indigo-making Rays, without diminishing much the number of the rest, the
rest must (by _Prop._ 6. _Part_ 2.) compound a pale yellow. But if the
Liquor be so much thicker as to stop also a great number of the
blue-making Rays, and some of the green-making, the rest must compound
an orange; and where it is so thick as to stop also a great number of
the green-making and a considerable number of the yellow-making, the
rest must begin to compound a red, and this red must grow deeper and
darker as the yellow-making and orange-making Rays are more and more
stopp'd by increasing the thickness of the Liquor, so that few Rays
besides the red-making can get through.
Of this kind is an Experiment lately related to me by Mr. _Halley_, who,
in diving deep into the Sea in a diving Vessel, found in a clear
Sun-shine Day, that when he was sunk many Fathoms deep into the Water
the upper part of his Hand on which the Sun shone directly through the
Water and through a small Glass Window in the Vessel appeared of a red
Colour, like that of a Damask Rose, and the Water below and the under
part of his Hand illuminated by Light reflected from the Water below
look'd green. For thence it may be gather'd, that the Sea-Water reflects
back the violet and blue-making Rays most easily, and lets the
red-making Rays pass most freely and copiously to great Depths. For
thereby the Sun's direct Light at all great Depths, by reason of the
predominating red-making Rays, must appear red; and the greater the
Depth is, the fuller and intenser must that red be. And at such Depths
as the violet-making Rays scarce penetrate unto, the blue-making,
green-making, and yellow-making Rays being reflected from below more
copiously than the red-making ones, must compound a green.
Now, if there be two Liquors of full Colours, suppose a red and blue,
and both of them so thick as suffices to make their Colours sufficiently
full; though either Liquor be sufficiently transparent apart, yet will
you not be able to see through both together. For, if only the
red-making Rays pass through one Liquor, and only the blue-making
through the other, no Rays can pass through both. This Mr. _Hook_ tried
casually with Glass Wedges filled with red and blue Liquors, and was
surprized at the unexpected Event, the reason of it being then unknown;
which makes me trust the more to his Experiment, though I have not tried
it my self. But he that would repeat it, must take care the Liquors be
of very good and full Colours.
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