Perhaps there are few natural subjects of observation which have been so long and so much debated as the luminosity of the waters of the sea. What we know with certainty on the subject may be reduced to the following simple facts. There are several luminous animals which, when alive, give out at pleasure a faint phosphoric light: this light is, in most instances, rather bluish, as in Nereis noctiluca, Medusa pelagica var. β (Forskäl, Fauna Ægyptiaco-arabica, s. Descriptiones animalium quæ in itinere orientali observavit, 1775, p. 109), and in the Monophora noctiluca, discovered in Baudin’s expedition, (Bory de St.-Vincent, Voyage dans les Iles des Mers d’Afrique, 1804, T. i. p. 107, pl. vi.) The luminous appearance of the sea is due partly to living animals, such as are spoken of above, and partly to organic fibres and membranes derived from the destruction of these living torch-bearers. The first of these causes is undoubtedly the most usual and most extensive. In proportion as travellers engaged in the investigation of natural phenomena have become more zealous in their researches, and more experienced in the use of excellent microscopes, we have seen in our zoological systems the groups of Mollusca and Infusoria, which become luminous either at pleasure or when excited by external stimulus, increase more and more. The luminosity of the sea, so far as it is produced by living organic beings, is principally due, in the class of Zoophytes, to the Acalephæ (the families of Medusa and Cyanea), to some Mollusca, and to a countless host of Infusoria. Among the small Acalephæ, the Mammaria scintillans offers the beautiful spectacle of, as it were, the starry firmament reflected by the surface of the sea. This little creature, when full grown, hardly equals in size the head of a pin. Michaelis, at Kiel, was the first to show that there are luminous siliceous-shelled infusoria: he observed the flashing light of the Peridinium (a ciliated animalcule), of the cuirassed Monad the Prorocentrum micans, and of a rotifera to which he gave the name of Synchata baltica. (Michaelis über das Leuchten der Ostsee bei Kiel, 1830, S. 17.) The same Synchata baltica was subsequently discovered by Focke in the Lagunes of Venice. My distinguished friend and Siberian travelling companion, Ehrenberg, has succeeded in keeping luminous infusoria from the Baltic alive for almost two months in Berlin. He shewed them to me in 1832 with a microscope in a drop of sea-water: placed in the dark I saw their flashes of light. The largest of these little infusoria were 1-8th, and the smallest from 1-48th to 1-96th of a Paris line in length (a Paris line is about nine-hundredths of an English inch): after they were exhausted, and had ceased to send forth sparkles of light, the flashing was renewed on their being stimulated by the addition of acids or of a little alcohol to the sea-water.