OPTICAL PHENOMENA. 535 



Frequently consists of a uniformly white arch, but it has often been seen tinted, the colours 

 differing only in intensity from those caused by the direct solar illuminations. Aristotle 

 states that he was the first observer of this interesting spectacle, and that he only saw 

 two in the course of fifty years ; but it must have been repeatedly witnessed, without 

 a record having been made of the fact. Thoresby relates an account received from a 

 friend, of an observation of the bow fixed by the moon in the clouds, while travelling in 

 the Peak of Derbyshire. She had then passed the full about twenty-four hours. The 

 evening had been rainy, but the clouds had dispersed, and the moon was shining very 

 clearly. This lunar iris was more remarkable than that observed by Dr. Plot, of which 

 there is an account in his History of Oxford, that being only of a white colour, but this 

 had all the hues of the solar rainbow, beautiful and distinct, but fainter. Mr. Bucke 

 remarks upon having had the good fortune to witness several, two of which were perhaps 

 as fine as were ever witnessed in any country. The first formed an arch over the vale of 

 Usk. The moon hung over the Blorenge ; a dark cloud was suspended over Myarth ; 

 the river murmured over beds of stones, and a bow, illumined by the moon, stretched 

 from one side of the vale to another. The second was seen from the castle overlooking 

 the Bay of Carmarthen, forming a regular semicircle over the river Towy. It was in a 

 moment of vicissitude ; and the fancy of the observer willingly reverted to the various 

 soothing associations under which sacred authority unfolds the emblem and sign of a 

 merciful covenant. 



5. Spectral Illusions. A series of curious and interesting phenomena, involving the 

 apparent elevation and approach of distant objects, the production of aerial images of ter- 

 restrial forms, of double images, their inversion, and distortion into an endless variety of 

 grotesque shapes, together with the deceptive aspect given to the desert-landscape, are 

 comprehended in the class of optical illusions. Different varieties of this singular visual 

 effect constitute the mirage of the French, the fata morgana of the Italians, the looming 

 of our seamen, and the glamour of the Highlanders. It is not peculiar to any particular 

 country, though more common in some than others, and most frequently observed near 

 the margin of lakes and rivers, by the sea-shore, in mountain districts and on level plains. 

 These phantoms are perfectly explicable upon optical principles, and though influenced 

 by local combinations, they are mainly referable to one common cause, the refractive and 

 reflective properties of the atmosphere, and inequalities of refraction arising from the 

 intermixture of strata of air of different temperatures and densities. But such appearances 

 in former times were readily converted by the imagination of the vulgar into supernatural 

 realities ; and hence many of the goblin stories with which the world has been rife, not 

 yet banished from the discipline to which childhood is subject, 



" As when a shepherd of the Hebrid Isles 

 Placed far amid the melancholy main, 

 (Whether it be lone fancy him beguiles, 

 Or that aerial beings sometimes deign 

 To stand, embodied, to our senses plain) 

 Sees on the naked hill, or valley low, 

 The whilst in ocean Phoebus dips his wain, 

 A vast assembly moving to and fro, 

 Then all at once in air dissolves the wondrous show." 



Pliny mentions the Scythian regions within Mount Imaus, and Pomponius Mela those 

 of Mauritania, behind Mount Atlas, as peculiarly subject to these spectral appearances. 

 Diodorus Siculus likewise refers to the regions of Africa, situated in the neighbourhood 

 of Gyrene, as another chosen site : " Even," says he, " in the severest weather, there are 

 sometimes seen in the air certain condensed exhalations that represent the figures of all 



