reek: x], F[Greek: o],
F[Greek: t]. And this being known, it is easy to determine what
thickness of Air is represented by G[Greek: ph], or by any other
distance of the Ruler from AH.

But farther, since by the 10th Observation the thickness of Air was to
the thickness of Water, which between the same Glasses exhibited the
same Colour, as 4 to 3, and by the 21st Observation the Colours of thin
Bodies are not varied by varying the ambient Medium; the thickness of a
Bubble of Water, exhibiting any Colour, will be 3/4 of the thickness of
Air producing the same Colour. And so according to the same 10th and
21st Observations, the thickness of a Plate of Glass, whose Refraction
of the mean refrangible Ray, is measured by the proportion of the Sines
31 to 20, may be 20/31 of the thickness of Air producing the same
Colours; and the like of other Mediums. I do not affirm, that this
proportion of 20 to 31, holds in all the Rays; for the Sines of other
sorts of Rays have other Proportions. But the differences of those
Proportions are so little that I do not here consider them. On these
Grounds I have composed the following Table, wherein the thickness of
Air, Water, and Glass, at which each Colour is most intense and
specifick, is expressed in parts of an Inch divided into ten hundred
thousand equal parts.

Now if this Table be compared with the 6th Scheme, you will there see
the constitution of each Colour, as to its Ingredients, or the original
Colours of which it is compounded, and thence be enabled to judge of its
Intenseness or Imperfection; which may suffice in explication of the 4th
and 18th Observations, unless it be farther desired to delineate the
manner how the Colours appear, when the two Object-glasses are laid upon
one another. To do which, let there be described a large Arc of a
Circle, and a streight Line which may touch that Arc, and parallel to
that Tangent several occult Lines, at such distances from it, as the
Numbers set against the several Colours in the Table denote. For the
Arc, and its Tangent, will represent the Superficies of the Glasses
terminating the interjacent Air; and the places where the occult Lines
cut the Arc will show at what distances from the center, or Point of
contact, each Colour is reflected.

_The thickness of colour'd Plates and Particles of_
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