DEAR MR. HOLDER,--I remember on one occasion, when sailing on the Indian Ocean, the night was dark, but the crest of every wave glowed with light. As our vessel moved swiftly through the water, dashing the foamy waves on each side of her bows, she left bright streaks of light that reached far behind us. Every undulation of the water was lit up with scintillating points of light; and the ocean round us was so luminous, it would for splendor vie with the finest pyrotechnic display. So intense was the glow, the hull and sails of the vessel were illuminated by it; and, as I gazed at the glorious spectacle, I was filled with wonder and delight. The scene changed constantly, sometimes less brilliant than others, then again every rope in the ship was lit up; this, I presume, from the animals being more or less numerous. At the same time, darting in every direction, could be seen numerous fishes, making distinct streaks of light. Luminous spots from one to two inches in diameter were observed some distance under the water. These were _medusæ_. We captured many in our nets, and placed them in buckets of water on the deck, where they still continued to emit phosphorescent light. The grand scene lasted most of the night, and was faintly visible till dawn of day. In the year 1868 or 1869, as I was dredging and collecting on the reefs near Port Louis, Mauritius, I met with one of the most singular sights it is possible to conceive. My Lascar crew gently moved my boat over the reefs, so that I could see any object in the water. The day was beautiful, not a cloud in the sky; but the bright sun shone down into the clear waters of the Indian Ocean, scarcely marked by a ripple on the surface. As the boat crossed over the shelving reef into deep water, what was my astonishment to find the depths alive with hundreds of millions of little creatures (which I supposed to be jelly-fish), actively moving about in the water, as far as the eye could penetrate. The little creatures, as they flitted about, emitted all the colors of the spectrum; the most brilliant diamond could not vie with the coruscations of light sent out by them. Such a scene must be witnessed to form any idea of its magnificence: the whole ocean was aglow with colored lights. I threw over my hand-net, and drew many thousands into the boat, which I placed in a large glass jar filled with sea-water, where I could examine them. They proved to be _young polyps of different species of the reef-building corals_. Those that I carried home were still luminous in the evening, and I thought I would pay a visit later, to see the effect at night; and it was truly great. The sea was one vast area of luminosity. The illumination extended for miles. Fishing-boats making for Port Louis harbor could be plainly seen a long way off by the phosphorescent