Causes of the Colors in the Sky and in Masses of Water.

The Manufacturer and Builder 6, 1870

The last number of the American Journal of Science, contains an account of investigations in regard to the causes of the blue color of Lake Leman, in Switzerland, by Dr. Hayes, of Boston. He found no coloring matter whatever, and the water being other-wise remarkably pure, he comes to the conclusion that the blue color is entirely due to reflection from the sky. But the fact is, that the water is much more blue than the sky, and has this color even when the sky is overcast with clouds, though not so intensely. It must not be lost sight of that a large mass of water will show "blue" color for the same reason that causes a large mass of very pure air to appear blue, that is, oblique illumination. It is simply by the property that the blue rays are more refrangible than the yellow, orange, and red, that this effect is produced. The latter have a greater tendency toward the straight direction, while the green, blue, and violet rays, being the more refrangible half of the spectrum, are thrown sidewise in greater abundance. Their mean effect is the sky-blue, the green and violet neutralizing one another to a certain degree, to produce with the prismatic blue the peculiar tinge of the sky, which is not a pure blue, but a mixed color, as all landscape painters know very well. With deep water we have the same case; only its blue is darker because the liquid is denser, and seen against a dark background. This color is also seen in the deep parts of the ocean, when so far from land that the clearness of the water can not be affected by mud from the bottom, or from coasts and rivers. Near the shore the sea-water appears green, because the solutions and earthy matters in suspension give it a yellow color, which, with the blue, makes green. When this matter in suspension is in such excess that the transparency is impeded, this blue is seen no mere, and the water is of an ochre yellow, as seen in the Hudson River, near New-York, in the Missouri, and in many other localities. If the matter in solution and suspension is a very slight quantity, the yellowish color is pure, and not no much due to matter in suspension; its amount is then well balanced with the natural blue, and the beautiful emerald green of our large American lakes is produced. This is peculiarly visible in Lake Erie and at Niagara Faals.

An argument in favor of the theory here given for the cause of the blue color of large masses of pure air and pure water, is that, by directly transmitted light, orange is the permanent color, as often seen at sunrise and sunset, when the air is dry, and when the sun is seen from a diving-bell. through a large mass of clear water. White light consists of six colors, separated one from another by the well-known prismatic experiment; they are in the following order:

Red,
Orange,
Yellow,
less refrangible.

Green,
Blue,
Violet,
more refrangible.

They are all separated more or less by air and by water ; but the first three have a tendency to move more in a straight line than the latter, which have a tendency to greater lateral deflection, which is the refraction. The mean result of the first three must be the orange color of our sunsets; the mean result of the latter three the blue color of our clear sky, and of pure, deep water.