Let us stand on the sea-shore on a cloudless night, with a full moon over the sea, and a swell on the water. Of course a long line of splendor will be seen on the waves under the moon, reaching from the horizon to our very feet. But are those waves between the moon and us actually more illuminated than any other part of the sea? Not one whit. The whole surface of the sea is under the same full light, but the waves between the moon and us are the only ones which are in a position to reflect that light to our eyes. The sea on both sides of that path of light is in perfect darkness—almost black. But is it so from shadow? Not so, for there is nothing to intercept the moonlight from it: it is so from position, because it cannot reflect any of the rays which fall on it to our eyes, but reflects instead the dark vault of the night sky. Both the darkness and the light on it, therefore—and they are as violently contrasted as may well be—are nothing but reflections, the whole surface of the water being under one blaze of moonlight, entirely unshaded by any intervening object whatsoever.
Now, then, we can understand the cause of the chiaro-scuro of the sea by daylight with lateral sun. Where the sunlight reaches the water, every ripple, wave, or swell reflects to the eye from some of its planes either the image of the sun or some portion of the neighboring bright sky. Where the cloud interposes between the sun and sea, all these luminous reflections are prevented, and the raised planes of the waves reflect only the dark under-surface of the cloud; and hence, by the multiplication of the images, spaces of light and shade are produced, which lie on the sea precisely in the position of real or positive light and shadows—corresponding to the outlines of the clouds—laterally cast, and therefore seen in addition to, and at the same time with, the ordinary or direct reflection, vigorously contrasted, the lights being often a blaze of gold, and the shadows a dark leaden gray; and yet, I repeat, they are no more real lights, or real shadows, on the sea, than the image of a black coat is a shadow on a mirror, or the image of white paper a light upon it.[191]
Are there, then, no shadows whatsoever upon the sea? Not so. My assertion is simply that there are none on clear water near the eye. I shall briefly state a few of the circumstances which give rise to real shadow in distant effect.
I. Any admixture of opaque coloring matter, as of mud, chalk, or powdered granite renders water capable of distinct shadow, which is cast on the earthy and solid particles suspended in the liquid. None of the seas on our south-eastern coast are so clear as to be absolutely incapable of shade; and the faint tint, though scarcely perceptible to a near observer,[192] is sufficiently manifest when seen in large extent from a distance, especially when contrasted, as your correspondent says, with reflected lights. This was one reason for my introducing the words—“near the eye.”
There is, however, a peculiarity in the appearances of such shadows which requires especial notice. It is not merely the transparency of water, but its polished surface, and consequent reflective power, which render it incapable of shadow. A perfectly opaque body, if its power of reflection be perfect, receives no shadow (this I shall presently prove); and therefore, in any lustrous body, the incapability of shadow is in proportion to the power of reflection. Now the power of reflection in water varies with the angle of the impinging ray, being of course greatest when that angle is least: and thus, when we look along the water at a low angle, its power of reflection maintains its incapability of shadow to a considerable extent, in spite of its containing suspended opaque matter; whereas, when we look down upon water from a height, as we then receive from it only rays which have fallen on it at a large angle, a great number of those rays are unreflected from the surface, but penetrate beneath the surface, and are then reflected[193] from the suspended opaque matter: thus rendering shadows clearly visible which, at a small angle, would have been altogether unperceived.
II. But it is not merely the presence of opaque matter which renders shadows visible on the sea seen from a height. The eye, when elevated above the water, receives rays reflected from the bottom, of which, when near the water, it is insensible. I have seen the bottom at seven fathoms, so that I could count its pebbles, from the cliffs of the Cornish coast; and the broad effect of the light and shade of the bottom is discernible at enormous depths. In fact, it is difficult to say at what depth the rays returned from the bottom become absolutely ineffective—perhaps not until we get fairly out into blue water. Hence, with a white or sandy shore, shadows forcible enough to afford conspicuous variety of color may be seen from a height of two or three hundred feet.
III. The actual color of the sea itself is an important cause of shadow in distant effect. Of the ultimate causes of local color in water I am not ashamed to confess my total ignorance, for I believe Sir David Brewster himself has not elucidated them. Every river in Switzerland has a different hue. The lake of Geneva, commonly blue, appears, under a fresh breeze, striped with blue and bright red; and the hues of coast-sea are as various as those of a dolphin; but, whatever be the cause of their variety, their intensity is, of course, dependent on the presence of sun-light. The sea under shade is commonly of a cold gray hue; in sun-light it is susceptible of vivid and exquisite coloring: and thus the forms of clouds are traced on its surface, not by light and shade, but by variation of color by grays opposed to greens, blues to rose-tints, etc. All such phenomena are chiefly visible from a height and a distance; and thus furnished me with additional reasons for introducing the words—“near the eye.”
IV. Local color is, however, the cause of one beautiful kind of chiaro-scuro, visible when we are close to the water—shadows cast, not on the waves, but through them, as through misty air. When a wave is raised so as to let the sun-light through a portion of its body, the contrast of the transparent chrysoprase green of the illuminated parts with the darkness of the shadowed is exquisitely beautiful.
Hitherto, however, I have been speaking chiefly of the transparency of water as the source of its incapability of shadow. I have still to demonstrate the effect of its polished surface.
Let your correspondent pour an ounce or two of quicksilver into a flat white saucer, and, throwing a strip of white paper into the middle of the mercury, as before into the water, interpose an upright bit of stick between it and the sun: he will then have the pleasure of seeing the shadow of the stick sharply defined on the paper and the edge of the saucer, while on the intermediate portion of mercury it will be totally invisible[195]. Mercury is a perfectly opaque body, and its incapability of shadow is entirely owing to the perfection of its polished surface. Thus, then, whether water be considered as transparent or reflective (and according to its position it is one or the other, or partially both—for in the exact degree that it is the one, it is not the other), it is equally incapable of shadow. But as on distant water, so also on near water, when broken, pseudo shadows take place, which are in reality nothing more than the aggregates of reflections. In the illuminated space of the wave, from every plane turned towards the sun there flashes an image of the sun; in the un-illuminated space there is seen on every such plane only the dark image of the interposed body. Every wreath of the foam, every jet of the spray, reflects in the sunlight a thousand diminished suns, and refracts their rays into a thousand colors; while in the shadowed parts the same broken parts of the wave appear only in dead, cold white; and thus pseudo shadows are caused, occupying the position of real shadows, defined in portions of their edge with equal sharpness: and yet, I repeat, they are no more real shadows than the image of a piece of black cloth is a shadow on a mirror.