Nov. 19, 1874] 



NATURE 



49 



ON MIRAGE* 



THE name of "Mirage " is applied to certain illusory 

 appearances due to excessive bending of the rays 

 of light in their passage through the atmosphere. These 

 appearances are by no means uniform. 



Sometimes, especially in hot countries, the observer 

 loses sight of the ground beyond a certain distance from 

 his position, and sees in its stead, what looks like a 

 sheet of water, either calm or with movements resembling 

 waves ; and if any distant objects are sufficiently lofty to 

 be seen above this apparent lake, their images are seen 

 beneath the objects themselves, inverted as if by reflection 

 in this imaginary water. The dry and hot soil of Egypt 

 is famous for the production of this form of the phe- 

 nomenon. It is also mentioned as of frequent occurrence 

 in the plains of Hungary, in the plain of La Crau in the 

 South of France, and in the fen districts of England when 

 dried up by the summer heat. It is also common in 

 Australia. The Deputy Surveyor-General of South 

 Australia once reported the existence of a large inland 

 lake, which on further examination turned out to be 

 nothing but a mirage. 



Another class of appearances are known (especially 

 among nautical men) under the name of looming. Distant 

 objects are said to loom when they appear abnormally 

 elevated above their true positions. This abnormal eleva- 

 tion not unfrequently brings into view objects which in 

 ordinary circumstances are beyond the horizon. It is 

 also frequently accompanied by an appearance of ab- 

 normal proximity (though this may perhaps be rather a 

 subjective inference from the unusual elevation and clear 

 visibility of the objects than a separate optical charac- 

 teristic), and it is further accompanied in many, though 

 not in all cases, by a vertical magnification, the heights of 

 objects being many times magnified in comparison with 

 their horizontal breadths, so as to produce an appearance 

 resembling spires, pinnacles, columns, or basaltic cliffs. 

 Some beautiful descriptions of these latter appearances, 

 with illustrative plates, are given in Scoresby's " Green- 

 land," the objects thus magnified being icebergs ; and a 

 very full and interesting account of the phenomena of 

 mirage, as observed in high latitudes, will also be found 

 in the '' Arctic Regions " of the same author. 



It is usually across water that looming is observed ; 

 and as a surface of water stands naturally in contrast with 

 a sandy desert or a surface of parched land, so also the 

 optical effects produced arc, in a manner, opposite. The 

 inverted images which are often presented in looming are 

 not beneath the object, as in the case of mirage on dry 

 land, but above it, as is formed by reflection in the sky. 

 The only examples that I have myself seen of mirage were 

 of this kind. They were seen across sheets of calm 

 water, the hills on the other side being seen with fictitious 

 hills upside down resting on the tops of the real hills. In 

 rare instances, two or even three of these images are 

 seen one above another, vertically over the real object ; 

 but these multiple images are usually too small to be 

 seen without the aid ot a telescope — the objects whose 

 images they are being so distant as to appear mere specks 

 to the naked eye. 



There is always more or less of change observable in 

 the images formed by mirage, and the changes are 

 greatest and most sudden when the images are most dis- 

 torted, as compared with the true forms of the objects. 

 The appearances also change with the height of the 

 observer's eye. Looming is seen to the greatest advan- 

 tage from an elevated position, such as the mast-head of 

 a ship. The mirage of dry land is sometimes visible at 

 any moderate height, but in other cases — especially in 

 countries which are not very hot — the range of height 

 from which it is visible is extremely limited. A very fine 

 mirage, recently observed in the fen districts, was only 



* A Paper re.-id by Prof. \. D. Everett, M.A., D.C.L., before the Belfast 

 atural Histor>' and Philosophical Society. 



seen when the observci- was on the top of the marsh w.all. 

 But this case seems to have been peculiar. It was ac- 

 companied by the further peculiarity that a strong wind 

 was blowing — the general rule being that mirage is only 

 seen in calm weather. Observers of mirage on the sands 

 of Morecambe Bay, and of the Devonshire coast, sta'.e 

 that it could frequently be only seen by stooping. 



Mirage is seldom seen in winter. The hot shining of 

 the sun seems to be an invariable antecedent ; and th s 

 is true even of, the polar regions, where Capt. Scoresby 

 attributes the phenomenon to " the rapid evaporation 

 which takes place in a hot sun from the surface of tte 

 sea, and the unequal density occasioned by partial con- 

 densations, when the moist air becomes chilled by passing 

 over considerable surfaces of ice." 



Time will not allow me to do much in the way of 

 quoting the very numerous records which exist. Scoresby's 

 accounts alone would almost suffice to occupy the 

 evening, and I would again refer to them as models of 

 accurate observation and effective description. 1 will 

 content myself with quoting nearly in full the account of 

 a mirage observed at Hastings and neighbouring parts of 

 the south coast of England in 1798, as given in the Phi- 

 losophical Transactions for that year, the narrator being 

 Mr. Latham, F.R.S. :— 



" On Wednesday last, July 26, about five o'clock in the 

 afternoon, whilst I was sitting in my dining-room at this 

 place (Hastings), which is situated ufon the parade, 

 close to the sea-shore, nearly fronting the south, my atten- 

 tion was excited by a great number of people running 

 down to the sea-side. Upon inquiring the reason, I was 

 informed that the coast of France was plainly to be dis- 

 tinguished with the naked eye. I immediately w'ent 

 down to the shore, and was surprised to find that, even 

 without the assistance of a telescope, 1 could very plainly 

 see the cliffs on the opposite coast, which at the nearest 

 part are between forty and fifty miles distant, and are not 

 to be discerned from that low situation by the aid of 

 the best glasses. They appeared to be only a few miles 

 off, and seemed to extend for some leagues along the 

 coast. I pursued my walk along the shore to the east- 

 ward, close to the water's edge, conversing with the 

 sailors and fishermen on the subject. They at first 

 could not be persuaded of the reality of the appearance, 

 but they soon became so thoroughly convinced, by the 

 cliffs gradually appearing more elevated and approaching 

 nearer, as it were, that they pointed out and named to 

 me the ditt'erent places they had been accustomed to visit, 

 such as the Bay, the Old Head or Man, the Windmill, 

 &c., at Boulogne, St. Valery, and other places on the 

 coast of Picardy, which they afterwards confirmed when 

 they viewed them through their telescopes. Their obser- 

 vations were, that the place:: appeared as near as if they 

 were sailing at a small distance into the harbours. 



" Having indulged my curiosity upon the shore for near 

 an hour, during which the ciift's appeared to be at some 

 times more bright and near, at others more faint, and at 

 a greater distance, but never out of sight, I went upon 

 the eastern cliff, w'hich is of a very considerable height, 

 when a most beautiful scene presented itself to my view ; 

 for I could at once see Dungeness, Dover cliffs, and the 

 French coast, all along from Calais, Boulogne, &c., to St. 

 Valdry, and, as some of the fishermen affirm-rd, as far to 

 the westward as Dieppe. By the telescope, the French 

 fishing-boats were plainly to be seen at anchor, and the 

 different colours of the land upon the heights, together 

 with the buildings, were perfectly discernible. This 

 curious phenomenon continued in the highest splendour 

 till past eight o'clock, . . . when it gradually vanished. 

 The day was extremely hot, . . . not a breath of wind 

 was stirring the whole of the day. ... A few days 

 afterwards I was at Winchelsea, and at several places 

 along the coast, where I was informed the above pheno- 

 menon had been easily visible. 



