_The transparent parts of Bodies, according to their several sizes,
reflect Rays of one Colour, and transmit those of another, on the same
grounds that thin Plates or Bubbles do reflect or transmit those Rays.
And this I take to be the ground of all their Colours._
For if a thinn'd or plated Body, which being of an even thickness,
appears all over of one uniform Colour, should be slit into Threads, or
broken into Fragments, of the same thickness with the Plate; I see no
reason why every Thread or Fragment should not keep its Colour, and by
consequence why a heap of those Threads or Fragments should not
constitute a Mass or Powder of the same Colour, which the Plate
exhibited before it was broken. And the parts of all natural Bodies
being like so many Fragments of a Plate, must on the same grounds
exhibit the same Colours.
Now, that they do so will appear by the affinity of their Properties.
The finely colour'd Feathers of some Birds, and particularly those of
Peacocks Tails, do, in the very same part of the Feather, appear of
several Colours in several Positions of the Eye, after the very same
manner that thin Plates were found to do in the 7th and 19th
Observations, and therefore their Colours arise from the thinness of the
transparent parts of the Feathers; that is, from the slenderness of the
very fine Hairs, or _Capillamenta_, which grow out of the sides of the
grosser lateral Branches or Fibres of those Feathers. And to the same
purpose it is, that the Webs of some Spiders, by being spun very fine,
have appeared colour'd, as some have observ'd, and that the colour'd
Fibres of some Silks, by varying the Position of the Eye, do vary their
Colour. Also the Colours of Silks, Cloths, and other Substances, which
Water or Oil can intimately penetrate, become more faint and obscure by
being immerged in those Liquors, and recover their Vigor again by being
dried; much after the manner declared of thin Bodies in the 10th and
21st Observations. Leaf-Gold, some sorts of painted Glass, the Infusion
of _Lignum Nephriticum_, and some other Substances, reflect one Colour,
and transmit another; like thin Bodies in the 9th and 20th Observations.
And some of those colour'd Powders which Painters use, may have their
Colours a little changed, by being very elaborately and finely ground.
Where I see not what can be justly pretended for those changes, besides
the breaking of their parts into less parts by that contrition, after
the same manner that the Colour of a thin Plate is changed by varying
its thickness. For which reason also it is that the colour'd Flowers of
Plants and Vegetables, by being bruised, usually become more transparent
than before, or at least in some degree or other change their Colours.
Nor is it much less to my purpose, that, by mixing divers Liquors, very
odd and remarkable Productions and Changes of Colours may be effected,
of which no cause can be more obvious and rational than that the saline
Corpuscles of one Liquor do variously act upon or unite with the tinging
Corpuscles of another, so as to make them swell, or shrink, (whereby not
only their bulk but their density also may be changed,) or to divide
them into smaller Corpuscles, (whereby a colour'd Liquor may become
transparent,) or to make many of them associate into one cluster,
whereby two transparent Liquors may compose a colour'd one. For we see
how apt those saline Menstruums are to penetrate and dissolve Substances
to which they are applied, and some of them to precipitate what others
dissolve. In like manner, if we consider the various Phaenomena of the
Atmosphere, we may observe, that when Vapours are first raised, they
hinder not the transparency of the Air, being divided into parts too
small to cause any Reflexion in their Superficies. But when in order to
compose drops of Rain they begin to coalesce and constitute Globules of
all intermediate sizes, those Globules, when they become of convenient
size to reflect some Colours and transmit others, may constitute Clouds
of various Colours according to their sizes. And I see not what can be
rationally conceived in so transparent a Substance as Water for the
production of these Colours, besides the various sizes of its fluid and
globular Parcels.