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
Z; and from the Center of the Circle ADF, through Z to the
Circumference, drawing the Right Line OY, the Place of the Point Y in
the Circumference shall shew the Colour arising from the Composition of
all the Colours in the given Mixture, and the Line OZ shall be
proportional to the Fulness or Intenseness of the Colour, that is, to
its distance from Whiteness. As if Y fall in the middle between F and G,
the compounded Colour shall be the best yellow; if Y verge from the
middle towards F or G, the compound Colour shall accordingly be a
yellow, verging towards orange or green. If Z fall upon the
Circumference, the Colour shall be intense and florid in the highest
Degree; if it fall in the mid-way between the Circumference and Center,
it shall be but half so intense, that is, it shall be such a Colour as
would be made by diluting the intensest yellow with an equal quantity of
whiteness; and if it fall upon the center O, the Colour shall have lost
all its intenseness, and become a white. But it is to be noted, That if
the point Z fall in or near the line OD, the main ingredients being the
red and violet, the Colour compounded shall not be any of the prismatick
Colours, but a purple, inclining to red or violet, accordingly as the
point Z lieth on the side of the line DO towards E or towards C, and in
general the compounded violet is more bright and more fiery than the
uncompounded. Also if only two of the primary Colours which in the
circle