r where
the Sun shines through the Glass of the Window, or by shutting the
Window that the Sun might shine through the Glass upon the Powder, and
by such other fit Means of increasing or decreasing the Lights wherewith
the Powder and Paper were illuminated, the Light wherewith the Powder is
illuminated may be made stronger in such a due Proportion than the Light
wherewith the Paper is illuminated, that they shall both appear exactly
alike in Whiteness. For when I was trying this, a Friend coming to visit
me, I stopp'd him at the Door, and before I told him what the Colours
were, or what I was doing; I asked him, Which of the two Whites were the
best, and wherein they differed? And after he had at that distance
viewed them well, he answer'd, that they were both good Whites, and that
he could not say which was best, nor wherein their Colours differed.
Now, if you consider, that this White of the Powder in the Sun-shine was
compounded of the Colours which the component Powders (Orpiment, Purple,
Bise, and _Viride AEris_) have in the same Sun-shine, you must
acknowledge by this Experiment, as well as by the former, that perfect
Whiteness may be compounded of Colours.

From what has been said it is also evident, that the Whiteness of the
Sun's Light is compounded of all the Colours wherewith the several sorts
of Rays whereof that Light consists, when by their several
Refrangibilities they are separated from one another, do tinge Paper or
any other white Body whereon they fall. For those Colours (by _Prop._
II. _Part_ 2.) are unchangeable, and whenever all those Rays with those
their Colours are mix'd again, they reproduce the same white Light as
before.

_PROP._ VI. PROB. II.

_In a mixture of Primary Colours, the Quantity and Quality of each being
given, to know the Colour of the Compound._

[Illustration: FIG. 11.]

With the Center O [in _Fig._ 11.] and Radius OD describe a Circle ADF,
and distinguish its Circumference into seven Parts DE, EF, FG, GA, AB,
BC, CD, proportional to the seven Musical Tones or Intervals of the
eight Sounds, _Sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_, _sol_,
contained in an eight, that is, proportional to the Number 1/9, 1/16,
1/10, 1/9, 1/16, 1/16, 1/9. Let the first Part DE represent a red
Colour, the second EF orange, the third FG yellow, the fourth CA green,
the fifth AB blue, the sixth BC indigo, and the seventh CD violet. And
conceive that these are all the Colours of uncompounded