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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 Pyrosoma 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 hemisphterica 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. Langstaffe 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. 



