When a layer of air rests quietly on another the plane of contact, if below the observer, reflects the sky in much the same manner as does a body of water. An object at this plane is seen both upright and inverted, thereby forming a mirage. If the plane of contact is materially above the eye of the observer the inversion occurs in the air. Occasionally inverted images of the shipping in the harbor are formed.[1]
The looming of objects—that is, bringing to sight objects that normally are below the horizon—is clearly a case of refraction. The rays of light which should pass above the observer are bent within reach of his vision.
According to legends a fairy named Morgana hovered around and about the southern coasts of Italy. This sprite used her powers romantically rather than maliciously to change the commonplace shoreline across the straits of Messina to a most wonderful landscape of turreted embattlements and castellate fortifications. Hence the name, Fata Morgana. The phenomenon apparently is produced by a horizontal layer of air, denser at the center than at its surface. It therefore becomes practically a cylindrical lens which magnifies in a vertical but not in a horizontal direction. It may be considered as a form of looming.
It is probable that the Brocken spectre is produced in part by a mass of air which acts as a lens. The traditional “heiligenschein,” or halo, is an example of diffraction, however.
- ↑ Many physicists hold that the inversion in such instances is due to refraction. In one popular textbook of physics a diagram graphically describes the method of refraction; but the diagram illustrates reflection. The mirage is considered in detail in Chapter XII.
