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 Light gradually
passing into one another, as they do when made by Prisms; the
Circumference DEFGABCD, representing the whole Series of Colours from
one end of the Sun's colour'd Image to the other, so that from D to E be
all degrees of red, at E the mean Colour between red and orange, from E
to F all degrees of orange, at F the mean between orange and yellow,
from F to G all degrees of yellow, and so on. Let _p_ be the Center of
Gravity of the Arch DE, and _q_, _r_, _s_, _t_, _u_, _x_, the Centers of
Gravity of the Arches EF, FG, GA, AB, BC, and CD respectively, and about
those Centers of Gravity let Circles proportional to the Number of Rays
of each Colour in the given Mixture be describ'd: that is, the Circle
_p_ proportional to the Number of the red-making Rays in the Mixture,
the Circle _q_ proportional to the Number of the orange-making Rays in
the Mixture, and so of the rest. Find the common Center of Gravity of
all those Circles, _p_, _q_, _r_, _s_, _t_, _u_, _x_. Let that Center be
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