IF you happened to cast your eye across this column yesterday, you may recall that Weather Eye concerned itself with the oriental peregrinations of an Irish expatriate as the sun descended on the British Empire. "I took passage on a small ship" his story went, "that ran along the Borneo coast past islands that were like jewels in seas that seemed to change colour from dark to blue to shining emerald green. At night the water shone with phosphorescence, and sometimes we could even make out the banks of rivers by the glow of millions of fireflies along them." Now, what is this talk of phosphorescence?
Phosphorescence of the sea is a luminous glow emanating from millions of tiny marine organisms, mostly of the species known as Noctiluca miliaris. These little creatures are about one fifth of a millimetre in length, just large enough to be seen by the naked eye as tiny separate dots, and their chemical make-up is such that they emit light when oxygen is dissolved in the water that surrounds them. This happens, for example, when the sea is churned up by breaking waves.
Phosphorescence is more frequent in coastal waters than in the middle of the ocean, and is to be seen at its most spectacular in the tropical oceans of the world. Passengers aboard a ship in those a.parts are often treated to a display of an almost continual shower of sparks of light, made up of a myriad of tiny luminous marine animals disturbed and oxidised by the ship's passage through the water. In the Indian Ocean, in particular, the entire sea seems luminous at times, and sometimes a system of enormous bands of light seem to rotate like the spokes of a wheel across its surface. This phenomenon, sometimes referred to as the "light wheels of the Indian Ocean", results from the combined optical effects of the phosphorescent bow waves generated by the vessel, the background winds waves, and the motion of the ship itself.
But even around our less exotic Irish coasts, phosphorescence of the sea is not at all uncommon. It is said that if you immerse your finger in the sea by day when these phosphorescent creatures are present in great numbers, you may feel a slight prickly sensation, and by this means you can foretell whether the breaking waves should glow that night or not. Then, if you stay well away from background lights and allow your eyes to become accustomed to the dark, you may see, as Shelly did,
. . . waves upon the shore
Like light dissolved in star-showers, thrown.