See also:

• MIRAGE (a French word, from mirer, to look at, se mirer, to be reflected)

MIRAGE (a See also:

• FRENCH
• FRENCH, DANIEL CHESTER (1850– )
• FRENCH, NICHOLAS (1604-1678)

French word, from mirer, to look at, se mirer, to be reflected)  , an See also:

• OPTICAL

optical illusion due to See also:

• VARIATIONS
• VARIATIONS, CALCULUS
• VARIATIONS, CALCULUS OF

variations in the refractive See also:

• INDEX

index of the See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

atmosphere . It embraces the phenomena of the visionary See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance of lakes in arid deserts, the images of See also:

• SHIPS

ships and icebergs, frequently seen as if inverted and suspended in the atmosphere in the Polar Regions, the Fata Morgana, and " looming " as witnessed in mists or fogs . In the See also:

• ARTICLE (from Lat. articulus, a joint)

article See also:

• REFRACTION
• REFRACTION (Lat. refringere, to break open or apart)

REFRACTION it is shown that a See also:

• RAY (Lat. raia)
• RAY (or WRAY, as he wrote his name till 1670), JOHN (1628-1705)

ray of See also:

• LIGHT

light traversing a homogeneous See also:

• MEDIUM

medium is deviated from its rectilinear path when it enters a medium of different refractive index; it is therefore readily seen that the path of a ray through continuously varying .See also:

• MEDIA

media is necessarily See also:

• CURVILINEAR

curvilinear, being compounded of an See also:

• INFINITE (from Lat. in, not, finis, end or limit; cf. findere, to deave)

infinite number of infinitesimally small rectilinear deviations . Our atmosphere is a medium of continuously varying refractive index . Meteorological optical phenomena, due to variations in the refractive index of the atmosphere, may be divided into See also:

• GROUPS
• GROUPS, THEORY

groups: (I) those due to the permanent or normal variation experienced as one ascends in the atmosphere, and (2) those due to sporadic variations occasioned by irregular See also:

• HEATING

heating . The first variation must be taken into See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account in correcting See also:

• GEODETIC

geodetic observations of heights and astronomical observations of the heavenly bodies; it also has a considerable bearing on the phenomena of the See also:

• TWILIGHT

twilight and the See also:

• AFTERGLOW

afterglow (See REFRACTION: § Astronomical; and TWILIGHT) . The second (or temperature) variation gives rise to phenomena which we proceed to discuss . A See also:

• COMMON

common type of See also:

• MIRAGE (a French word, from mirer, to look at, se mirer, to be reflected)

mirage is the appearance of an isolated See also:

• LAKE
• LAKE, GERARD LAKE, 1ST VISCOUNT (1744-1808)

lake frequently seen in hot sandy deserts, as in the See also:

• SAHARA

Sahara, See also:

• TURKESTAN
• TURKESTAN, or HAZRET

Turkestan, &c . The explanation is as follows: The See also:

• SAND
• SAND, GEORGE (1804-1876)

sand, being abnormally heated by the See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar rays, causes the neighbouring See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air to expand, consequently its See also:

• DENSITY (Lat. densus, thick)

density, and therefore its refractive index, is diminished, and attains a minimum value in the lowest layers . It increases as we ascend and reaches a maximum at a certain height, and then decreases according to the normal variation . Any See also:

• OBJECT

object viewed across such an See also:

• AREA

area is seen by two sets of rays: one set passing near the See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth and assuming a curved path See also:

• CONVEX

convex to the See also:

• HORIZON (Gr. 6pfTwv, dividing)

horizon, the second set more remote from the earth and See also:

• CONCAVE

concave to the horizon . The object thus appears See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double, an See also:

• IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

image being seen mirrored in the sand .

