« PreviousContinue »
Radiant heat emitted by hot bodies, is polarized by glass surfaces, when reflected at an angle equal to that at which light is polarized. Metallic surfaces have a similar action on both kinds of rays.
The chemical rays may be polarized by glass surfaces, and possess all the general physical properties of the luminous rays.
ART. X. On the Colours of Waters. By G. W. Jordan, Esq., F. R. S.
THE colours of waters in the scenes amid which they are diffused, and of the atmosphere, and bodies existing in the atmosphere from which they are derived, have heretofore only incidentally been referred to; never, that I know of, generally stated or accounted for. Poetry has her blue and green seas, and Geography her black, white, red, and yellow seas.
The internal colour of the sea on the British coasts is green, from vegetable colouring matter brought down into it by the waters of the land. Beyond the limits of this green colour, at distances remote from land, the waters of the ocean have generally been considered as blue, from the almost constant exhibition of this colour at its surface. But neither the waters of the coast, nor of the ocean, are essentially or exclusively green or blue.
Water and sea water are in themselves perfectly transparent and colourless. The colours which they exhibit in their states of accumulation, as rivers, lakes, and seas, depend upon, and are produced by other bodies, from without and from within the waters, and are as various as those bodies and their reflections. They are not of the water; those from without are rather of the air, being reflected by the air incumbent on the surface of the water, and the water itself being altogether incapable of reflecting from and by its surface of external incidence; those from within by bodies within the waters.
Transparent bodies are of a transparency more or less perfect, of a trasparency colourless or coloured. All transparent bodies are incapable of reflecting by, or from their first surfaces, and therefore the coloured transparency of any body appears only by light which has entered into, and passed out of the body. The colours thus exhibited, are either essential or accidental, and are to be distinguished from those of prismatic refraction. Flint glass is colourless, crown glass essentially light green, yet both as prisms, exhibit all the varieties of colour in refracted light. In the opal, solution of lignum nephritium, and other semi-trasparent bodies, the colours are obviously produced by the inflections of particles diffused through the bodies, rendering them at various thicknesses variously coloured, and partially and finally opaque. The atmosphere is essentially colourless, and appears of various colours, from the inflections and reflections of bodies uncombinedly diffused throughout it. Sea water, therefore, which is in itself perfectly transparent, and at times by mixture imperfectly trasparent, exhibits no colours of its own. The colours reflected from its surface, not by its surface, are reflected by the incumbent air itself, perfectly or imperfectly transparent; and are the colours of all the bodics so reflected, of the sun's light or of bodies illuminated by that light, or of the sun's light changed in and by its passage through the air. The colours transmitted from within its substance, are of bodies of the mineral or vegetable kingdom carried into it by waters from the land. The flame of a candle, or any other object seen by reflection at the surface of red wine, experiences no change of colour, receives no tint from the liquor, the light seen has never entered it, and is not reflected by it, but by the air incumbent on it. Whatever light has entered and passed through or out of it, receives and carries with it the red tints of the wine. If this red may be considered essential to the wine, yet none of it is reflected at its surface of external incidence, and the case is the same with all transparent coloured bodies in air.
When light arrives at the confines of two transparent bodies,
as air and water, or water and air, it is divided into two portions varying in intensities or quantities with its different incidences respectively. When light in air arrives at the confines of air and water, the quantities of light reflected by the air, reflected from, but not by the water, vary according to the following proportions:-at 90, so, 70, 60, 50, degrees from the reflecting surface, the quantities vary from a little below to a little above of the whole, at 40 to at 30 to, at 20 to, at 15 to, at 10 to, at 5 to, at 2 to 3, at 1 to 7, at
to, and finally to the whole light. The reflection from quicksilver is about of the whole light. Accordingly, at these low incidences, the appearances produced by the air at the surface of the water become visibly specular, resembling those of quicksilver and metallic mirrors, as are also the total reflections of light from within at the further surfaces of glass and of other transparent bodies.
