222 M. W. Beetz on the Colour of Water. 



courses it Las traversed great distances, it is blue. It also ap- 

 pears green when it has passed through the moderately thin 

 section of a wave (which it may indeed have reached by reflexion 

 from another wave). 



I spoke just now of the reddish-yellow colour in the almost 

 dry places which has been noticed by so many observers. This 

 colour depends entirely on the nature of the ground. Most 

 frequently it consists of whitish sand, or whitish pebbles. If 

 these were absolutely white, if they reflected colours in a diffused 

 manner to the same extent, the reddish colour would not occur. 

 A new porous clay-cell of a Grove's battery may appear quite 

 white, while, when it is moistened with water, it is of a rusty yellow 

 or flesh-red colour. Hence its surface acquires the property of 

 reflecting red light to a preponderating extent. If, now, the 

 substances which constitute the sea-bottom have the same 

 property, the bottom will appear red in those parts in which it 

 is covered with quite thin layers of water. If the thickness of 

 the layer of water increases, fewer red rays reach the bottom ; 

 the returning rays are again partially absorbed by the water, 

 and thus the red colour is continually disappearing, although 

 the forms of substances lying on the ground can always be 

 distinctly perceived. 



Moreover, this red colour is much increased by contrast. In 

 the dry places of the Aar I have often observed that the bright 

 red, which they show, dimininishes considerably when they are 

 viewed, not near the beautiful green of the deeper water, but 

 through an isolated tube. 



There might seem to be a fact in disaccordance with the 

 statement, that sea-water in thin layers is green, and blue in 

 thicker; a white object, for instance an oar, appears of a di- 

 stinctly pure blue when immersed at even a very inconsiderable 

 depth below the level of the Achensee, while it is of an intense 

 green below the Tegern- or Konigsee. The light which impinges 

 upon the white surface of the oar, has had to traverse a much 

 more considerable distance than that from the surface of the 

 water; it comes from the side through a considerable mass of 

 water, in which it has assumed the characteristic colour of the 

 lake. But if the same white surface is brought near the bank 

 and turned towards it, and is at the same depth as in the former 

 case, it is seen in the Achensee to be of an almost unaltered 

 white, while in the Tegernsee it is always somewhat greenish ; 

 for the colour of the blue water is only perceptible at great 

 distances, that of the green at very small ones. 



This surprising strong coloration in consequence of laterally 

 incident light, led me to the proposal which Arago has made, 

 to investigate the true colour of water in transmitted light. 



