A very muddy river, as the Arno for instance at Florence, is seen during sunshine of its own yellow color, rendering all reflections discolored and feeble. At twilight it recovers its reflective power to the fullest extent, and the mountains of Carrara are seen reflected in it as clearly as if it were a crystalline lake. The Mediterranean, whose determined blue yields to hardly any modifying color in daytime, receives at evening the image of its rocky shores. On our own seas, seeming shadows are seen constantly cast in purple and blue, upon pale green. These are no shadows, but the pure reflection of dark or blue sky above, seen in the shadowed space, refused by the local color of the sea in the sunlighted spaces, and turned more or less purple by the opposition of the vivid green.
We have seen, however, above, that the local color of water, while it comparatively refuses dark reflections, accepts bright ones without deadening them. Hence when a shadow is thrown [§ 9. Modification of dark reflections by shadow.]across a space of water of strong local color, receiving, alternately, light and dark reflections, it has no power of increasing the reflectiveness of the water in the bright spaces, still less of diminishing it; hence, on all the dark reflections it is seen more or less distinctly, on all the light ones it vanishes altogether.
Let us take an instance of the exquisite complexity of effect induced by these various circumstances in co-operation.
Suppose a space of clear water showing the bottom under a group of trees, showing sky through their branches, casting shadows on the surface of the water, which we will suppose also to possess some color of its own. Close to us, we shall see the bottom, with the shadows of the trees clearly thrown upon it, and the color of the water seen in its genuineness by transmitted light. Farther off, the bottom will be gradually lost sight of, but it will be seen in the dark reflections much farther than in the light ones. At last it ceases to affect even the former, and the pure surface effect takes place. The blue bright sky is reflected truly, but the dark trees are reflected imperfectly, and the color of the water is seen instead. Where the shadow falls on these dark reflections a darkness is seen plainly, which is found to be composed of the pure clear reflection of the dark trees; when it crosses the reflection of the sky, the shadow of course, being thus fictitious, vanishes.
Farther, of course on whatever dust and other foulness may be present in water, real shadow falls clear and dark in proportion to the quantity of solid substance present. On very muddy rivers, real shadow falls in sunlight nearly as sharply as on land; on our own sea, the apparent shadow caused by increased reflection, is much increased in depth by the chalkiness and impurity of the water.
Farther, when surface is rippled, every ripple, up to a certain variable distance on each side of the spectator, and at a certain angle between him and the sun, varying with the size and shape of the ripples, reflects to him a small image of the sun. Hence those dazzling fields of expanding light so often seen upon the sea.
Any object that comes between the sun and these ripples, takes from them the power of reflecting the sun, and in consequence, all their light; hence any intervening objects cast apparent shadows upon such spaces of intense force, and of the exact shape, and in the exact place of real shadows, and yet which are no more real shadows than the withdrawal of an image of a piece of white paper from a mirror is a shadow on the mirror. Farther, in all shallow water, more or less in proportion to its shallowness, but in some measure, I suppose, up to depths of forty or fifty fathoms, and perhaps more, the local color of the water depends in great measure on light reflected from the bottom. This, however, is especially manifest in clear rivers like the Rhone, where the absence of the light reflected from below forms an apparent shadow, often visibly detached some distance from the floating object which casts it.
[§ 10. Examples on the water of the Rhone.]
The following extract from my own diary at Geneva, with the subsequent one, which is a continuation of that already given in part at Venice, will illustrate both this and the other points we have been stating.
"Geneva, 21st April, Morning.