of those Colours are written below.
[Illustration: FIG. 6.]
But farther, to define the Latitude of these Colours in each Ring or
Series, let A1 design the least thickness, and A3 the greatest
thickness, at which the extreme violet in the first Series is reflected,
and let HI, and HL, design the like limits for the extreme red, and let
the intermediate Colours be limited by the intermediate parts of the
Lines 1I, and 3L, against which the Names of those Colours are written,
and so on: But yet with this caution, that the Reflexions be supposed
strongest at the intermediate Spaces, 2K, 6N, 10Q, &c. and from thence
to decrease gradually towards these limits, 1I, 3L, 5M, 7O, &c. on
either side; where you must not conceive them to be precisely limited,
but to decay indefinitely. And whereas I have assign'd the same Latitude
to every Series, I did it, because although the Colours in the first
Series seem to be a little broader than the rest, by reason of a
stronger Reflexion there, yet that inequality is so insensible as
scarcely to be determin'd by Observation.
Now according to this Description, conceiving that the Rays originally
of several Colours are by turns reflected at the Spaces 1I, L3, 5M, O7,
9PR11, &c. and transmitted at the Spaces AHI1, 3LM5, 7OP9, &c. it is
easy to know what Colour must in the open Air be exhibited at any
thickness of a transparent thin Body. For if a Ruler be applied parallel
to AH, at that distance from it by which the thickness of the Body is
represented, the alternate Spaces 1IL3, 5MO7, &c. which it crosseth will
denote the reflected original Colours, of which the Colour exhibited in
the open Air is compounded. Thus if the constitution of the green in the
third Series of Colours be desired, apply the Ruler as you see at
[Greek: prsph], and by its passing through some of the blue at [Greek:
p] and yellow at [Greek: s], as well as through the green at [Greek: r],
you may conclude that the green exhibited at that thickness of the Body
is principally constituted of original green, but not without a mixture
of some blue and yellow.
By this means you may know how the Colours from the center of the Rings
outward ought to succeed in order as they were described in the 4th and
18th Observations. For if you move the Ruler gradually from AH through
all distances, having pass'd over the first Space which denotes little
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