SCIENCE. 



79 



deep waters to an inherent selective reflecting property 

 of its molecules, by which they reflected the blue rays of 

 light more copiously than the other rays of the solar 

 spectrum. Since the researches of Soret, Tyndall and 

 others, this selective reflection has been transferred to 

 the finely-divided particles which are known to be held 

 in suspension in greater or less abundance, not only in 

 all natural waters, but even in the most carefully dis- 

 tilled water. When the depth of water is sufficiently 

 great to preclude any solar rays reaching the bottom, 

 then the various shades of blue which are perceived under 

 similar conditions of sunshine, will depend upon the at- 

 tenuation and abundance of materials held in suspension ; 

 the purity and delicacy of the tint increasing with the 

 smallness and the degree of diffusion of the suspended 

 particles. Moreover, it is evident that Tyndall is quite 

 correct in assigning to " true molecular absorption " 

 some agency in augmenting " the intense and exceptional 

 blueness " of certain waters ; for it is obvious that the 

 " blue of scattering by small panicles " must be purified 

 by the abstraction of the less refrangible rays, which al- 

 ways accompany the blue during the transmission of the 

 scattered light to the observer. It seems to be very cer- 

 tain that were water perfectly free from suspended matter 

 and coloring substances in solution, and of uniform den- 

 sity, it would scatter no light at all. "But," as Tyndall 

 remarks, " an amount of impurity so infinitesimal as to be 

 scarcely expressible in numbers, and the individual parti- 

 cles of which are so small as wholly to elude the micro- 

 scope," may be revealed in an obvious and striking man- 

 ner when examined by a powerfully concentrated beam 

 of light in a darkened chamber. If the wa'ers of the 

 lakes and seas were chemically pure and optically homo- 

 geneous, absolute extinction of the traversing solar rays 

 would be the consequence if they were deep enough. So 

 that to an observer floa'ing on the surface, such waters 

 would appear as "black as ink," and apart from a slight 

 glimmer of ordinary light reflected from the surface, no 

 light, and hence no color would reach the eye from the 

 body cf the liquid. According to Tyndall, "in very clear 

 and very deep sea water, this condition is approximately 

 fulfilled, and hence the extraordinary darkness of such 

 water." In some places, when looked down upon, the 

 water " was of almost inky blackness — black qualified by 

 a trace of indigo." But even this trace of indigo he 

 ascribes to the small amount of suspended matter, which 

 is never absent even in the purest natural water, throw- 

 ing back to the eye a modicum of light before the travers- 

 ing rays attain a depth necessary for absolute extinction. 

 He adds : " An effect precisely similar occurs under 

 the moraines of the Swiss glaciers. The ice is here ex- 

 ceptionally compact, and owing to the absence of the 

 internal scattering common in bubbled ice the light 

 plunges into the mass, is extinguished, and the perfectly 

 clear ice presents an appearance of pitchy blackness " 

 (" Hours of Exercise in the Alps ; " " Voyage to Algeria 

 to Observe the Eclipse," Am. Ed., N. Y., 1871, pp. 463- 

 470). In like manner the waters of certain Welsh tarns, 

 which are reputed to be bottomless, are said to present 

 an inky hue. And it is more than probable that the 

 waters of the Silver spring, whose exceptional trans- 

 parency has been previously indicated, would, if they were 

 sufficiently deep, present a similar blackness, or absence 

 of all color by diffuse reflection. 



It remains for us to explain the cause of the green 

 tints which the waters of certain lakes and seas assume 

 under peculiar circumstances. These green colors mani- 

 fest themselves under the following conditions, viz : '(a.) 

