250 THE POPULAR SCIENCE MONTHLY. 



the softer tones. But below the dam the water is in lively motion 

 with endless little racing wavelets, and here the green color comes 

 out so vividly that the reflection almost wholly vanishes. Yet 

 another fact is revealed by the movement. Only with a perfectly 

 smooth mirror surface are the outlines of the reflected objects 

 fully clear and sharp. The reflection is then so perfect that one 

 often hardly knows whether he sees the objects themselves or 

 only their mirrored images on the water ; and the lines between 

 water and shore are quite effaced. The slightest movement causes 

 the outline of the mirror picture to appear notched ; clearer lines 

 from the horizon creep into the darker colors of the picture, but 

 notches from these, too, spring out over the lines which the out- 

 line should have. This phenomenon is so common that we notch 

 the borders of water reflections in colored pictures, as well as in 

 those drawn only in black. I have no doubt that a relationship 

 lies at the bottom of this phenomenon like the fact observed by 

 Colladon, and now often remarked, that water in motion carries 

 the light along with it. A stream of water, flowing through a 

 dark tube out of an illuminated receiver, carries the light along, 

 whether it be white or colored, and shines ; why should not mov- 

 ing waves exhibit the same effect ? 



But enough of these painters' impressions, which, as we have 

 said, are neglected by the physicist, but are still of the highest 

 significance for the beholder as well as for the artist, and, as may 

 result from our representation, are dependent on various factors, 

 among which, besides mirroring, the real color of the water is to 

 be considered. 



Let us go a little closer into this matter. 



Pure or colorless water containing salts in solution is beauti- 

 fully blue and perfectly transparent, at least to a certain depth. 

 It is, hence, clear that with the color of all objects visible at this 

 depth, and constantly reflecting the rays of light, is associated under 

 the water a blue tone, more intensive as the depth at which the 

 object lies is greater. The gravels on the shore of a lake or the 

 sea become, when seen through the blue water, as if they were 

 observed through a pane of blue glass ; and since all shore figures, 

 with trifling exceptions, are of a yellowish color, they will shine 

 of a more or less green color, and the water on the shore will like- 

 wise appear green. 



I here lay aside all physical deductions concerning the nature 

 of color. We know that it is not, as was once thought, a property 

 of bodies, but that a transparent body like water, for example, 

 shows a distinct color, because it lets certain colored rays through, 

 but not others, and that a solid body reflects the rays which we 

 perceive, but to a certain extent absorbs the others. The discussion 

 of the nature of color is not of very great importance for our essay. 



