill 
beautifully. It has been my experience that the angle of refraction 
of these larger mirages is much flatter than that of the small ones, 
so that they must be viewed from a greater distance. Willcox Dry 
Lake bed mirages in southeastern Arizona are an illustration of this. 
With some 50 square miles of almost completely flat surface, the bed 
offers very frequent showings. Easily seen from the highway, it is 
as worthy of a stop as many of the beautiful real lakes. It is an 
experience to remember to see the great dry lake bed of early morn- 
ing give away to the seeming beautiful blue water with changing 
lights and shadows through the day, and a fading out toward evening. 
Many of the mirage lakes not only show refiections of nearby trees 
and hills, but have islands, capes, and bold headlands, even very real- 
looking ships sometimes, and, rarely, a shimmering white city on the 
far shore. These mirage effects can stand quite a bit of wind. Rip- 
pling effects are not uncommon, and I have had reports of waves 
seeming to be breaking on the shore. A railroad trainman told me 
of having his train seemingly isolated on an island, wheels of the rear 
cars deep in the mirage water. There have been reports of planes 
coming down to land on some of these lakes, specifically one south of 
Las Vegas, Nev., and of flocks of ducks sweeping over for the same 
