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
Bodies.

For if Bodies by Reflexion cannot in the least change the Colour of any
one sort of Rays, they cannot appear colour'd by any other means than by
reflecting those which either are of their own Colour, or which by
mixture must produce it.

But in trying Experiments of this kind care must be had that the Light
be sufficiently homogeneal. For if Bodies be illuminated by the ordinary
prismatick Colours, they will appear neither of their own Day-light
Colours, nor of the Colour of the Light cast on them, but of some middle
Colour between both, as I have found by Experience. Thus red Lead (for
instance) illuminated with the ordinary prismatick green will not appear
either red or green, but orange or yellow, or between yellow and green,
accordingly as the green Light by which 'tis illuminated is more or less
compounded. For because red Lead appears red when illuminated with white
Light, wherein all sorts of Rays are equally mix'd, and in the green
Light all sorts of Rays are not equally mix'd, the Excess of the
yellow-making, green-making and blue-making Rays in the i