A PERPLEXING PHENOMENON — MIRAGE 



515 



observer is the i'aet that from just helow 

 the lower transition stratum and from 

 just above the upper transition hiyer, 

 double, triple, or nuihiple imai>(>s of the 

 distant object can be seen. 



In Johns Hopkins University, Pro- 

 fessor R. W. Wood has constructed in 

 his physical laboi'atory an apparatus 

 which reproduces on a small scale cer- 

 tain mirage effects which are very often 

 seen in deserts.^ This apparatus con- 

 sists of three slabs of blackboard slate 

 (each a meter long, 20 cms. wide, and 

 1 cm. thick), mounted on iron tripods 

 and carefully brought into alignment. 

 The surface is sprinkled with sand, to 

 prevent reflection. A mirror mounted 

 so as to reflect the sky when viewed 

 from the opposite end of the desert, is 

 so arranged that the artificial sky comes 

 down to the level of the sanded surface. 

 In front of it is mounted a chain of 

 mountains cut out of pasteboard (with 

 peaks varying from 1 to 2 cms. in height 

 and valleys which come down to the 

 sanded plain). The desert is heated by 

 gas jets. If we look along the sand, the 

 eye an inch or two above the plane of 

 the surface, we shall see, as the desert 



1 R. W. Wood, Physical Optics, p. 70, MacMillan 

 Co., New York. 1905. 



Note. — In the figures on the preceding page the line 

 abed represents a wave front of light after traveling in a 

 horizontal direction a short distance through syrup and 

 water on the one hand and alcohol and water on the 

 other. Syrup and alcohol are optically denser than 

 water, hence the wave front will become bent more and 

 more as it moves forward as indicated by the lines 

 a'b'c'd' and a"b"c"d" . The concave be portion of the 

 wave front is rendered convex after passing the point o 

 and gives rise to an inverted image. The size of the in- 

 verted image depends upon the distance of the eye from 

 the region marked o, so that if this distance is too small, 

 the vertical magnification will become indefinitely large 

 and the image will not be seen. Thus an inverted 

 image may be seen oidy when the eye is removed a 

 considerable distance from the tank. In addition to the 

 inverted image two other images are to be seen Be- 

 tween positions 2 and 4 three portions of the original 

 wave front enter the eye: (1) the straight portion ab 

 giving rise to an undeviated and undistorted image; 

 (2) the convex portion of the wave, erf, giving rise to a 

 depressed erect image; and (3) the concave portion be, 

 giving rise to an inverted image superimposed upon this 

 and undistorted. 



warms up, what appears to be a bril- 

 liant pool of water on the sand and 

 inverted images of the mountains. 



When we read accounts of mirages by 

 well-known travelers, we note that the 

 effects produced in the mirages are not 

 always the same. An analysis of some 

 of these accounts will show us the varied 

 physical conditions under which mirages 

 occur and will suggest a possible classi- 

 fication of tliem. 



(1) — ■ Professor Busch, who was the first 

 to make a scientific study and to record 

 data with regard to mirages, saw over a 

 verdant plain eight miles from Bremen, 

 on October 5, 1779, the ordinary image of that 

 town and a second image below, very dis- 

 tinct but upside down. 



(2) — When Bonaparte invaded Egypt 

 with his French army in 1798, Monge, one of 

 the learned men attached to that expedition, 

 observed that the delta of the Nile forms a 

 vast horizontal plain, the uniformity of which 

 is broken only by gentle eminences upon 

 which are built the villages. At morning 

 and evening there is no change in the aspect 

 of the country; but when the sun has heated 

 the surface of the plain it seems, at a certain 

 distance off, to be inundated; the villages 

 look like islands in the middle of an immense 

 lake, and below each village is to be seen its 

 inverted image. To complete the illusion, 

 the ground vanishes, and the vault of heaven 

 is reflected in still water. It is easy to under- 

 stand the cruel disappointment of the 

 French army. Exhausted by fatigue, driven 

 forward by a devouring thirst under the burn- 

 ing sky, these men fancied they had reached 

 a great pool of still water in which they saw 

 reflected the shadow of the villages and the 

 palm trees ; but as they gradually approached, 

 the limits of this seeming inundation re- 

 treated, and the imaginary lake drew back 

 and finally melted away altogether. The 

 same illusion was repeated in the case of the 

 next village. 



(,3) — During the French Army expedition 

 of May, 1837, to Algeria, M. Bonnefont, a 

 scientist attached to the expedition, observed 

 a flock of flamingoes about three miles and a 

 half off. As they started to fly, they assumed 

 such enormous dimensions as to give the idea 

 of Arab horsemen defiling one after the other. 



