172 FRAGMENTS OP SCIENCE. 



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

 the solar beam plunges deeper into the sea, orange fol- 

 lows red, yellow follows orange, green follows yellow, 

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

 enough, follows green. Absolute extinction of the 

 solar beam would be the consequence if the water were 

 deep and uniform. If it contained no suspended mat- 

 ter, 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 natu- 

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

 light from the limiting surfaces of strata of different 

 densities. A modicum of light is thus thrown back 

 to the eye, before the depth necessary to absolute ex- 

 tinction has been attained. An effect precisely similar 

 occurs under the moraines of glaciers. The ice here is 

 exceptionally 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.* 



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. 



* I learn from a correspondent that certain Welsh tarns, 

 which are reputed bottomless, have this inky hue. 



