ion pronounces it to be thicker.
Then we enter the Bay of Biscay, where the indigo resumes its power,
and where the home examination shows the greatly augmented purity of
the water. A second specimen of water, taken from the Bay of Biscay,
held in suspension fine particles of a peculiar kind; the size of them
was such as to render the water richly iridescent. It showed itself
green, blue, or salmon-coloured, according to the direction of the
line of vision. Finally, we come to our last two bottles, the one
taken opposite St. Catherine's lighthouse, in the Isle of Wight, the
other at Spithead. The sea at both these places was green, and both
specimens, as might be expected, were pronounced by the home
examination to be thick with suspended matter.

Two distinct series of observations are here referred to--the one
consisting of direct observations of the colour of the sea, conducted
during the voyage from Gibraltar to Portsmouth: the other carried out
in the laboratory of the Royal Institution. And here it is to be
noted that in the home examination I never knew what water was placed
in my hands. The labels, with the names of the localities written
upon them, had been tied up, all information regarding the source of
the water being thus held back. The bottles were simply numbered, and
not till all of them had been examined, and described, were the labels
opened, and the locality and sea-colour corresponding to the various
specimens ascertained. The home observations, therefore, must have
been perfectly unbiassed, and they clearly establish the association
of the green colour with fine suspended matter, and of the ultramarine
colour, and more especially of the black-indigo hue of the Atlantic,
with the comparative absence of such matter.

So much for mere observation; but what is the cause of the dark hue of
the deep ocean? [Footnote: A note, written to me on October 22, by my
friend Canon Kingsley, contains the following reference to this point:
'I have never seen the Lake of Geneva, but I thought of the brilliant
dazzling dark blue of the mid-Atlantic under the sunlight, and its
black-blue under cloud, both so solid that one might leap off the
sponson on to it without fear; this was to me the most wonderful thing
which I saw on my voyages to and from the West Indies.']

A preliminary remark or two will clear our way towards an explanation.
Colour resides in white light, appearing when any constituent of the
wh