Among the many revelations of modern science, none have a more absorbing interest than those relating to the illumination of the deep sea. Until within a few years the ocean has been a sealed book. The surface forms only were known; and it was assumed that, owing to the enormous pressure, lack of sunlight, and consequent darkness, Nature, at least in the abyssal depths, was at fault, and this vast region was devoid of life and incapable of supporting it. Recent investigations, however, have shown the reverse, and that this great area, with its plateaux, its mountain ranges, and its isolated, coral-capped peaks, whose valleys are now known to lie miles in ocean depths, teems with living forms, and, far from being the dismal realm we had supposed, is a region of surpassing wonder; which we may, _in fancy_, term that lower firmament, where float sparkling, gleaming constellations, meteor-like disks and globes with trailing luminosity, single stars and _nebulæ_ of living lights. The phosphorescence of the sea is no new discovery, and those who have visited the seashore at night must have witnessed this phenomenon. The region of coves and beaches along the shores of Eastern Massachusetts, around Nahant particularly, is a favorable one for its full display. As the waves come rolling in upon the rocks, or upon the long, expansive shingle, in tidal measure, we see the foaming crest, seemingly igniting all along the line, more and more intense in brilliancy, when, with a roar, it breaks, masses of scintillating liquid upon the sands. We glide over the smooth portions of this sea, our boat leaving a golden train; and every dip of oar, or the dash of some affrighted fish, creates an equally vivid display. Even when not disturbed, looking down into the calm, clear depths, the same phenomenon is witnessed. Pale, ghostly forms are seen here and there, moving slowly about, while the seeming silvery atoms suggest the _nebulæ_ of this submarine sky. Deeper yet, the bottom shows weird splendors. The great kelps are bedecked with mystic lights, and gleam like diamond’s flash from ledge and rock. These wonderful exhibitions of submarine illumination are due to the presence of luminous creatures, or in some cases to large animals swimming through immense numbers of small phosphorescent bodies, so appearing as light-givers themselves. In nearly every branch of the animal kingdom we shall find these _living lights_; some marvellously brilliant, others glowing with dim rays, and all contributing often to wondrous illumination, far-reaching or circumscribed. [Illustration: PLATE I. LUMINOUS PROTOZOANS. _Noctilucæ_ (in milk). _Noctiluca miliaris_ (magnified 100 diameters). _N. miliaris_ (slightly magnified). LUMINOUS PORTION OF NOCTILUCA (highly magnified).]