of the North American lakes the eye can perceive objects on the bottom at the depth of several hundred metres. Visibility extends to no such depths in either the Lake of Geneva or the Mediterranean Sea. The water of the Lake of Geneva is more transparent in winter than in summer, but in this lake, as well as in the sea-waters that have been thus far examined, the extreme limits of visibility are at forty-five, and at most fifty metres' depth. Observations in diving apparatus have shown that one is there as in a blue cloud, and can only see some seven or eight metres in a horizontal direction, in exceptional cases twenty metres, and at most twenty-five metres. But the seeing man can dive with the apparatus only to a depth of thirty metres, and, although he can not see clearly, he is surrounded by diffuse light.
The light from above must therefore penetrate more deeply. A more closely approximate measurement has been made by such means as sinking sensitized photographic plates into the water, and exposing them to the light at fixed depths, or by sinking substances which are chemically acted upon, changed, or destroyed by light, so that the measure of the alteration may at the same time furnish the measure of the strength of the acting light. Photographic experiments have shown that a depth of four hundred metres in the Mediterranean Sea is the average limit to which a blackening of the plate can be verified.
Thus light penetrates to ten times as great a depth as our eye, and this is an important point—a whole zone, three hundred metres in thickness, receives light and thus also sends up rays which our eyes can not immediately distinguish, but in all probability perceives through the mixture of the color tones which they produce. It is known that there are other differences than those of blindness to certain colors in the eyes of men, and that our organs may be trained to an extraordinary degree of delicacy in the observation of the finer tints. I once visited the Gobelins tapestry factory in Paris in company with some painters; the workmen could distinguish with ease and indubitably tints which looked identical to our unskilled eyes. There must, to return to our subject, radiate up from that depth to the surface, light, of a bluish color, which makes far less impression on our eyes than the colors called warm, yellow and red, which—especially the latter—are absorbed by the water.
It was formerly believed that total darkness reigned in the greater depths of a thousand metres and more, and that the collected colors of deep water were seen on a black ground. But, in the light of the recent deep-sea investigations, this idea must be given up, along with the other one that once prevailed, that there is no animal life in great depths. Most animals living in dark caves have atrophied or no eyes; there are also living
