25 2 THE POPULAR SCIENCE MONTHLY. 



of the North American lakes the eye can perceive objects on the 

 bottom at the depth of several hundred metres. Visibility ex- 

 tends to no such depths in either the Lake of Geneva or the Med- 

 iterranean Sea. The water of the Lake of Geneva is more trans- 

 parent 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, al- 

 though 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 sub- 

 stances 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 hun- 

 dred 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 proba- 

 bility 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 or- 

 gans may be trained to an extraordinary degree of delicacy in the 

 observation of the finer tints. I once visited the Gobelins tapes- 

 try factory in Paris in company with some painters ; the work- 

 men 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 lat- 

 ter 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 col- 

 lected 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 



