482 
Proceedings of the Royal Society 
then referred to. It is shown that when the sky is covered with 
white clouds, the surface reflection is so strong as to mask the colour 
of the water, and that when the sky is deep blue the sky-reflected 
light intensifies and deepens the apparent colour of the water. The 
importance of the surface-reflected light is best seen when the sky 
is covered with clouds, and glowing with a colour different from that 
of the water, as at sunset when the clouds are richly coloured all 
over the sky and deeply down towards the horizon. The water will 
then, especially if calm, appear like a sea of molten metal glowing 
with sky-reflected light, so powerful and brilliant as entirely to 
overpower the light internally reflected by the water. 
Pure water having been shown to be perfectly transparent to the 
more refrangible rays, and as it absorbs the red rays, water, when 
looked at from the side on which the light is falling, must necessarily 
be dark, and cannot reflect any perceptible amount of blue light. We 
must, therefore, look to the solid particles in suspension in the 
water as the cause of the light reflected by water, and while the 
selective absorption of the water principally determines its colour, 
its brilliancy is entirely determined by the suspended particles. 
It is shown why it is that though we have waters of many colours, 
that yet we only observe the colour when it is blue or green, and 
never when it is yellow. Amongst other reasons given is the much 
less brilliancy of yellow waters, this less brilliancy being due to the 
less transparency of the yellow waters compared to blue ; only the 
reflecting particles near the surface are active in the yellow water, 
whereas the particles to a considerable depth in the blue can reflect 
their light to the surface. This is one reason why Loch Lomond is 
so much darker than the Lake of Geneva. Loch Lomond certainly 
has fewer and less powerful reflecting particles than the Lake of 
Geneva, but it is darker also, because only the particles to a much 
less depth can reflect their light to surface. 
The waters of our northern seas, when provided with proper 
reflecting particles, such as air-bells and white particles, are shown to 
be much bluer than they generally appear. The brightness and 
blueness of the waters off the coast of Cornwall, are shown to be 
due to the beaches along this coast being at many points covered 
with a whitish coloured sand, which gets mixed up with the water 
by the action of the waves. As the water of the sea is constantly 
