_Exper._ 17. For if in the homogeneal Lights obtained by the solution of
the Problem proposed in the fourth Proposition of the first Part of this
Book, you place Bodies of several Colours, you will find, as I have
done, that every Body looks most splendid and luminous in the Light of
its own Colour. Cinnaber in the homogeneal red Light is most
resplendent, in the green Light it is manifestly less resplendent, and
in the blue Light still less. Indigo in the violet blue Light is most
resplendent, and its splendor is gradually diminish'd, as it is removed
thence by degrees through the green and yellow Light to the red. By a
Leek the green Light, and next that the blue and yellow which compound
green, are more strongly reflected than the other Colours red and
violet, and so of the rest. But to make these Experiments the more
manifest, such Bodies ought to be chosen as have the fullest and most
vivid Colours, and two of those Bodies are to be compared together.
Thus, for instance, if Cinnaber and _ultra_-marine blue, or some other
full blue be held together in the red homogeneal Light, they will both
appear red, but the Cinnaber will appear of a strongly luminous and
resplendent red, and the _ultra_-marine blue of a faint obscure and dark
red; and if they be held together in the blue homogeneal Light, they
will both appear blue, but the _ultra_-marine will appear of a strongly
luminous and resplendent blue, and the Cinnaber of a faint and dark
blue. Which puts it out of dispute that the Cinnaber reflects the red
Light much more copiously than the _ultra_-marine doth, and the
_ultra_-marine reflects the blue Light much more copiously than the
Cinnaber doth. The same Experiment may be tried successfully with red
Lead and Indigo, or with any other two colour'd Bodies, if due allowance
be made for the different strength or weakness of their Colour and
Light.
And as the reason of the Colours of natural Bodies is evident by these
Experiments, so it is farther confirmed and put past dispute by the two
first Experiments of the first Part, whereby 'twas proved in such Bodies
that the reflected Lights which differ in Colours do differ also in
degrees of Refrangibility. For thence it's certain, that some Bodies
reflect the more refrangible, others the less refrangible Rays more
copiously.
And that this is not only a true reason of these Colours, but even the
only reason, may appear farther from this Consideration, that the Colour
of homogeneal Light cannot be changed by the Reflexion of natural
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