172 FRAGMENTS OF SCIENCE. 



visual rays, the red are first extinguished. As the solai 

 beam plunges deeper into the sea, orange follows red, 

 yellow follows orange, green follows yellow, and the 

 various shades of blue, where the water is deep enough, 

 follow green. Absolute extinction of the solar beam 

 would be the consequence if the water were deep and 

 uniform. If it contained no suspended matter, such 

 water would be as black as. ink. A reflected glimmer of 

 ordinary light would reach us from its surface, as it 

 would from the surface of actual ink; but no light, 

 hence no colour, would reach us from the body of the 

 water. 



In very clear and deep sea-water this condition is 

 approximately fulfilled, and hence the extraordinary 

 darkness of such water. The indigo, already referred 

 to, is, I believe, to be ascribed in part to the suspended 

 matter, which is never absent, even in the purest natural 

 water ; and in part to the slight reflection of the light 

 from the limiting surfaces of strata of different densi- 

 ties. A modicum of light is thus thrown back to the 

 eye, before the depth necessary to absolute extinction 

 has been attained. An effect precisely similar occurs 

 under the moraines of glaciers. The ice here is ex- 

 ceptionally compact, and, owing to the absence of the 

 internal scattering common in bubbled ice, the light 

 plunges into the mass, where- it is extinguished, the 

 perfectly clear ice presenting an appearance of pitchy 

 blackness. 1 



The green colour of the sea has now to be accounted 

 for; and here, again, let us fall back upon the sure 

 basis of experiment. A strong white dinner-plate had 

 a lead weight securely fastened to it. Fifty or sixty 

 yards of strong hempen line were attached to the plate. 



1 I learn from a correspondent that certain Welsh tarns, which 

 arc reputed bottomless, have this inky hue. 



