527. What is the cause of the optical illusions frequently observed in nature?
There are various kinds of natural optical illusions:—
The mirage, in which landscapes are seen reflected in burning sands.
The fata morgana, in which two or three reflections of objects occur at the same time.
The ærial spectra, or ærial reflections, &c.
Fig. 16.—ILLUSTRATING THE APPEARANCE OF PHANTOM SHIPS.
The optical illusions above enumerated owe their origin to various atmospheric conditions, in which refractions and reflections are multiplied by the different densities of atmospheric layers. They chiefly occur in hot countries, where, from the varying effects of heat, the conditions of atmospheric refraction and reflection frequently prevail in their highest degree.