we had already passed, where we beheld a precisely similar appearance. It was, in fact, _the Mirage_, a prodigy to which every one of us were then strangers, although it afterwards became more familiar. Yet upon no future occasion did we ever behold this extraordinary illusion so marvellously displayed. The view of it enabled us to imagine the horrible despondency to which travellers must sometimes be exposed, who, in traversing the interminable desert, destitute of water, and perishing with thirst, have sometimes this deceitful prospect before their eyes." Another traveller adds a particular which is well worthy of notice. "The most singular quality of this vapour is its power of reflection. When a near observer is a little elevated, as on horseback, he will see trees and other objects reflected as from the surface of a lake. The vapour, when seen at a distance of six or seven miles, appears to lie upon the earth like an opaque mass; and it certainly does not rise many feet above the ground, for I observed that, while the lower part of the town of Abusheher was hid from the view, some of the more elevated buildings, and the tops of a few date-trees, were distinctly visible." CHAPTER X. HOW PARLEY SUPPOSES THESE APPEARANCES TO BE PRODUCED. If you wish to understand the manner in which it is most probable that these illusions are produced, you must closely attend to what I am going to tell you. I will make my explanation as simple as I can, but I shall be able to teach you nothing, unless you do your part by paying attention. You know that vision is produced by rays of light passing from the object seen, to the eye of the spectator. These rays have a tendency to form straight lines, and they would be perfectly straight, if they had only to pass through empty space; but this cannot be the case near the surface of the earth, as they there have to go through air, and frequently through other transparent substances. When these rays meet with a substance which they cannot go through, and which is therefore called opaque, such as a man or a house, they are turned back or _reflected_, and strike the eye of the person who may be looking in that direction, so as to cause vision. But when they meet with a body which they can pass through, called therefore transparent, such as water or glass, they are turned aside out of their original course or _refracted_; and this refraction takes place in different degrees, according to the density of the substance. Thus, in looking at objects through air, you do not see anything exactly in its real position; but in looking at them through water, the variation between the reality and the appearance is still greater, because the density of water being greater than that of air, it will refract more. To prove this you need only put a stick obliquely into water, and it will look as if it were broken or bent at the surface of the water. [Illustration]