A PERPLEXING PHENOMENON — MIRAGE 



519 



sity of the air layers develops most often 

 in the desert or on extensive plains in 

 the summer time. On cold autumn 

 mornings a heated layer may arise over 

 large bodies of water. Slight variations 

 in the supply of the heat may produce 

 grotesque changes in form, as in observa- 

 tion (3), or produce unsteadiness of the 

 image as in observations (3) and (4). 

 In this form of mirage, called "inferior 

 mirage," the inverted object appears 

 to be reflected on a surface of water; 

 in reality it is an inverted image of the 

 sky and of objects which rise above the 

 horizon [graphically represented on pages 

 517 and 520]. 



When the images lie in a horizontal 

 instead of a vertical plane, a phase of 

 inferior mirage called "lateral mirage" 

 exists. It is generally developed in the 

 air opposite extensive walls, board 

 fences or high cliffs having a southern 

 exposure. 



In the lower portion of the tank experi- 

 ment the syrup and water media produce 

 between positions 2 and 4- [upper figure, 

 page 514], three images in a vertical plane, 

 with the middle one inverted. The 

 lowest image is the ordinary one, the 

 others have been lifted above their true 

 positions. In observations (5), (6), (7), 

 (8), and (9), a like order of images 

 shovild occur, the inverted image and the 

 higher erect one appearing as if sus- 

 pended in the sky. The lower erect one 

 should rest on the surface of the earth. 

 In number (8) the upper erect image is 

 mentioned; in observations (5), (6), 

 (7), and (9), either it was not recorded 

 or it was not seen. Sometimes it may 

 be absent. 



In the tank experiment the rarer of 

 the two lower media lies on top and the 

 denser below. To have the same rela- 

 tions in the atmosphere, it is necessary 

 for a zone of rarefied air to be sand- 

 wiched in between normal air above and 



denser air below. Should the rarefied 

 zone reach to great heights, as is sug- 

 gested in observation (5), no mechani- 

 cal unstability of the air would arise, 

 since the denser layers are ever;)avhere 

 below the less dense. The illusions will 

 consequently be far steadier and afford 

 much better optical images in this 

 variety, called "superior mirage," than 

 in inferior mirage. Both erect and 

 inverted images of objects, even below 

 the horizon, may be seen in the sky, 

 since in superior mirage the path of the 

 light waves through the rarer medium is 

 concave toward the earth and when the 

 rarefied zone is very high it may be 

 abnormally concave. 



A superior mirage may arise where 

 warm air passes over a frozen sea or ice 

 cap and may be local in its distribution. 

 A superior mirage was seen in Paris 

 between the hours of three and four of a 

 December morning in 1869 [seepage 521]. 

 The Arctic explorer Scoresby noted that 

 typical examples of this kind were never 

 observed on the sea closer than fifteen 

 miles. Observation (6) illustrates this 

 point [see also figures, pages 517 and 522]. 



Should the zones of rarefied air in 

 superior mirage be increased to two, 

 three, four or more such zones, a series 

 of images all in a vertical plane would 

 appear. The basal and highest ones 

 would be erect images, and the inter- 

 mediate, iuA'erted ones. Such an effect 

 is called "multiple mirage." Observa- 

 tion (9) is a record of one. Mr. Albert 

 Operti, the artist with Peary in 1906, 

 sketched a multiple mirage which he 

 saw. It consists of four inverted images 

 between two erect images, of a distant is- 

 land and the surrounding ice floes [page 

 512]. The objects which gave rise to the 

 mirage were not seen at first, but as the 

 ship drew nearer to them, they were 

 sketched in position. 



When the images produced by inferior. 



