mirage

   The term mirage comes from the French verb se mirer, which means to reflect or to be reflected. It is unknown who introduced the term. It appears in the title of a paper by the French physicist Gaspard Monge, Comte de Péluse (17461818), who described the phenomenon in 1799 on the basis of observations made during a 30-day expedition to Egypt with the army of Napoleon Bonaparte (1769-1821). While Monge mentions the term in passing, which suggests that it was well known at the time, his paper would seem to be the oldest known scientific document in which it appears. The phenomenon itselfhas been known and described since ancient times. Today mirage is used as an umbrella term for a group of * physical illusions attributed to differences in the refractive index of the atmosphere, which are in turn attributed to differences in temperature between adjacent layers of air. The mirage is defined by the American Meteorological Society (AMS) as "a refraction phenomenon wherein an image of some distant object is made to appear displaced from its true position because of large vertical density variations near the surface; the image may appear distorted, inverted, or wavering." Especially in calm weather, the interface between warm and cold air near the surface of the ground or water may act as a refracting lens, bending light rays from the sky, and thus producing an image of a distant object or group of objects. These objects appear in an erect or an inverted position, and they may or may not appear disproportionally large or distorted. How it is possible that long-distance mirages - representing a city, a boat or a mountain, hundreds or even a thousand miles away - can be perceived without any diminution in the object's apparent size is not fully understood. Like other physical illusions, mirages are objective *illusions which can be perceived by any person in possession of adequate visual acuity, and which can also be photographed. Mirages are commonly classified as * inferior mirage, * superior mirage, and * double mirage. The inferior mirage is one with a relative position below a perceived distant object or the horizon. Some examples of inferior mirages are desert mirages and * highway mirages, which may both present in the form of distant pools of water, oil, or blue sky covering the surface of the ground. Inferior mirages tend to be unstable and to vanish as one approaches. They may be vibrating, vertically extended (i.e. 'towering'), or flattened (i.e. 'stooping'). The term superior mirage refers to one with a relative position above a distant object or the horizon. The resulting image is attributed to the presence of relatively hot air over a cold surface. Superior mirages, also known as arctic mirages, may present in the form of erect or inverted images, or a combination of the two, depending on the distance of the perceived object and the temperature gradients involved. When various temperature layers are present, mirages may merge, sometimes giving rise to multiple images. The term double mirage refers to a combination of the superior and inferior mirages. This type of mirage is less common. An even rarer type is the * lateral mirage, in which a distant object is perceived as if it were displaced sideways. The term * fata morgana is used to denote a superior mirage with a relatively high complexity. Mirages can appear deceptively realistic, as witness the 1913 Crocker Land Expedition, undertaken to map and explore a landmass referred to as Crocker Land, which was characterized at the time as "the world's last geographical problem". The expedition, organized by the American explorer Donald Baxter MacMillan (1874-1970), and sponsored by the American Museum of Natural History, the American Geographical Society, and the University of Illinois' Museum of Natural History, was unsuccessful, in the sense that Crocker Island turned out be an illusory landmass inferred from a recurring mirage. Conceptually and phenomenologically, the mirage should not be confused with the * desert hallucination, which has a propensity to occur during the night.

   References

   Burleigh, N. (2007). Mirage: Napoleon's scientists and the unveiling of Egypt.New York,NY: HarperCollins Publishers.

   Flammarion, C. (1873). The atmosphere.Trans-lated by Pitman, C.B. Edited by Glaisher, J. London: Sampson Low, Marston, Low, & Searle.

   Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.

   Monge, G. (1799). Sur le phénomène d'optique, connu sous le nom de Mirage. Mémoires sur l'Egypte, 1, 64-79.