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NOTES ON RANGE IN DEPTH OF MARINE ALGZ. 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 direct sunlight for several months 
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. Forbes 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. 
Laminarian, from low-water mark to 15 or 20 fathoms. 3rd. The Me- 
dian zone,} from 15 or 20 fathomsto 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 Alge (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 Filum, Cut- 
leria multifida, Zonaria parvula, Polysiphonia parasitica, Chylocladia 
* < Journal of aH innean Societ; vol. 
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t He used the te coralline Zo the latter very loose or incor- 
rect if applied to the e Corallinide of algolo, 
