478 
Proceedings of the Royal Society 
reproduced, while further out the waters were deeper blue, rivalling 
in brilliancy those of the Mediterranean. The work done by the 
waves along the shores of the Mediterranean, in manufacturing the 
light-reflecting particles, is for the Lake of Geneva, done by the 
grinding of the glacier mills and streams of the Ehone valley. 
The silt brought down by the Ehone was found to be composed of 
clean white particles, like fine white sand. Many of the particles 
are thin polished plates, and when examined by means of a con- 
centrated beam of light, while in suspension in water, are seen to 
flash brilliantly in the strong light. This white solid matter 
brought down by the Ehone is constantly being deposited all over 
the bottom of the lake, and it is this whitish deposit which gives to 
the Lake of Geneva one of its peculiarities. The light reflected 
from the whitish bottom causes the water of the lake, all along the 
shore to appear of a peculiar light blue-green colour, and enables us 
to judge of the depth of the lake at depths far beyond that to which 
we can see the bottom. We can only see a white surface of 15 cm 
square to a depth of about 7 m., yet the light reflected from the 
bottom affects the appearance of the water at depths far beyond 
7 m., showing that light penetrates by diffusion in these waters to 
far greater depths than it can directly. 
The brilliancy and beauty of the Lake of Geneva would thus 
appear to be due to the purity and transparency of its waters, 
coupled with the presence of an enormous amount of white reflecting 
surfaces, both in suspension in its water and deposited all over the 
bottom of the lake, the effect being intensified by the brilliancy of 
the reflecting particles. 
The colour of the water in Lake Bourget was found to be similar 
to that in the Lake of Geneva, though at the time it was visited it 
was slightly more turbid. A white surface could not be seen to so 
great a depth as in Geneva, and the water, even in the middle of the 
lake, when examined by means of a concentrated beam of sunlight, 
was found to be very full of suspended light-reflecting particles 
similar to those brought down by the Ehone. 
The examination of the water in these lakes was confined to the 
coloured surface experiments, and to a spectroscopic examination of 
the internally reflected light. The results were all similar to those 
in the Mediterranean. 
