62 On Water and Air. [January, 
to the eye. When this green silk is placed in the green part 
of the spedtrum it is, you see, a vivid green. Now I place 
it on the red part, and there it is simply black. Thus the 
colour of a body depends partly upon the colours quenched 
by the body, and partly upon the colours sent back to the eye. 
Now, Mr. Cottrell has here a little apparatus — simply a 
cell — and his objedt in handing it to me is that I may show 
you the influence of depth in producing colour. Water is 
so feebly coloured that, in order to make its colour evident 
to you, I have been obliged to have that long tube. You 
see the colour sometimes on the glaciers of Switzerland, in 
beautiful blue pools of water. Perhaps very many of the 
boys present will, when they grow older, visit the coast of 
Naples. Very likely they will go to the end of the Island of 
Capri, and there they will find coming down the steep pre- 
cipitous coast, at a certain place, a little arch, just large 
enough to admit a boat. You pass underneath this arch, 
and you find yourself under a huge grotto, the walls of which 
shimmer, in magic beauty, with the most delicate azure 
light. The reason is that, in consequence of the smallness 
of the passage, the light is unable to get into the grotto by 
the entrance, but it first plunges into the blue sea, and then 
comes up again and illuminates the cavern, so that it has to 
pass through twice the depth of the water before it reaches 
the grotto, and, in consequence of the depth of water through 
which it has to pass, it gets coloured blue. When I say 
that the light is coloured blue, I mean that its red and 
yellow and orange and all the other rays except the blue are 
cut off by the water, and only the blue remains. Now, the 
experiment I am going to show you is simply to illustrate 
the influence of depth. Here is a cell composed of a thin 
chamber ; here is one somewhat thicker ; and here is one 
thicker still. The spedtrum is on the screen, and if I simply 
interpose this thin cell, containing a light blue liquid, in the 
path of the beam you see that it has very little effedt indeed 
upon the spedtrum ; and the reason is that the stratum is 
so thin that it cannot rob the light of its rays. Now, I will 
pour more of this blue liquid into the thicker cell. You 
observe now that by thickening the layer I have cut away 
the red ray altogether. You see how little light remains if 
I make the layer still thicker; and if I went on in this way 
I should adfually cut away the whole spedtrum with a 
substance which, when used in a thin layer, has hardly 
any effedt at all upon the light. In a drinking-glass the 
water is not in a sufficient mass to give a colour to the 
light. But in the tanks at Canterbury it is a most delicate 
