thrown back to the eye, before the depth necessary to
absolute extinction has been attained. An effect precisely similar
occurs under the moraines of glaciers. The ice here is exceptionally
compact, and, owing to the absence of the internal scattering common
in bubbled ice, the light plunges into the mass, where it is
extinguished, the perfectly clear ice presenting an appearance of
pitchy blackness. [Footnote: I learn from a correspondent that certain
Welsh tarns, which are reputed bottomless, have this inky hue.]

The green colour of the sea has now to be accounted for; and here,
again, let us fall back upon the sure basis of experiment. A strong
white dinner-plate had a lead weight securely fastened to it. Fifty
or sixty yards of strong hempen line were attached to the plate.

My assistant, Thorogood, occupied a boat, fastened as usual to the
davits of the "Urgent," while I occupied a second boat nearer the
stern of the ship. He cast the plate as a mariner heaves the lead,
and by the time it reached me it had sunk a considerable depth in the
water. In all cases the hue of this plate was green. Even when the
sea was of the darkest indigo, the green, was vivid and pronounced. I
could notice the gradual deepening of the colour as the plate sank,
but at its greatest depth, even in indigo water, the colour was still
a blue-green. [Footnote: In no case, of course, is the green pure, but
a mixture of green and blue.]

Other observations confirmed this one. The "Urgent" is a screw
steamer, and right over the blades of the screw was an orifice called
the screw-well, through which one could look from the poop down upon
the screw. The surface-glimmer, which so pesters the eye, was here in
a great measure removed. Midway down, a plank crossed the screw-well
from side to side; on this I placed myself and observed the action of
the screw underneath. The eye was rendered sensitive by the
moderation of the light; and, to remove still further all disturbing
causes, Lieutenant Walton had a sail and tarpaulin thrown over the
mouth of the well. Underneath this I perched myself on the plank and
watched the screw. In an indigo sea the play of colour was
indescribably beautiful, and the contrast between the water, which had
the screw-blades, and that which had the bottom of the ocean, as a
background, was extraordinary. The one was of the most brilliant
green, the other of the deepest ultramarine. The surface of the water