thereabouts, and by consequence equal to 16/79 parts of an Inch, and its
true Semi-diameter equal to 8/79 parts. Now as the Diameter of the
Sphere (182 Inches) is to the Semi-diameter of this fifth dark Ring
(8/79 parts of an Inch) so is this Semi-diameter to the thickness of the
Air at this fifth dark Ring; which is therefore 32/567931 or
100/1774784. Parts of an Inch; and the fifth Part thereof, _viz._ the
1/88739 Part of an Inch, is the Thickness of the Air at the first of
these dark Rings.
The same Experiment I repeated with another double convex Object-glass
ground on both sides to one and the same Sphere. Its Focus was distant
from it 168-1/2 Inches, and therefore the Diameter of that Sphere was
184 Inches. This Glass being laid upon the same plain Glass, the
Diameter of the fifth of the dark Rings, when the black Spot in their
Center appear'd plainly without pressing the Glasses, was by the measure
of the Compasses upon the upper Glass 121/600 Parts of an Inch, and by
consequence between the Glasses it was 1222/6000: For the upper Glass
was 1/8 of an Inch thick, and my Eye was distant from it 8 Inches. And a
third proportional to half this from the Diameter of the Sphere is
5/88850 Parts of an Inch. This is therefore the Thickness of the Air at
this Ring, and a fifth Part thereof, _viz._ the 1/88850th Part of an
Inch is the Thickness thereof at the first of the Rings, as above.
I tried the same Thing, by laying these Object-glasses upon flat Pieces
of a broken Looking-glass, and found the same Measures of the Rings:
Which makes me rely upon them till they can be determin'd more
accurately by Glasses ground to larger Spheres, though in such Glasses
greater care must be taken of a true Plane.
These Dimensions were taken, when my Eye was placed almost
perpendicularly over the Glasses, being about an Inch, or an Inch and a
quarter, distant from the incident Rays, and eight Inches distant from
the Glass; so that the Rays were inclined to the Glass in an Angle of
about four Degrees. Whence by the following Observation you will
understand, that had the Rays been perpendicular to the Glasses, the
Thickness of the Air at these Rings would have been less in the
Proportion of the Radius to the Secant of four Degrees, that is, of
10000 to 10024. Let the Thicknesses found be therefore diminish'd in
this Proportion, and they will become 1/88952 and 1/89063, or (to use
the nearest round Number) the 1/89000th Part of an Inch. This is the
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