In the case of pigments, then, the light is reflected at the limiting surfaces of the particles, but it is in part absorbed within the particles.  The reflection is necessary to send the light back to the eye; the absorption is necessary to give the body its colour.  The same remarks apply to flowers.  The rose is red, in virtue, not of the light reflected from its surface, but of light which has entered its substance, which has been reflected from surfaces within, and which, in returning through the substance, has had its green extinguished.  A similar process in the case of hard green leaves extinguishes the red, and sends green light from the body of the leaves to the eye.

All bodies, even the most transparent, are more or less absorbent of light.  Take the case of water.  A glass cell of clear water interposed in the track of our beam does not perceptibly change any one of the colours of the spectrum.  Still absorption, though insensible, has here occurred, and to render it sensible we have only to increase the depth of the water through which the light passes.  Instead of a cell an inch thick, let us take a layer, ten or fifteen feet thick:  the colour of the water is then very evident.  By augmenting the thickness we absorb more of the light, and by making the thickness very great we absorb the light altogether.  Lampblack or pitch can do no more, and the only difference in this respect between them and water is that a very small depth in their case suffices to extinguish all the light.  The difference between the highest known transparency and the highest known opacity is one of degree merely.

If, then, we render water sufficiently deep to quench all the light; and if from the interior of the water no light reaches the eye, we have the condition necessary to produce blackness.  Looked properly down upon, there are portions of the Atlantic Ocean to which one would hardly ascribe a trace of colour:  at the most a tint of dark indigo reaches the eye.  The water, in fact, is practically black, and this is an indication both of its depth and purity.  But the case is entirely changed when the ocean contains solid particles in a state of mechanical suspension, capable of sending the light impinging on them back to the eye.

Throw, for example, a white pebble, or a white dinner plate, into the blackest Atlantic water; as it sinks it becomes greener and greener, and, before it disappears, it reaches a vivid blue green.  Break such a pebble, or plate, into fragments, these will behave like the unbroken mass:  grind the pebble to powder, every particle will yield its modicum of green; and if the particles be so fine as to remain suspended in the water, the scattered light will be a uniform green.  Hence the greenness of shoal water.  You go to bed with the black water of the Atlantic around you.  You rise in the morning, find it a vivid green, and correctly infer that you are crossing the Bank of Newfoundland.  Such water is found charged with fine matter in a state of mechanical suspension.  The light from the bottom may sometimes come into play, but it is not necessary.  The subaqueous foam, generated by the screw or paddle-wheels of a steamer, also sends forth a vivid green.  The foam here furnishes a reflecting surface, the water between the eye and it the absorbing medium.