purple.
This Iris was immediately encompassed with a second, whose Colours were
in order from the inside outwards, purple, blue, green, yellow, light
red, a red mix'd with purple.
Then immediately follow'd the Colours of the third Iris, which were in
order outwards a green inclining to purple, a good green, and a red more
bright than that of the former Iris.
The fourth and fifth Iris seem'd of a bluish green within, and red
without, but so faintly that it was difficult to discern the Colours.
_Obs._ 3. Measuring the Diameters of these Rings upon the Chart as
accurately as I could, I found them also in the same proportion to one
another with the Rings made by Light transmitted through the two
Object-glasses. For the Diameters of the four first of the bright Rings
measured between the brightest parts of their Orbits, at the distance of
six Feet from the Speculum were 1-11/16, 2-3/8, 2-11/12, 3-3/8 Inches,
whose Squares are in arithmetical progression of the numbers 1, 2, 3, 4.
If the white circular Spot in the middle be reckon'd amongst the Rings,
and its central Light, where it seems to be most luminous, be put
equipollent to an infinitely little Ring; the Squares of the Diameters
of the Rings will be in the progression 0, 1, 2, 3, 4, &c. I measured
also the Diameters of the dark Circles between these luminous ones, and
found their Squares in the progression of the numbers 1/2, 1-1/2, 2-1/2,
3-1/2, &c. the Diameters of the first four at the distance of six Feet
from the Speculum, being 1-3/16, 2-1/16, 2-2/3, 3-3/20 Inches. If the
distance of the Chart from the Speculum was increased or diminished, the
Diameters of the Circles were increased or diminished proportionally.
_Obs._ 4. By the analogy between these Rings and those described in the
Observations of the first Part of this Book, I suspected that there
were many more of them which spread into one another, and by interfering
mix'd their Colours, and diluted one another so that they could not be
seen apart. I viewed them therefore through a Prism, as I did those in
the 24th Observation of the first Part of this Book. And when the Prism
was so placed as by refracting the Light of their mix'd Colours to
separate them, and distinguish the Rings from one another, as it did
those in that Observation, I could then see them distincter than before,
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