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NOTES ON RANGE IN DEPTH OF MARINE ALGjE. 149
The influence of the law, that in water there is a limit of obliquity
beyond which transmission into the air cannot occur, giving rise to
total reflection, and the unequal absorption exerted on the different
separable rays of light, can only be hinted at here in relation to this
subject. According to Bouguer, sea-water at a depth of 700 feet loses
all transparency. Mr. H. Wild, in a recent number of Poggendorff's
' Annalen,' states that light, in traversing 5 metres in depth, has its
intensity reduced to one-third. He, however, adds that the transpa-
rency of water at low temperatures is greater than at higher.
Further, it is very notable that in high northern latitudes, where
thick ice covers the surface of the sea during great part of the year,
and where, moreover, the absence of dii-ect sunlight for several mouths
together produces very peculiar conditions, nevertheless seaweeds
abound, the number of species not much more than fifty, but some of
large size, and most of them individually plentiful.*
The late Professor E. Porbes adopted the following zones in relation
to the distribution of marine Organisms on the British shores :— 1st.
Littoral zone, comprehending the space between tide marks. 2nd. The
Laminarian, from low-water mark to 15 or 20 fathoms. 3rd. The Me-
dian zone, f from 15 or 20 fathoms to 50. 4th. The Infra-median ; and
5th. The Abyssal. In the first two of these seaweeds are abundant
they are rare in the lower part of the median zone, and very rare indeed
beyond it.
In recording habitats of British Algce (as in ' Phycologia ' of the
late Professor Harvey), the expression "cast up from deep water"
is often used ; it is somewhat indefinite, nevertheless, as many delicate
species are thrown on shore in very perfect condition,—they cannot
have come from any great distance ; and if we examine the tidal chart
in Johnston's ' Physical Atlas,' where depths round the British and
Irish shores are also given, it will be seen that the line of 10 fathoms
on the general coast is very narrow, but is wider in bays and arms of
the sea ;
and as these localities yield many species, 10 fathoms may be
considered a common bathymetrical range. The following may be
mentioned as reaching to or beyond 15 fathoms :
—
Chorda Filnm, Cul-
ler ia hiuUifida, Zonaria parvula, Polysiphunia parasitica, Chylocludia
* ' Journal of the Lhmean Society,' vol. ix.
+ He used the term median or coralline zone, the latter very loose or incorrect
if applied to the Corallinida of algologists.
;