The desert

by John C. Van Dyke

How produced.

The bending of the light-rays to either side instead of up or down, as following the perpendicular, may or may not be of frequent occurrence. I do not even know if the butte appearance 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 distance 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 weather. 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 density of the air will give the appearance of waves or slight undulations on the water. In either case the illusion is quite perfect. Water lying in such a bed would reflect the exact color of the sky over it; and what the eyes really see in this desert picture is the reflection of the sky not from water but from strata of thick air.

Objects in the water.

This illusion of water is probably seen more perfectly in the great dry lake-beds of the desert where the ground is very flat and there is no vegetation, than elsewhere. In the old Coahuila Valley region of the Colorado the water comes up very close to you and the more you flatten the angle of reflection by flattening yourself upon the ground, the closer the water approaches. The objects in it which people imagine look like familiar things are certainly very near. And these objects—wild-fowl, bushes, tufts of swamp grass, islands, buttes—are frequently bewildering because some of them are right side up and some of them are not. Some are reversed in the air and some are quietly resting upon the ground.

Confused mirage.

The swimming wolf.

It happens at times that the whole picture is confused by the light-rays being both reflected and refracted, and in addition that the rays from certain objects come to us in a direct line. The ducks, reeds, and tufts of grass, for instance, are only clods of dirt or sand-banked bushes which are detached at the bottom by heavy drifts of air. We see their tops right side up by looking through the air-layer or some broken portion of it. But in the same scene there may be trees upside down, and mountains seen in reflection, drawn out to stupendous proportions. In the Salton Basin one hot day in September a startled coyote very obligingly ran through a most brilliant water-mirage lying directly before me. I could only see his head and part of his shoulders, for the rest of him was cut off by the air-layer; but the appearance was that of a wolf swimming rapidly across a lake of water. The illusion of the water was exact enough because it was produced by reflection, but there was no illusion about the upper part of the coyote. The rays of light from his head and shoulders came to me unrefracted and unreflected—came as light usually travels from object to eye.

Colors and shadows in mirage.

But refracted or reflected, every feature of the water-mirage is attractive. And sometimes its kaleidoscopic changes keep the fancy moving at a pretty pace. The appearance and disappearance of the objects and colors in the mirage are often quite wonderful. The reversed mountain peaks, with light and shade and color upon them, wave in and out of the imaginary lake, and are perhaps succeeded by undulations of horizon colors in grays and pinks, by sunset skies and scarlet clouds, or possibly by the white cap of a distant sierra that has been caught in the angle of reflection.

Trembling air.