modest investigators who have occupied themselves with the chemical constitution and physical properties of sea-water.
Sea-water, it is well known, when it is not muddy, is one of the clearest of all natural waters. When we walk along the shore at low tide, it is often difficult, unless we are careful, to keep from stepping into the occasional pools on the rocks, the water in the little hollows being so transparent as to be invisible. The question of the color of this water deserves serious examination, and labors on the subject are not wanting. The most notable ones are those of Father Secchi, of Professor Tyndall, and the more recent researches of M. W. Spring and M. Soret.
Father Secchi made his experiments in 1865, on board a Pontifical corvette. A number of disks, formed by stretching variously-colored cloths over iron hoops, the largest twelve feet in diameter, were let down at a time when the conditions of the weather were most favorable for transparency. The largest disk, which was painted white, became invisible at the depth of about forty-two metres, while the smaller disks and a delf plate, distorted by refraction, went out of eight at smaller depths. The disappearance seemed to depend upon the confusion of the image, which was broken up in every direction. The largest disk, the considerable surface of' which offered more resistance to the distortion, finally ceased to be perceived, because its color, turning in succession to light green, blue, and dark blue, became at last as dark as the surrounding medium. Disks, painted yellow or red, were lost to sight still more quickly, or under not more than twenty metres of water. Repetitions of similar experiments gave co-ordinate results; and it may be stated, as a general rule of average, that the practical limit of submarine vision, under favorable circumstances, is at twenty-five metres under the surface.
It was found, by spectroscopic examinations of the light reflected from the differently' colored disks, that the yellow was enfeebled and extinguished first, and next the red, under the increasing thickness of the overlying water. By the gradual disappearance of these two colors, a white object is made to pass through green to blue—the tint which all such objects finally assume when sunk under salt-water. Each of the three simple colors—yellow, red, and blue, or violet—has its distinct part among the solar rays. Yellow is luminous, red is calorific, and violet-blue provokes chemical reactions. Water, in a very thick mass, is neither transparent nor diathermanous; but, being penetrable to the blue, indigo, and violet rays, it is diactinic. These radiations, too, will, of course, gradually lose their energy, and become extinguished at last in a very deep stratum of liquid; but the limit is extremely remote.
According to the theory propounded by Professor Tyndall, the sea-waves present three principal hues—blue, green, and yellow. The indigo-blue waters are the purest, while the yellow ones contain muddy
