CHLOROPHYLL AND LIGHT INTENSITY. 389 



dow and without warmth. The red becomes darker, purple, finally black, and 

 impenetrable night holds you enchained ". This description is founded doubtless 

 on the account of divers of the olden time, according to which red light should 

 predominate in the abysses of the ocean. These accounts must, however, be 

 retained only to the following extent. The cliffs and the rocky bottom to which 

 the divers descended might have been richly carpeted with red Floridese, possibly 

 also just then the strata of water above were filled with those unicellular red 

 algse, which cause the so-called " flowers of the sea ". In the neighbourhood of 

 the mouth of the Tejo at times a superficial area of sixty million of square 

 metres is coloured scarlet by Protococcus Atlanticus, a unicellular alga, 40,000 

 of which cover only a square millimetre ; and Trichodesmium Erythrceum, 

 another microscopic alga consisting of bundles of delicate articulated threads in 

 innumerable milliards, fills the watery strata in the Red Sea as well as in the 

 Indian and Pacific Oceans, so that there immeasurable stretches of water receive 

 a dingy red colouring. When these algse make their appearance the sea is said 

 to blossom, and at those times the depths may appear to the diver as shrouded 

 in a reddish-yellow twilight. At times the same colour has even been observed 

 in the Lake of Geneva when its waters had been disturbed; it is due to the 

 fact that the blue rays of the incident light are weakened by the fine atoms 

 suspended in the water. With respect to this occurrence, we may consider that 

 the above-mentioned accounts of divers are not the results of intentional decep- 

 tion, but only refer to particular cases. They cannot be applied universally. As 

 a matter of fact, the colour of sea-water, in direct as well as in reflected light, 

 is blue, and the diver who carries on his work at the bottom of the untroubled 

 and non- blossoming sea, is not surrounded there by red, but by blue light. 

 The greater the quantity of salt contained in the water, the deeper the blue. This 

 blue nowhere appears so beautiful and so deep in tint as in the Dead Sea, and in 

 the region of the Gulf Stream and of the Kurosiur, where the water is particularly 

 rich in dissolved salts, and also has in the upper strata a comparatively high tem- 

 perature. The blue colour of the water is explained thus: from among the rays 

 which are characterized by different wave-lengths and different refrangibility 

 (which, taken together, form colourless daylight, and which we admire separated 

 in the colours of the rainbow), the red, orange, and yellow are absorbed in their 

 passage through the water, and only those rays which are characterized by high 

 refrangibility, viz. the blue, are allowed to pass through. The rays on the further 

 side of the red, not perceptible to our eyes, the so-called dark heat-rays, are like- 

 wise absorbed in their passage through the water, and an object at some depth under 

 water would therefore only be reached by rays of high refrangibility, particularly 

 blue rays. The conditions of illumination for plants growing in the depths of the 

 ocean are consequently in reality quite unfavourable. It is not only that a portion 

 of the light falling on the surface of the water is reflected, and the other portion is 

 weakened by its passage through the water, but besides, those rays which are 

 necessary to the formation of organic matter by the chlorophyll-granules in the 



