Ghostly ships play a role in Ulysses through
allusions to the wildly fanciful tales of two musical versions
of The Flying Dutchman,
but they are also seen through the skeptical lens of Bloom's
amateur science. Near the end of Nausicaa, as he lies
looking out toward Howth with darkness coming on, a strange
shape appears in the sky: "Were those nightclouds there all
the time? Looks like a phantom ship. No. Wait. Trees are
they? An optical illusion. Mirage." If the amorphous
shape he is trying to make out in the evening sky is a
ship, Bloom knows enough to explain the seemingly supernatural
vision as a known scientific phenomenon.
The optical illusions known as "superior mirages" are seen most often in the Arctic, but in cold waters like those around Ireland they can occur under just the right conditions, when a temperature inversion traps colder air along the sea's surface under a blanket of warmer air. The greater refractive index of the cold air bends light downwards, making a ship appear higher than it really is. The image can appear either right-side up or upside down. Such images are usually more stable than "inferior mirages," which are caused by a layer of hot summer air lying close to the ground: those tend to shimmer, vibrate, and lengthen vertically or horizontally.
By the end of the 19th century seamen's tales of ghost ships
were being countered with this scientific understanding. In
"When We Must Not Believe Our Eyes," a chapter in Frank R.
Stockton's Round-about Rambles in Lands of Fact and Fancy
(1910), the author assures his young readers that many scary
things which our eyes tell us are true are in fact not real.
He describes an incident when "awe-stricken sailors saw a
large ship, with all sails set, gliding over what seemed to be
a placid ocean, for beneath the ship was the reflection of it.
The news soon spread through the vessel that a phantom-ship
with a ghostly crew was sailing in the air over a
phantom-ocean, and that it was a bad omen, and meant
that not one of them should ever see land again."
The captain, who knows how disjunct layers of air can produce optical illusions, tells his men that "this strange appearance was caused by the reflection of some ship that was sailing on the water below this image, but at such a distance they could not see it. There were certain conditions of the atmosphere, he said, when the sun's rays could form a perfect picture in the air of objects on the earth, like the images one sees in glass or water, but they were not generally upright, as in the case of this ship, but reversed—turned bottom upwards. This appearance in the air is called a mirage." Directed by the captain to climb to the foretop, one of the sailors reports that "he could see on the water, below the ship in the air, one precisely like it. Just then another ship was seen in the air, only this one was a steamship, and was bottom-upwards, as the captain had said these mirages generally appeared. Soon after, the steamship itself came in sight. The sailors were now convinced, and never afterwards believed in phantom-ships" (277-79).