102 THE WONDER OF LIFE 



to be visible at about 3 fathoms, but we have to multiply 

 this by two since the light has to travel up again from the 

 plate, so that a common average for light-penetration is 6 

 fathoms. In very clear water, as in the Lake of Constance 

 in winter, the figure may rise to over 12 fathoms. And 

 this must be further extended if we take the chemical rays 

 into account, for silver chloride paper is affected at 55 

 fathoms and silver bromide paper at over 90 fathoms. 



The diverse coloration of freshwater basins raises a 

 number of interesting and difficult questions. Chemically 

 pure water is said to have an azure blue colour. The 

 addition of numberless impalpable dust particles pro- 

 duces a yellowish tint, which along with the primitive blue 

 gives green. Thus we have to thank the dust for the 

 colour of the lake as well as of the clouds overhead. But 

 the green is often in part due to millions of unicellular 

 Algae. A tawny yellow, familiar in the rivers of the Scottish 

 Highlands, may be produced by abundance of dissolved 

 organic matter humic acid and the like. A most remark- 

 able iridescence of water is sometimes seen when the sur- 

 face is covered with millions of the translucent moulted 

 cuticles of water-fleas, but the splendour of this has to be 

 seen to be believed. The practical importance of the colour 

 of the water is in connexion with its penetrability by light ; 

 the blue water is most penetrable, the green less, the yellow 

 still less. 



Various Lacustrine Regions. The littoral or shore 

 area of the lake may be broad or narrow according to the 

 configuration of the lake. Like the corresponding sea- 

 shore area, it is subject to great vicissitudes diurnal and 

 seasonal, it is often full of movement, it is strongly illumined, 

 it has a rich vegetation, and it is often crowded with 



