220 M. W. Beetz on the Colour of Water. 



other, and correspond to the different numbers of reflexions. In 

 making some experiments, I had, at first, so placed the mirrors 

 that the uncovered glass surfaces were opposite each other. The 

 light must then, at each reflexion, pass twice through the glass 

 plates themselves. If the liquid contained no liquid, then the 

 image appeared almost white after six to eight reflexions ; but 

 still, on comparing the subjective images with one another, it 

 could be seen that each following one had a somewhat yellowish 

 tint. I supposed that this coloration was to be ascribed to the 

 tolerably thick layer of glass which the light had to traverse, 

 and therefore turned the mirrors, which were once more 

 polished on the silvered side. Yet even in this case each fol- 

 lowing image showed a yellower colour, though in a less degree. 

 The colour must therefore be ascribed to the special colour of 

 the silver, from which part of the light is reflected diffusely. 

 Yet when the polish is good, it is so unimportant that it does 

 not disturb further observation. 



If the box is half filled with distilled water and the entire slit 

 d illuminated, the lower part of the picture on the receiving 

 plate is seen to be blue, while the upper part remains white. 

 Looking through the slit d' at the upper part of the box, the 

 entire range of more and more yellowish pictures is seen ; look- 

 ing through the lower part, each following picture appears of a 

 darker blue, with a very feeble tinge of green. The phenomenon 

 is just the same when water from the deep blue Achensee is 

 poured into the vessel; if this is replaced by water from the 

 Tegernsee, after a few reflexions the images appear of an intense 

 yellowish green (not bluish green), although my box was only 

 10 inches long. If garden earth is drenched with water, which 

 is allowed to drain off, and this is filtered and mixed, first in 

 small and then in larger quantities, with distilled water, the 

 colour of the images passes first into yellowish green and then 

 steadily into a brown colour, just as was to be expected from 

 Wittstein's experiments. The colours in question in these ex- 

 periments are also those in transmitted light. 



What, then, are the phenomena which have evoked the idea 

 of the dual colour of water ? 



Newton based his view upon an experiment of Halley. As the 

 latter, on a sunny day, had descended in a diving-bell to a 

 great depth in the water, the upper surface of his hand, which 

 was directly illuminated through the sea-water and through a 

 window in the bell, appeared of a rose-red, but the water below 

 him and the under surface of his hand, which was illuminated 

 by the rays reflected from the lower water, was green. The expe- 

 riment is manifestly erroneously interpreted. The rays which 

 came from below are clearly not reflected by the water, but trans- 



