_
SEA WATER
227
that if aphotographic plate was exposed for eighty minutes
at a depth of 3,280 feet it was blackened by the light rays :
but a plate exposed for 120 minutes at 5,578 feet was not
affected (seePlzite _‘2_). .
-
-
A most excellent example of the absorption of light by
the sea water is furnished by the famous cave at Capri,
known as the Blue Grotto of“ Capri.
Within this cave
everything is enveloped in the pùrest blue light.
The
explanation is to be found in the fact that the only light
that can enter the cave itself has to pass rst beneath the
water which practically lls its-narrow entrance.
In its
'
passage through the water much of the red light and some
of the yellow and green is absorbed; and the. only light
that can come once more above the surface of the
,
to illuminate the interior of the caVeis composed to a very
_
large extefitŸof—blueïrays. 5ÏThe blue Colour of the sea also
.
owes its origin to this phenomenon, for the colour of the
water is due to the reection of light uPwards from the
_
-'—j“
‘
small particles suspended in the water itself. The light
‘
reaching a particle at a given depth is thus reected up—
wards:and has to pass once more through the depth of
water it has already traversed in its doWnward journéy.-«
Much of the red and yellow light will become absorbed on
.
this upward journey if—this has not already happened on
-‘—
_its dowriward
and it is mostly blue and green
light which can survive
above the surface once
more and give the sea its typical colour; As all these raÿs
,
of light are being, absorbed'in their downward passage it
is natural—that the actual strength of the light is gradually
diminishing the deeper one
In the open ocean the
'
strength 'of light is already too weak at a depth of rod -
fathoms
to support much plant life (see Plate 2), and
below this depth few living plankton plants are to be
.
found.
Nevertheless
this
upper
layer of water,Q
Ioc
SEA WATER
227
that if aphotographic plate was exposed for eighty minutes
at a depth of 3,280 feet it was blackened by the light rays :
but a plate exposed for 120 minutes at 5,578 feet was not
affected (seePlzite _‘2_). .
-
-
A most excellent example of the absorption of light by
the sea water is furnished by the famous cave at Capri,
known as the Blue Grotto of“ Capri.
Within this cave
everything is enveloped in the pùrest blue light.
The
explanation is to be found in the fact that the only light
that can enter the cave itself has to pass rst beneath the
water which practically lls its-narrow entrance.
In its
'
passage through the water much of the red light and some
of the yellow and green is absorbed; and the. only light
that can come once more above the surface of the
,
to illuminate the interior of the caVeis composed to a very
_
large extefitŸof—blueïrays. 5ÏThe blue Colour of the sea also
.
owes its origin to this phenomenon, for the colour of the
water is due to the reection of light uPwards from the
_
-'—j“
‘
small particles suspended in the water itself. The light
‘
reaching a particle at a given depth is thus reected up—
wards:and has to pass once more through the depth of
water it has already traversed in its doWnward journéy.-«
Much of the red and yellow light will become absorbed on
.
this upward journey if—this has not already happened on
-‘—
_its dowriward
and it is mostly blue and green
light which can survive
above the surface once
more and give the sea its typical colour; As all these raÿs
,
of light are being, absorbed'in their downward passage it
is natural—that the actual strength of the light is gradually
diminishing the deeper one
In the open ocean the
'
strength 'of light is already too weak at a depth of rod -
fathoms
to support much plant life (see Plate 2), and
below this depth few living plankton plants are to be
.
found.
Nevertheless
this
upper
layer of water,Q
Ioc