I am indebted for the foregoing estimates to the labours of those celebrated opticians, Bouguer and Boscovich; who, however, forgetting the presence of the air in their experiments, ascribe those reflections to the water alone, which are by the air; and to the quicksilver alone, those which are partly by the quicksilver and partly by the air.
Upon these varying quantities of light, and upon the changes of form in the waters, and by the waters given to the surfaces of air incumbent on them, depend all the colours exhibited at these surfaces by reflection. These colours belong not to the water, are not of its essence, are extrinsic and adventitious. Other colours there are of waters adventitious also, though observed within them, and although internal not essential.
When light in water arrives at the confines of water and air, if it be incident at an angle of inclination to the surface less than 41-30, it is totally reflected back within the water, and none passes out. If this angle be increased continually, small portions of light begin to escape into the air of intensities at first so evanescent, as not to be perceived until the incidences within the water are considerably increased. Of these, the quantities so transmitted, compared with those of
light reflected within the air, those increasing, and these diminishing, as the respective incidences are increased, come to something like an equality with them at between 60 or 70 degrees of internal incidence in water, and 50 or 60 of external transmission into air, and thus render it extremely difficult to see or to distinguish light passing out of water into air, at less angles of internal incidence within the water than between 70 and 90 from the surface, or within 20 from the perpendicular, at distances from the spectator extending to about triple the altitude of the eye above the surface of the water.
When the water is at rest, its surface is apparently specular when viewed at small angles of elevation above it, but this appearance is changed and removed when the vision is at larger angles, and any internal colours which may be in the water mingle themselves in increasing, and considerable quantities with the reflected light. But as when the angles of observation with the surface are small, these internal colours become dilute and evanescent, and finally non-apparent, the specular appearance becomes complete, and exhibits together with the images of surrounding objects, that blackness or absence of colour from the surface in other directions by which a judgment is formed of the good or bad qualities of the largest artificial specula or looking glasses. Indeed the light thus reflected, compared with that reflected from quicksilver, is, as before stated, equal and superior thereto.
These observations and estimates apply to the surfaces of water and air perfectly plane and at rest. When by surfaces thus constituted objects above and around are represented, an inverted picture of all these things is exhibited within the water, and the colours of objects are removed to their respective distances within, and are not properly or immediately referred to the surface or to the water itself. By agitations alone are the colours of reflected light brought to the confines of the surfaces of water and air, and are made to appear as colours of the water which indeed they never enter, and by which they are not in fact reflected, and from which reflected light never derives any colours whatever.
If the water be at rest and coloured within by any internal mixtures, this inverted picture is seen at considerable horizontal distances, and small angles from the surface, and is more or less bright or vivid as these respectively increase or diminish, its colours being then unmixed with any internal from the water; but as these distances decrease and angles increase, the light reflected in air diminishing, and the light transmitted from within the water increasing, this last begins to be perceived, to mix with, and to change, and finally to obscure the other with which it is mixed. When the surface of water is calm, and that of the adjacent air consequently undisturbed, every object seen by reflection as the sun or the moon is referred to its proper place within and below the water, and is single. But when undulations exist, each undulation constituting a separate reflector, the object is brought up to the surface, at distances equal to half the radices of the circle of curvature of each undulation, many images thereof are produced, and the object being sufficiently broad, the undulations of adequate size, and frequency and proximity, the different images become united; and in the case of sun or moon, form one long uninterrupted line of brilliant light, such as is very frequently observed at the surfaces of agitated waters, and of which painters have not failed to avail themselves and to introduce for splendid effect.
Upon this power of bringing objects and their colours to the surface, and upon the power which undulations further possess by change of inclination of surface, to exhibit only the most powerful of prevailing reflections, upon these principles of transmission and reflection, and their modifications by undulation of water and elevation of view, depend all the colours and variations of colours of waters.
When the sun shines in a cloudless sky at altitudes more or less considerable above the horizon, the atmosphere exhibits the following appearances:
All around the body of the sun and to considerable distances from it, a circular appearance of white light may be seen, diminishing in intensity as the distances from the sun's body