In many cases, where, from their excessive activity, it is difficult to catch these objects for a sufficient length of time in the field of view to study their parts, we have found it useful to diminish their powers of motion. This may be done by slightly warming the water, by suffering it to stand for a few hours in the glass, or by the addition of a small quantity of spirits, and probably of other sub stances. But slight injuries are sufficient to kill them ; and, as they then become invisible, the observer must be on his guard not to exceed in the application of these means.
From the necessity of thus using a large glass to con tain them, and from the freedom of motion thence allow ed, it is evident that a high magnifying power cannot be applied. It is scarcely possible, indeed, to make effective use of one greater than that produced by a simple lens of half an inch focal distance; and as, with this power, the field of view is very contracted, it is often convenient to have two other lenses at hand, of one inch and of two inches in focal distance. The very minute ones may, in deed, occasionally be secured under a compound micro scope, in a single drop of water ; but the observer will be disappointed much oftener than he will succeed, in his attempts to examine them in this way ; partly from the chance of his failing to find any in many successive por tions of water thus separated, and partly for the reasons that have just been stated.
Having thus far proved, as we imagine, that both the general luminous appearance and the bright scintillations emitted by the waters of the sea, are produced by the smaller or more imperfect and obscure marine animals; and having described all the general circumstances relat ing to them which are most important, we shall, as far as lies in our power, give a list of those that have been as certained to be luminous, reserving the consideration of the fishes to a future part of this essay. That this list is not larger, must be attributed to the inattention and pre judices of the observers, which we have already noticed ; and that it is longer in number than in names, arises from the discovery of many new animals by Dr. Alacculloch, which he has not yet arranged or described, or rather has not published. That this list will ultimately be extended to perhaps every inhabitant of the ocean, we entertain no doubt ; but While opinions are yet unsettled on this sub ject, we think it best to limit ourselves to such particulars as have really been ascertained. We shall place these
animals without any order ; as there is not a sufficient number of them to render an arrangement of any use, and as we have rather chosen to name them according to the authorities by which their luminous properties have been ascertained. Having very little confidence in the principles of arrangement by which many of the new genera have been formed, and considering that the whole subject requires a thorough revisal and reform, we shall use indiscriminately the ancient and the new names, just as they happen to occur. Naturalists will easily know the animals that are meant, under any nomenclature.
Among the crustaceous animals, several species have been found luminous by different observers, such as the Galathea amplectens of Fabricius, by Sir Joseph Banks, and the Astacus fulgens of the same author. Several lu minous species have also been found in the divisions of Gammaridex. The Beroe fulgens, of a genus now most improperly united to the Medusx, derives its name from its luminous property; and Sir C. Giesecke informs us, that he has found, in Greenland, many undcscribed spe cies of the same genus, which are also luminous. To these Dr. Macculloch has added eight new species, all equally possessed of this property. Some Beroes were also found to be luminous in Captain Tuckey's voyage. Sir C. Giesecke also mentions a Cyclops, which he calls brevicornis, possessed of the powers of phosphorescence. That is a genus, however, respecting which naturalists are not agreed ; as, indeed, they seem little satisfied, in general, with each other's arrangements respecting these animals. But, in the mean time, and for want of a better place, Dr. Macculloch has added to Sir C. Giesecke's ani mals thirty-three new species, and all of them highly lu minous. The Monoculus lynceus is also said, by the same naturalist, to be luminous; and, in that genus, Dr. Macculloch has introduced five new species, all lumi nous. The Nereis noctiluca has been often described as luminous; appearing, indeed, as we formerly remarked, lit have been the animal in which this property was first very generally noticed ; and, into that genus also, the last named observer has introduced three or, four new and luminous species. The Limulus noctilucus is describ ed as a phosphorescent animal in Captain Horsburgh's voyage.