Further, a great many of the animals that yield distinct and bright sparks, are either of microscopic dimensions, so as to be utterly invisible to the naked eye, or, if not so very minute, are rendered thus invisible by the combina tion of their transparency with their very limited dimen sions. Those which produce a general diffused light are, in particular, thus microscopic, while they are also abun dant, so as to crowd the water like the common infusoria. Among those which we have examined, there d re innu merable animals, and of many species, that do not reach to the hundredth of an inch in dimensions ; there are even many not larger than the two hundredth of that measure, and perhaps many more that we have never discovered, still smaller. The transparency and low refractive den sity of these contribute, no less than their minuteness, to render them difficult to discover, while there are difficul ties in managing the lenses, the light, and the objects themselves, which throw no small additional impediments in the way of the investigation.
It is not possible, in the first place, to conduct these inquiries by means of the compound microscope in the usual manner, and in a single drop of water; because, although such animals may abound in the sea, it is perfect ly easy to take out many successive drops without entrap ping one. Moreover such a drop dries quickly under the lens, even while the lights and foci are arranging ; and whenever that happens, all the animals which it may in clude disappear for ever, so small is the quantity of solid matter which they contain. They must therefore be sought for in a large glass, where they may swim at liberty by the aid of a simple lens. Even here it is difficult to find them, though we should have previously ascertained that they were present, and that for many reasons. The wa ter, in the first place, is commonly turbid where they abound, or even where they exist. Those also alone come into view, which approach so near to the fore part of the glass as materially to diminish the column of water be tween them and the eye ; and thus also they often escape observation, while the spectator is surprised to find that he can discover nothing in the light, when, in the dark, the water has abounded in luminous sparks. When a lens is used, it is also only in the observer's power to gain a sight of those which pass immediately through its focal point ; so that, in this case also, he is apt to underrate their num bers, or, if rare, to doubt their existence altogether. By the mere light of day it is scarcely possible to discover them at all, and that of the sun is too bright for the eye to bear. It is not then very surprising, if, under these diffi culties, incautious, or hasty, or inexpert observers, should fail to discover them even where they most abound.
Another cause yet has tended to deceive naturalists re specting the presence of minute animals in sea-water, and that is their speedy solution or disintegration after death. That many of them consume much oxygen, and that all con sume some, is from analogy probable. Hence it is, probably, that they shortly die when confined to a limited quantity of water. This is very remarkable in the case of the me dusa; and thus, if, where smaller animals are present, the water is laid by till the morning for examination, nothing will be found but milky fibres floating about in it.
To these various and neglected sources of error, we must yet add two more, by which the luminous animals themselves contrive to deceive the observer. There is no doubt that the light is under the command of the animals, and that from this, perhaps from other causes also, it is subject to intermissions. Thus, whether from caprice, or fa tigue, or a voluntary effort, or some other causes yet un known, they often refuse to show their light even when vio lently agitated or injured; if we are right respecting the final causes of this property, which we shall hereafter suggest, any injury is precisely what would tend to make them ex tinguish it, for the purpose of escaping from the pursuit of their enemies. Hence, although they have been lumi nous when in the sea, or have even shown their lights af ter being taken, they afterwards refuse to shine again in spite of all our attempts. Thus a naturalist, finding that his prize abounds with animals, and that he cannot never theless procure any light from them, is apt, perhaps natu rally enough, to imagine that the cause of the cence lies elsewhere, not in the animals, but in the sea water itself.
The last of these sources of deception which we shall here point out, arises from the nature and magnitude of the light compared to that of the animal ; and this operates in two ways. A naturalist who happens to know that a dead medusa, lying on the beach, is luminous all over, ex pects to find the same animal displaying the same light when alive. Yet the two cases are perfectly distinct ; and this is an important fact to notice, as it relates to the light itself, independently of the object now under considera tion. The luminous part of even a large medusa, or other animal, is sometimes extremely small; a single bright spot, perhaps not equal to an hundreth part of its dimen sions. Hence the observer who has seen in the water a bright spot, not larger than a pin's head, and has taken a medusa of three or four inches in diameter, cannot ima gine that it proceeds from such an animal, particularly when he cannot induce it to make a display again ; and thus he returns to his favourite hypothesis of luminous water. He is equally subject to be deceived by a circum stance precisely the reverse of this. In many of the very minute animals, front its brilliancy and power of radiation, as happens in a star, the spot of light appears as far to exceed the animal in dimensions. It is by no means uncommon, thus, in some of the luminous species, to find that a light, which seems in the sea as large as a pea, is caused by a creature not the twentieth of an inch in dia meter. In this way an observer brings on board a bucket of water, which he sees, when over the ship's side, to be full of such large lights, and examining it afterwards by candle-light, finds nothing but a crowd of transparent crea tures, nearly microscopic, and of course, under these cir cumstances, unwilling to shine, or exhibiting no light. Thus he falsely concludes that the animals which he is . inspecting are not the cause of these lights, and once more returns to his favourite mysterious property of sea water.