230 Mr Blackadder's Remarks on an Optical Phenomenon, fyc. 



ed as in a concave mirror, is against this supposition, and ren- 

 ders it probable that the appearances were produced by un- 

 equal refraction. 



In that state of the air which gives rise to unequal refrac- 

 tion, the natural appearance of objects undergoes surprising 

 modifications. They are seen diminished, magnified, multi- 

 plied, and transformed, so as to bid defiance to description ; 

 mountains seem levelled or divided, and plains raised into 

 mountains. On the occasion referred to, some part of the 

 island of Sicily, lying to the west of ^Etna, may have had its 

 appearance thus modified, and the aerial creation may have 

 had such a resemblance to that mountain as to have been mis- 

 taken for its image. If the observed image was evidently at 

 a greater distance than any part of the island, every allowance 

 being made for ocular deception, the appearances may still be 

 traced to unequal refraction. Mr Hughes states that the 

 island of Malta, which is distant about 140 miles, has been 

 seen from the summit of iEtna.* At the time of the year when 

 he and his party visited that mountain, its shadow at sunrise 

 is nearly in a line with the solitary island of Pontellaria, which 

 is distant about 180 miles. If, then, Malta has been seen, whe- 

 ther aided by looming or otherwise, at the distance of 140 

 miles, Pontellaria may also be seen, through the influence of 

 unequal refraction, though it be forty miles farther off. Pontel- 

 laria thus elevated, and rendered visible at the extremity of 

 the shadow of JEtna, may have been the source of deception. 

 As the diurnal altitude of the sun increased, causing the sha- 

 dow of the mountain to diminish, the state of the air upon 

 which the unequal refraction depended would likewise be 

 changed, and the image would consequently disappear. It 

 may readily be determined, either by those who have witnes- 

 sed, or who may yet have an opportunity of witnessing, the 



* It appears, that, from the summit of Ben Lomond, which is 3191 feet 

 ahove the level of the sea, the Isle of Man is visible, though distant about 

 130 miles. The Peak of TenerifFe, 12358 feet in height, has been descri- 

 ed at a still greater distance from the deck of a ship. Mount JEtna, by 

 barometrical measurement, is 10936 feet in height ; and mountains at a 

 greater distance than the Island of Pontellaria are said to have been dis- 

 tinguished from its summit. 



