"I have fathered a rival," chuckled the Emperor Napoleon, "who will sooner or later humble England's pride". The source of his amusement was the sale of the colony of Louisiana to Thomas Jefferson, President of the United States, for the bargain price of $15 million.
Louisiana, nowadays, is small. But at that time the name applied to a vast expanse of land extending from the Mississippi to the Rocky Mountains, and from the Gulf of Mexico to Canada. The Louisiana Purchase, as it was called, doubled the size of the United States in 1803, paving the way for that country to realise the emperor's ambitions and become one of the most powerful nations in the world.
Not long before the purchase, however, Louisiana would not have been Napoleon's to sell. It belonged to Spain. Half a century or so previously, Capt Antonio de Ulloa had landed at New Orleans to take office as the first Spanish governor of the colony, and although his somewhat stormy sojourn in the New World has long since been forgotten, his name lives on in meteorology; he was the first to draw attention to an optical phenomenon which bears his name, the Circle of Ulloa or Ulloa's Ring as it is sometimes called.
Ulloa's Ring is a "fogbow", closely related to the rainbow, but consisting of an arc of luminous white light without the spectral colours. The ordinary rainbow is seen when a passing shower provides a curtain of raindrops on the side of the sky directly opposite the sun. If the shower happens to be in the correct place at the appropriate time to make the angles right, light from the sun is reflected from inside the raindrops and directed back towards the eye. As part of the process, the white sunlight is broken into its constituent spectral colours and the result is the familiar multicoloured pattern we call a rainbow.
It is because drops of rain are relatively large that the colours of the rainbow stand out in such clear concentric semicircles. But if, say, early in the morning, there lies ahead of us, not a shower of raindrops but a bank of fog or cloud, the bow will have a different look. Droplets of fog or cloud are a mere 100th of the size of those of rain; the same internal reflection takes place, but the colours of the resulting arc overlap with one another and recombine to form the colour white. The result is Ulloa's ring, a luminous semicircular "fogbow", bright-white, without the distinctive colours of the rainbow.