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XVII. — Account of an Optical Illusion called the 
Fairy Islands, seen on the North Coast of 
Ireland, 
In a Letter from Mr Samuel Thomas Gregg to 
Mr Stewart. 
(Read I5th December 1827J 
Sir, 
Oaving been an eye-witness, with five others, of the fol- 
lowing optical phenomena, commonly called the Fairy Is- 
lands, in that part of the country where they are seen by 
the peasantry, but which I take to be the effects of the 
mirage, I thought an account of them might be interest- 
ing to your scientific friends, and I now send the particu- 
lars, that they may be added to the facts already accumu- 
lated on this subject. 
The appearances in question occurred in September, 
and the morning of the day on which they were seen was 
so extremely hazy, that the Londonderry steam-packet 
could not proceed on her voyage till 12 o'clock noon, 
when the mist cleared off*, and the day turned out fine, 
with a gentle breeze. The wind now changed to the east, 
and we were informed by the people that these phenomena 
are only seen when it blows from that quarter. A party, 
consisting of my father, brother, three sailors, and myself. 
