have produced Whiteness by mixing the Colours of
Prisms. If now the Colours of natural Bodies are to be mingled, let
Water a little thicken'd with Soap be agitated to raise a Froth, and
after that Froth has stood a little, there will appear to one that shall
view it intently various Colours every where in the Surfaces of the
several Bubbles; but to one that shall go so far off, that he cannot
distinguish the Colours from one another, the whole Froth will grow
white with a perfect Whiteness.

_Exper._ 15. Lastly, In attempting to compound a white, by mixing the
coloured Powders which Painters use, I consider'd that all colour'd
Powders do suppress and stop in them a very considerable Part of the
Light by which they are illuminated. For they become colour'd by
reflecting the Light of their own Colours more copiously, and that of
all other Colours more sparingly, and yet they do not reflect the Light
of their own Colours so copiously as white Bodies do. If red Lead, for
instance, and a white Paper, be placed in the red Light of the colour'd
Spectrum made in a dark Chamber by the Refraction of a Prism, as is
described in the third Experiment of the first Part of this Book; the
Paper will appear more lucid than the red Lead, and therefore reflects
the red-making Rays more copiously than red Lead doth. And if they be
held in the Light of any other Colour, the Light reflected by the Paper
will exceed the Light reflected by the red Lead in a much greater
Proportion. And the like happens in Powders of other Colours. And
therefore by mixing such Powders, we are not to expect a strong and
full White, such as is that of Paper, but some dusky obscure one, such
as might arise from a Mixture of Light and Darkness, or from white and
black, that is, a grey, or dun, or russet brown, such as are the Colours
of a Man's Nail, of a Mouse, of Ashes, of ordinary Stones, of Mortar, of
Dust and Dirt in High-ways, and the like. And such a dark white I have
often produced by mixing colour'd Powders. For thus one Part of red
Lead, and five Parts of _Viride AEris_, composed a dun Colour like that
of a Mouse. For these two Colours were severally so compounded of
others, that in both together were a Mixture of all Colours; and there
was less red Lead used than _Viride AEris_, because of the Fulness of its
Colour. Again, one Part of red Lead, and four Parts of blue Bise,
composed a dun Colour verging a little to purple, and by adding to this
a certain