44 TRANSLATIONS AND SELECTED ARTICLES. 
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 dis- 
turb 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; looking through the lower 
part, each following picture appears of a darker blue, with a very fee- 
ble tinge of green. ‘The phenomenon is just the same when water 
from the deep blue Achensee is poured into the vessel ; if this is re- 
placed 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 experiments are also those in trans- 
‘mitted 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 lat- 
ter, 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 illu- 
minated 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 experiment is manifestly erroneously in- 
terpreted. The rays which came from below are clearly not reflected 
by the water, but transmitted ; they are reflected indeed from foreign 
substances in the water, especially from the sea-bottom. The further 
distant this is, that is, the deeper the sea at the given place, the 
deeper will be the colour of the water—deep green when the water 
has a green, deep blue when it is blue (in transmitted light.) “The rays © 
which fell from above into the bell must also show the color of water, 
but to a much smaller extent, because the layer of water which they 
traverse is, in any case, much less thick than that which the rays 
