There is another illusion—a harmless one—that has not to do with perspective but with shadow and local color. The appearance is that of shadows cast down along the mountain’s side by the ridges or hogbacks. Any little patch of shadow is welcome on the desert, particularly upon the mountains which are always so strongly flooded with light. But this is only a counterfeit presentment. The ridges have no vegetation upon them to hold in place the soil and rocks and these are continually breaking away into land-slips. The slips or slides expose to view streaks of local color such as may be seen in veins of iron and copper, in beds of lignite or layers of slate. It is these streaks and patches of dark color that have broken away and slipped down the mountain side under the ridges that give the appearance of shadows. They have the true value in light, and are fair to look upon even though they are deception. The weather-beaten rocks of a talus under a peak may create a similar illusion, but the shadow effect loses a velvety quality which it has when seen under the ridges.
Illusion of lava-beds.
Appearance of cloud-shadow.
The illusion of a cloud-shadow resting upon the foot-hills or in the valley, is frequently produced by the local color of lava-beds. Lava may be of almost any color, but when seen close to view it is usually a reddish-black. At a distance, however, and as a mass, its beds have the exact value of a cloud-shadow. Any eye would be deceived by it. The great inundations of lava that have overrun the plains and oozed down the foot-hills and around the lomas (particularly on the Mojave) look the shadow to the very life. The beds are usually hedged about on all sides by banks of fine sand that seem to stand for sunlight surrounding the shadow, and thus the deception is materially augmented. Many times I have looked up at the sky to be sure there was no cloud there, so palpable is this lava shadow-illusion.
Mirage.
Definition.
But perhaps the most beautiful deception known to the desert is the one oftenest seen—mirage. Everyone is more or less familiar with it, for it appears in some form wherever the air is heated, thickened, or has strata of different densities. It shows on the water, on the grass plains, over ploughed fields or gravel roads, on roadbeds of railways; but the bare desert with its strong heat-radiation is primarily its home. The cause of its appearance—or at least one of its appearances—is familiar knowledge, but it may be well to state it in dictionary terms: “An optical illusion due to excessive bending of light-rays in traversing adjacent layers of air of widely different densities, whereby distorted, displaced, or inverted images are produced.”[5]
Need of explanation.
This is no doubt the true explanation of that form of mirage in which people on Sahara see caravans in the sky trailing along, upside down, like flies upon the ceiling; or on the ocean see ships hanging in the air, masts and sails downward. But the explanation is very general and is itself in some need of explanation. Perhaps then I may be pardoned for trying to illustrate the theory of mirage in my own way.
Refraction of light-rays.