The See also:

• SKY (M. Eng. skie, cloud; O. Eng. skua, shade; connected with an Indo-European root sku, cover, whence " scum," Lat. obscurus, dark, &c.)

sky appears as a shining lake; mountains or palms may be similarly reflected, but it is to be noted that the images are inverted (see fig.) . Similar atmospheric conditions sometimes prevail in the air over large. bodies of See also:

• WATER

water on See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

cold autumn mornings . These phenomena have been experimentally realized by R . W . See also:

• WOOD, ANTHONY A2 (1632-1695)
• WOOD, JOHN GEORGE (1827—1889)
• WOOD, MRS HENRY [ELLEN] (1814—1887)
• WOOD, SEARLES VALENTINE (1798—188o)

Wood (Phil . Mug., 1899, vol. xlvii.), who viewed See also:

• OBJECTS

objects over a See also:

• SERIES (a Latin word from serere, to join)

series of heated ,See also:

• SLATE (properly CLAY SLATE; in M. Eng. slat or sclat, from O. Fr. esclat, a small piece of wood used as a tile; esclater, to break into pieces, whence modern Fr. eclat, the root being seen also in Ger. schleissen, to split)

slate slabs . Another type of mirage, frequently observed at See also:

• SEA (in O. Eng. sae, a common Teutonic word; cf. Ger. See, Dutch Zee, &c.; the ultimate source is uncertain)
• SEA, COMMAND OF THE

sea in the See also:

• NORTHERN

northern latitudes, is presented in the appearance of ships and icebergs as if inverted and suspended in the clouds . This is due to a stratum of hot air at some distance above the sea level, the rays of light near the horizon being practically See also:

• HORIZONTAL

horizontal, while those at greater elevations are fairly concave . It may happen that the See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change in density is so See also:

• GREAT

great that only the upper rays reach the See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye; we are then met with the curious illusion of seeing inverted ships in the clouds, although nothing is visible on the ocean . The Fata Morgana, frequently seen in the Straits of See also:

• MESSINA

Messina, consists of an apparent See also:

• VERTICAL (from Lat. vertex, highest point)

vertical See also:

• ELONGATION

elongation of an object situated on the opposite See also:

• SHORE

shore . The See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

distribution of density is similar to that attending a See also:

• DESERT

desert mirage, but the transition is not so abrupt . The object is really viewed through a horizontallystratified medium consisting of a central See also:

• SHEET

sheet of maximum refractive index, over- and under-laid by sheets of decreasing refractive See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power .

The See also:

• SYSTEM

system consequently acts as a continuous See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

lens, magnifying the object in a vertical direction . If, in addition to this horizontal stratification, the atmosphere varies similarly in vertical planes, then the object would be magnified both horizontally and vertically . These conditions sometimes prevail in misty or foggy See also:

• WEATHER (O. Eng. weder; the word is common to Teutonic languages; cf. Du. weder, Dan. veir, Icel. ve8r, and Ger. Wetter and Gewitter, storm; the root is wa- to blow, from which is derived " wind ")

weather, more particularly at sea, and thus give rise to the phenomena known as " looming." A famous example is the Brockengespenst or " spectre of the See also:

• BROCKEN
• BROCKEN, SPECTRE OF THE (so named from having been first observed in 178o on the Brocken)

Brocken." The See also:

• CHROMATIC (Gr. xpcoµaruc6s, coloured, from xpwµa, colour)

chromatic halos which frequently encircle these images are due to diffraction . (See See also:

• CORONA (Lat. for " crown ")

CORONA.) It is interesting to See also:

• NOTE
• NOTE (Lat. nota, mark, sign, from noscere, to know)

note that lenses formed on non-homogeneous material, having the maximum refractive index along the central See also:

• AXIS (Lat. for " axle ")

axis, have been prepared, and reproduce the effects caused by abnormal distribution of the density of the atmosphere . The mathematical investigation of this subject was worked out by Gaspard See also:

• MONGE, GASPARD (1746-1818)

Monge . For this aspect and further details, both descriptive and experimental, see J . Pernter, Meteorologische Optik (1906); E . Mascart, Traite d'optique (1899–1903); R . W . Wood, See also:

• PHYSICAL

Physical See also:

• OPTICS

Optics (1905) ; R . S . See also:

• HEATH
• HEATH, BENJAMIN (1704-1766)
• HEATH, WILLIAM (1737—1814)

Heath, Geometrical Optics .