There is an angle at which the proportion of rays reflected to rays refracted becomes so much against the latter that the illumination of the bottom falls off very abruptly.
The transparency of the water along the littoral varies enormously. In times of rain, it is clouded by swollen streams pouring into it; in dry and still weather it becomes nearly as clear as in the open sea. There are also capricious and sudden changes caused by currents from the land or from the open sea, which are capable of producing great effects in a few hours. Experiments on the penetration of light, to have any value, should be made very far out.
When the water is comparatively clear, it still absorbs so much light that at thirty metres' depth, if the sky is clouded, one can not see distinctly enough to collect small animals. In a horizontal direction one can not distinguish a rock more than seven or eight metres off. When the sun is shining and the water is very clear, we can see a bright object at twenty or even perhaps at twenty-five metres. But in usual conditions we have to content ourselves with half these numbers. These facts, verified many times in the descents which I have executed with the diving apparatus of my laboratory at Nice during the last three years, appear to me important from several points of view.
It is evident that a submarine boat can not see its way under these conditions. Slow as may be its movement, there will not be time for it to retreat if it sees some obstacle rising in front of it; for it would not be more than ten metres away from the impediment at the moment of perceiving it. It will always have to take its directions before going down, and to sail only upon a ground the relief of which has been carefully explored. Submarine navigation will thus always be confined to limits which the genius of man—since it can not change the transparency of water—will never be able to enlarge.
These observations are also of great interest from a biological point of view. We can see every day that agile marine animals living in the illuminated strata of the waters—fishes, lobsters, and cephalopods—are in the habit, when they are frightened, of giving themselves up to a very rapid flight and quickly stopping. They feel that a few metres are enough to put them out of the range of vision of their pursuer. Some even take the pains to add to the obscurity of the water by discharging their ink, as the squids do, or stirring up the mud, after the manner of many fishes. Marine animals may well be near-sighted; for of what use to them is a long vision when they can at most see only a few metres away? Hence their crystalline lens is bulged into a nearly spherical shape. They live in a world of surprises, and, as it were, in a perpetual fog. The nets we stretch for them would
