of blue and violet in the middle of the indigo, and by the extreme
violet) are to one another very nearly as the sixth lengths of a Chord
which found the Notes in a sixth Major, _sol_, _la_, _mi_, _fa_, _sol_,
_la_. But it agrees something better with the Observation to say, that
the thicknesses of the Air between the Glasses there, where the Rings
are successively made by the limits of the seven Colours, red, orange,
yellow, green, blue, indigo, violet in order, are to one another as the
Cube Roots of the Squares of the eight lengths of a Chord, which found
the Notes in an eighth, _sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_,
_sol_; that is, as the Cube Roots of the Squares of the Numbers, 1, 8/9,
5/6, 3/4, 2/3, 3/5, 9/16, 1/2.
_Obs._ 15. These Rings were not of various Colours like those made in
the open Air, but appeared all over of that prismatick Colour only with
which they were illuminated. And by projecting the prismatick Colours
immediately upon the Glasses, I found that the Light which fell on the
dark Spaces which were between the Colour'd Rings was transmitted
through the Glasses without any variation of Colour. For on a white
Paper placed behind, it would paint Rings of the same Colour with those
which were reflected, and of the bigness of their immediate Spaces. And
from thence the origin of these Rings is manifest; namely, that the Air
between the Glasses, according to its various thickness, is disposed in
some places to reflect, and in others to transmit the Light of any one
Colour (as you may see represented in the fourth Figure) and in the same
place to reflect that of one Colour where it transmits that of another.
[Illustration: FIG. 4.]
_Obs._ 16. The Squares of the Diameters of these Rings made by any
prismatick Colour were in arithmetical Progression, as in the fifth
Observation. And the Diameter of the sixth Circle, when made by the
citrine yellow, and viewed almost perpendicularly was about 58/100 parts
of an Inch, or a little less, agreeable to the sixth Observation.
The precedent Observations were made with a rarer thin Medium,
terminated by a denser, such as was Air or Water compress'd between two
Glasses. In those that follow are set down the Appearances of a denser
Medium thin'd within a rarer, such as are Plates of Muscovy Glass,
Bubbles of Water, and some other thin Substances terminated on all sides
with air.
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