the green hue is not perceptible, unless viewed from such a distance that the rays of light emitted obliquely from the white surface have traversed a considerable thickness of the liquid before reaching the eye of the observer.

The experiments with the submerged white dinner-plate, in testing the transparency of the water, incidentally manifested, to some extent, the influence of depth on the color of the water.  The white disk presented a bluish-green tint at the depth of from nine to twelve meters; at about fifteen meters it assumed a greenish-blue hue, and the blue element increased in distinctness with augmenting depth, until the disk became invisible or undistinguishable in the surrounding mass of blue waters.  The water intervening between the white disk and the observer did not present the brilliant and vivid green tint which characterized that which is seen in the shallow portions of the Lake, where the bottom is white.  But this is not surprising, when we consider the small amount of diffused light which can reach the eye from so limited a surface of diffusion.

In studying the chromatic tints of these waters, a hollow pasteboard cylinder, five or six centimeters in diameter, and sixty or seventy centimeters in length, was sometimes employed for the purpose of excluding the surface reflection and the disturbances due to the small ripples on the water.  When quietly floating in a small row-boat, one end of this exploring tube was plunged under the water, and the eye of the observer at the other extremity received the rays of light emanating from the deeper portions of the liquid.  The light thus reaching the eye presented essentially the same variety of tints in the various portions of the Lake as those which have been previously indicated.

Hence it appears that under various condition—­such as depth, purity, state of sky and color of bottom—­the waters of this Lake manifest nearly all the chromatic tints presented in the solar spectrum between greenish-yellow and the darkest ultramarine-blue, bordering upon black-blue.

It is well known that the waters of oceans and seas exhibit similar gradations of chromatic hues in certain regions.  Navigators have been struck with the variety and richness of tints presented, in certain portions, by the waters of the Mediterranean Sea, the Atlantic and Pacific Oceans, and especially those of the Caribbean Sea.  In some regions of the oceans and seas, the green hues, and particularly those tinged with yellow, are observed in comparatively deep waters, or, at least, where the depths are sufficiently great to prevent the bottom from being visible.  But this phenomenon seems to require the presence of a considerable amount of suspended matter in the water.  In no portion of Lake Tahoe did I observe any of the green tints, except where the light-colored bottom was visible.  This was, probably, owing to the circumstance that no considerable quantity