these Glasses was plane, and the other spherical, their Intervals at
those Rings must be in the same Progression. I measured also the
Diameters of the dark or faint Rings between the more lucid Colours, and
found their Squares to be in the arithmetical Progression of the even
Numbers, 2, 4, 6, 8, 10, 12. And it being very nice and difficult to
take these measures exactly; I repeated them divers times at divers
parts of the Glasses, that by their Agreement I might be confirmed in
them. And the same method I used in determining some others of the
following Observations.
_Obs._ 6. The Diameter of the sixth Ring at the most lucid part of its
Orbit was 58/100 parts of an Inch, and the Diameter of the Sphere on
which the double convex Object-glass was ground was about 102 Feet, and
hence I gathered the thickness of the Air or Aereal Interval of the
Glasses at that Ring. But some time after, suspecting that in making
this Observation I had not determined the Diameter of the Sphere with
sufficient accurateness, and being uncertain whether the Plano-convex
Glass was truly plane, and not something concave or convex on that side
which I accounted plane; and whether I had not pressed the Glasses
together, as I often did, to make them touch; (For by pressing such
Glasses together their parts easily yield inwards, and the Rings thereby
become sensibly broader than they would be, did the Glasses keep their
Figures.) I repeated the Experiment, and found the Diameter of the sixth
lucid Ring about 55/100 parts of an Inch. I repeated the Experiment also
with such an Object-glass of another Telescope as I had at hand. This
was a double Convex ground on both sides to one and the same Sphere, and
its Focus was distant from it 83-2/5 Inches. And thence, if the Sines of
Incidence and Refraction of the bright yellow Light be assumed in
proportion as 11 to 17, the Diameter of the Sphere to which the Glass
was figured will by computation be found 182 Inches. This Glass I laid
upon a flat one, so that the black Spot appeared in the middle of the
Rings of Colours without any other Pressure than that of the weight of
the Glass. And now measuring the Diameter of the fifth dark Circle as
accurately as I could, I found it the fifth part of an Inch precisely.
This Measure was taken with the points of a pair of Compasses on the
upper Surface on the upper Glass, and my Eye was about eight or nine
Inches distance from the Glass, almost perpendicularly over it, and the
Glass was 1/6 of an Inch thick, and thence it is easy to collect that
the true Diameter of the Ring between the Glasses was greater than its
measur'd Diameter above the Glasses in the Proportion of 80 to 79, or
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