474 
Proceedings of the Royal Society 
changed in the same way as when seen through a blue transparent 
medium, such as a piece of glass. The white changed to blue. 
The red darkened very rapidly as it descended, a very small 
depth of water being sufficient to destroy all the colour. At a 
depth of about 2 m. a very brilliant red was so darkened as to 
appear a dark brick red. Yellow changed to green, by the water 
absorbing the red component of the yellow. An orange, as it sunk 
in the water, appeared to become more and more unripe, while a 
lemon became quite green. The purple surface quickly changed to a 
dark blue or violet by the selective absorption of the water. These 
changes, being all due to the cutting out of the red component of the 
colours, showed the water to have a selective absorption for the rays 
of the red end of the spectrum. 
If the water had been coloured blue by selective reflection, then 
those test colours would all have appeared deficient in blue when 
sunk in the water, as the fine particles would reflect and scatter 
the blue rays. Experiments are described which show that when 
these colours are sunk in water coloured blue by reflection from 
small particles, that white changes to yellow, while yellow simply 
deepens in colour, and purple grows redder. 
All these different methods of experimenting show this water to 
be a blue transparent medium, and that it acts in the same way as a 
solution of a blue salt or as a blue tinted glass. It is then shown that 
this selective absorption theory is not enough to account for the 
different colour phenomena seen in water. A piece of blue glass or 
a blue solution have but little colour when viewed from the side 
on which the light is falling, and it seems certain that light will 
penetrate pure water till it is all absorbed, and the water will look 
dark and colourless. Something more, and that of great importance, 
is obviously necessary to explain the different colour phenomena 
seen in different waters, and in the same water at different times. 
If the water of the Mediterranean, when brilliantly coloured, 
is examined by means of a concentrated beam of light, it is found 
to be full of fine solid particles in suspension. It is shown that 
it is to this dust of the sea — so to speak — that the Mediterranean 
owes its fine and varied colouring. The particles of this aquatic 
dust are large, and reflect not only the blue rays, like the supposed 
particles of the selective reflection theory, but they reflect rays of 
