ncident 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
Thickness of the Air at the darkest Part of the first dark Ring made by
perpendicular Rays; and half this Thickness multiplied by the
Progression, 1, 3, 5, 7, 9, 11, &c. gives the Thicknesses of the Air at
the most luminous Parts of all the brightest Rings, _viz._ 1/178000,
3/178000, 5/178000, 7/178000, &c. their arithmetical Means 2/178000,
4/178000, 6/178000, &c. being its Thicknesses at the darkest Parts of
all the dark ones.

_Obs._ 7. The Rings were least, when my Eye was placed perpendicularly
over the Glasses in the Axis of the Rings: And when I view'd them
obliquely they became bigger, continually swelling as I removed my Eye
farther from the Axis. And partly by measuring the Diameter of the same
Circle at several Obliquities of my Eye, partly by other Means, as also
by making use of the two Prisms for very great Obliquities, I found its
Diameter, and consequently the Thickness of the Air at its Perimeter in
all those Obliquities to be very nearly in the Proportions express'd in
this Table.

-------------------+--------------------+----------+----------
Angle of Incidence |Angle of Refraction |Diameter |Thickness
on | into | of the | of the
the Air. | the Air. | Ring. | Air.
-------------------+--------------------+----------+----------
Deg. Min. | | |
| | |
00 00 | 00 00 | 10 | 10
| | |
06 26 | 10 00 | 10-1/13 | 10-2/13
| | |
12 45 | 20 00 | 10-1/3 | 10-2/3
| | |
18 49 | 30 00 | 10-3/4 | 11-1/2
|