Page:Proceedings of the Royal Society of London Vol 1.djvu/401

Rh of thirty-ﬁve feet below the surface of the sea. As this light occa- sionally disappears entirely. and reappears after a considerable inter- val. Spallanzani supposed the animal to be luminous only when in motion, and that the cessation depends upon its being at perfect rest.

The Pyrosama Atlanticum, discovered by Peron, seems conﬁned to certain latitudes, and its light, like that of the former, is supposed to be visible only during motion.

Mr. Macartney has himself discovered one Beroe not before ob- served, and two Medusae, unless one of the latter be, in fact, the Medusa hemisphcericu of Gronovius and of Muller, who did not per- ceive it to be luminous. The second Medusa, from its extreme mi- nuteness and brilliancy. he calls Medusa scintillans : on account of its smallness it cannot be separated from the water but by straining through a cloth. When a small number of them are put into clear water, it is difﬁcult to distinguish them while separate, on account of their minuteness and transparency; but as they gradually collect at the surface of the water, they then appear together of a dusky straw colour.

It is to this species of Medusa that the author is inclined to ascribe many phenomena of illumination of large portions of the sea which have been described by navigators.

In some instances the sea has been compared to a plain of snow. Capt. Horsburgh saw it of an uniform white colour, like milk, on the Malabar coast, and says it has frequently that appearance in the Banda Sea. The same was observed by Mr. Langstaife in a voyage from New Holland to China; and it was ascertained by him to arise from numerous minute bodies of the size of small pins' heads, which, when lifted out of the water by adhering to the hand, were found connected together as a chain.

At Margate the author has seen these in great abundance; and in certain parts of Milford Haven they are generally so numerous, that on one occasion he separated, by straining, a pint of Medusae from a gallon of the water.

Mr. Macartney next examines the particular structure of those insects which possess distinct organization for the production of light; as, the various species of Lampyris, Elater, Fulgora, and Pausus.

The light of the genus Lampyris resides generally in the last rings of the abdomen. In the common glow-worm these rings are very transparent, and there is diffused over their internal surface a yellowish substance that has been compared to paste; but the author observes that the matter is organized. He also remarks, that in the last ring of the abdomen there are two small sacs that emit a light brighter than the rest, and apparently less under the control of the will, as it is more permanent.

In the Elater noctilucus and Elater ignitus, there is a similar soft yellowish substance underneath the corselet. But in those of the genus Fulgora, which the author has had opportunities of examining, this peculiar matter was not distinctly observable.