 In the finest blue water, when the depth is so small as 

 to allow the transmitted light to be reflected from a bot- 

 tom which is more or less white. Thus, a white sandy 

 bottom or white rocks beneath the surface of the Lake 

 of Geneva, or the Bay of Naples, or of Lake Tahoe, will, 

 if the depth is not too great or too small, impart a beau- 

 tiful emerald green to the waters above them. {b.) In 



the finest blue water, when a white object is looked a 

 through the intervening stratum of water. In the blue 

 waters of the sea this is frequently seen in looking at the 

 white bellies of the porpoises, as they gambol about a 

 ship or steamer. In a rough sea, the light which has 

 traversed the crest of a wave and is reflected back to 

 the eye of the observer from the white foam on the re- 

 mote side, sometimes crowns it with a beautiful green 

 cap. In March, 1869, I observed this phenomenon in 

 the magnificent ultramarine waters of the Carribbean 

 sea. A stout white dinner plate secured to a sounding 

 line, presents various tints of green as it is let down into 

 the blue water. Such experiments were made by Count 

 Xavier de Maistre, in the Bay of Naples, in 1832 ; by 

 Prof. Tyndall, in the Atlantic ocean, in December, 1870, 

 and by the writer in Lake Tahoe, in August and Septem- 

 ber, 1873. (c.) In waters of all degrees of depth when 

 a greater amount of solid matter is held in suspension 

 than is required to produce the blue color of the purer 

 deep waters of lakes and seas. Thus, Tyndall, in his 

 " Voyage to Algeria to observe the Ec'ipse," in Decem- 

 ber, 1870, collected 19 bottles of water from various 

 places in the Atlantic ocean between Gibraltar and 

 Spithead. These specimens were taken from the sea at 

 positions where its waters presented tints varying from 

 deep indigo blue, through bright green to yellow greeD. 

 After his return to England, he directed the concentrated 

 beam from an electric lamp through the several speci- 

 mens of water and found that the blue waters indicated 

 the presence of a small amount of suspended matter ; the 

 bright green a decidedly greater amount of suspended 

 particles, and the yellow green was exceeding thick with 

 suspended corpuscles. He remarks : " My home ob- 

 servations, I think, clearly establish the association of the 

 green color of sea water with fine suspended matter, and 

 the association of the ultramarine color, and more 

 especially of the black indigo hue of sea water, with the 

 comparative absence of such matter." (" Hours of 

 Exercise in the Alps ;" " Voyage to Algeria to observe 

 the Eclipse," Ed. cit. ante, pp. 464 et 467.) 



There is one feature which is common to all of the 

 three above indicated conditions, under which the green 

 color manifests itself in the waters of the lakes and seas, 

 viz : When a white or more or less light-colored reflecting 

 surface is seen through a stratum of intervening water of 

 sufficient purity and thickness. Condition (c,) is obvi- 

 ously included ; for it is evident that a background of 

 suspended particles may, under proper conditions, form 

 such a reflecting surface. 



Inasmuch as under these several conditions, more or 

 less of transmitted light is reflected back to the eye of the 

 observer, it is evident that the rays which reach him carry 

 with them the chromatic modifications due to the com- 

 bined influence of the selective absorption of the water 

 itself, and the selective reflection from the smaller 

 suspended particles. Hence, the chromatic phenomena 

 presented, being produced by the mingling of these rays 

 in various proportions, must manifest complex combina- 

 tions of tints, under varying circumstances relating to 

 color of bottom, depth of water, and the amount and 

 character of the suspended matter present. In the ex- 

 planations of the green color of certain waters by the 

 older physicists, we recognize the full appreciation of the 

 influence of selective reflections in the productions of the 

 phenomena ; but they seem to have overlooked the im- 

 portant effects of the molecular absorption. We have 

 seen that Sir I. Newton regarded the green tints of sea- 

 water as due to the more copious reflection of the violet, 

 blue and green rays, while those constituting the red end 

 of the spectrum are allowed to penetrate to greater 

 depths. ("Optics, loc. cit. ante.") Sir H. Davy ascribes 

 it, in part, to the presence of iodine and bromine in the 

 waters, imparting a yellow tint, which, mingled with the 

 blue color from pure water, produced the sea-green. 

 I ("Salmonia, Collected Works," Vol. 9, p. 201.) In like 



