In a species of cancer seen by Smith in the Gulf of Guinea, the light (which seemed to be emitted by the brain) was of a deep blue colour when the animal was at rest ; but when it moved, bright coruscations of silvery light were darted from it in all directions. The light of some centipedes inhabiting the islands of the Pacific is of a beautiful emerald-green colour. It is connected with a mucous matter covering the animal, which may be rubbed off by the fingers, and communicates to them a smell not unlike that of muriatic acid.
Sometimes the light proceeding from the sea is so white and dull as to give the effect of a sea of milk. This is frequently seen in the Gulf of Guinea, and seems to be caused some times by the presence of numerous Salpe and Scyllari, at other times by the admixture of the debris of fishes and other marine animals re cently dead.
An extraordinary series of phenomena con nected with a particular display of the lumi nousness of the sea, is reported by Mr. Hen derson as having occurred in the Atlantic, (lat. 2° long. 20' NV.) on the 5th March, 1821.. About 9 p.m. the sea appeared unusu ally luminous. Every person who kept his eye fixed upon it for but a short time was immedi ately affected with giddiness, headache, pain in the eyeballs, and slight sickness. Although these symptoms varied in intensity amongst the spectators, yet there was not one on board who did not feel some degree of them; and all im puted them to the effect of the light proceeding from the surface of the ocean. Mr. Henderson remarks : " For my own part, the headach, &c. which followed immediately my looking at the water, was particularly severe, nor did it go off until morning. The effects I experienced were like those produced by smoking too much tobacco."' There have been recorded some accounts of very intense light produced over a great extent of the ocean's surface by luminous animals, but it does not appear that any other voyagers have experienced physical effects from the light such as are described by Mr. Henderson. The great intensity with which it is occasionally produced by marine animals, however, is well illustrated by the descriptions that are given of the moral emotions with which it inspires the beholders. Witness, for instance, Mr.Bonnycastle'sdescrip tion of a scene which he met with in the Gulf of St. Lawrence, (7th Sept. 1826.) While it was very dark, a brilliant light, like that of the Aurora, was seen to shoot suddenlyfrom the sea, in a particular quarter. It spread thence over
the whole surface of the water between the two shores of the Gulf; and shortly there was pre sented " one blazing sheet of awful and most brilliant light." " Long tortuous lines of light showed many large fishes darting about as if in consternation at the scene." The light was suf ficient to enable one to see the most minute objects on the ship's deck. On drawing up a bucketful of the water, and stirring it with the hand, it presented "one mass of light, not in sparkles as usual, but in actual coruscations."-f Messrs. Quoy and Gaimard state that in handling luminous marine animals while alive, they have always been sensible of an odour pro ceeding from them similar to that which is per ceived around a highly charged electrical appa ratus.
The only observation with which we are ac quainted that seems to indicate the evolution of heat in connexion with the light of animals, is that reported by Macartney, who states that he found the thermometer raised by two or three degrees when placed in contact with a group of living glow-worms shining, or even with their light-giving sacs cut off. The repetition of this experiment, however, has not produced the same result in the hands of others : they saw no rise of the thermometer.
III. Circumstances in which light is given out, and by which its intensity is affected.— It is not known whether there be any lumi nous animals that give out light in all circum stances, and at every period of their existence, in their natural situations. So far as observa tion extends, certain molluscs, and some of the species of dater appear to shine without inter mission. But most of the other light-giving animals with which we are acquainted use their peculiar function only occasionally, and that, for the most part, under some kind of excite ment or irritation, natural or artificial. In the absence of more direct means of investigation, we may, perhaps, attain to some measure of acquaintance with the nature and analogies of animal light by inquiring into those sources of irritation under which it is given out. Here, however, we are met by the difficulty of fig contradictory statements of facts made ferent observers. So that our exact knowledge on the subject is still insufficient to admit of any satisfactory conclusions being drawn. What is known on this point may be conveni ently considered under the two following heads.