The cause of this diffused phosphorescence was long the subject of curiosity, and was long unknown, but more than a hundred years ago (in 1764) the light was stated by M. Kigaut to proceed from a minute and very lowly organism, now known as Noctiluca miliaris; and subsequent researches have confirmed this opinion.  This Noctiluca is a spherical form of not more than one-fiftieth of an inch in size, with a slight depression or indentation at one point, marking the position of a mouth leading to a short digestive cavity, and having close beside it a filament, by means of which it probably moves about.  The sphere is filled with protoplasm, in which there is a nucleus and one or more gaps, or “vacuoles.”  Such is nearly all the structure that can be discerned with the aid of the microscope in this simple organism.

Nevertheless, this lowly form is the chief cause of that diffused phosphorescence which is sometimes seen over a wide extent of the ocean.  How innumerable the individuals belonging to this species must therefore be, may be left to the imagination.  Probably the Noctiluca is not rivalled in this respect even by miscroscopic unicellular algae which compose the “red snow.”

By filtering sea-water containing Noctilucae its light can be concentrated, and it has been found that a few teaspoonfuls will then yield light enough to enable one to read holding a book at the ordinary distance from the eyes—­about ten inches.

A singular and highly remarkable case of diffused marine phosphorescence was observed by Nordenskioeld during his voyage to Greenland in 1883.  One dark night, when the weather was calm and the sea smooth, his vessel was steaming across a narrow inlet called the Igaliko Fjord, when the sea was suddenly observed to be illumined in the rear of the vessel by a broad but sharply-defined band of light, which had a uniform, somewhat golden sheen, quite unlike the ordinary bluish-green phosphorescence of the sea.  The latter kind of light was distinctly visible at the same time in the wake of the vessel.  Though the steamer was going at the rate of from five to six miles an hour, the remarkable sheet of light got nearer and nearer.  When quite close, it appeared as if the vessel were sailing in a sea of fire or molten metal.  In the course of an hour the light passed on ahead, and ultimately it disappeared in the remote horizon.  The nature of this phenomenon Nordenskioeld is unable to explain; and unfortunately he had not the opportunity of examining it with the spectroscope.

If we come now to consider the more partial phosphorescence of the sea, we find that it is due to animals belonging to almost every group of marine forms—­to Echinoderms, or creatures of the sea-urchin and star-fish type, to Annelid worm, to Medusidae, or jelly-fish, as they are popularly called, including the “great domes” and the “silvery disks” of the passage above quoted from Professor Martin Duncan, to Tunicates, among which is the Pyrosoma, to Mollusks, Crustaceans, and in very many cases to Actinozoa, or forms belonging to the type of the sea anemone and the coral polyp.