ACCOUNT OF AN OPTICAL ILLUSION, &C. 389 
went out on a boating excursion, from the White Rocks 
near Portrush, and on our return, at 5 oVlock in the after- 
noon, we first saw this optical deception make its appear- 
ance close to the Skerry Islands, which are situated in the 
sea, about four miles distant from the main land. We 
now saw a second set of Islands, at an extraordinary eleva- 
tion, apparently SOO yards above the level of the sea, 
breaking off by degrees in the centre, the top branching 
into appearances resembling old castles, towers, spires, with 
men and cattle running over them ! It sometimes also pre- 
sented an appearance, which I can only liken (though it 
may appear whimsical) to a regiment of dragoons gallop- 
ing and manoeuvring ! On our first observing the phe- 
nomena, the sailors cried out, " There''s the Fairy Islands,*" 
— the name given to these optical illusions by the pea- 
santry. The appearances continued for the space of an 
hour and a half, and we remained a considerable time 
on our oars observing them, — but the evening began to 
close in with a thick mist, and the objects vanished from 
our sight. 
The Skerry Islands where these phenomena occurred, 
are a small group of rocks a few miles out in the sea, and 
nearly opposite Portrush, being about six miles distant 
from the Gianf s Causeway, Downhill, and Dunluce Castle. 
I mention these particulars with a view to explain the suc- 
cessive appearances. This phenomenon seems to be caused 
by the irregular refractions which the rays of light occa- 
sionally suffer, by passing through the different strata of 
the lower atmosphere. Now, there are old castles and 
spires on the coast opposite the Skerry Islands, where 
these optical deceptions occurred, and to account for the 
appearances just mentioned, the rays of light in passing 
through the lower strata of the air, should describe curves, 
whose final tangents point in the visual directions of the 
