| |
24 30 | 40 00 | 11-2/5 | 13
| | |
29 37 | 50 00 | 12-1/2 | 15-1/2
| | |
33 58 | 60 00 | 14 | 20
| | |
35 47 | 65 00 | 15-1/4 | 23-1/4
| | |
37 19 | 70 00 | 16-4/5 | 28-1/4
| | |
38 33 | 75 00 | 19-1/4 | 37
| | |
39 27 | 80 00 | 22-6/7 | 52-1/4
| | |
40 00 | 85 00 | 29 | 84-1/12
| | |
40 11 | 90 00 | 35 | 122-1/2
-------------------+--------------------+----------+----------

In the two first Columns are express'd the Obliquities of the incident
and emergent Rays to the Plate of the Air, that is, their Angles of
Incidence and Refraction. In the third Column the Diameter of any
colour'd Ring at those Obliquities is expressed in Parts, of which ten
constitute that Diameter when the Rays are perpendicular. And in the
fourth Column the Thickness of the Air at the Circumference of that Ring
is expressed in Parts, of which also ten constitute its Thickness when
the Rays are perpendicular.

And from these Measures I seem to gather this Rule: That the Thickness
of the Air is proportional to the Secant of an Angle, whose Sine is a
certain mean Proportional between the Sines of Incidence and Refraction.
And that mean Proportional, so far as by these Measures I can determine
it, is the first of an hundred and six arithmetical mean Proportionals
between those Sines counted from the bigger Sine, that is, from the Sine
of Refraction when the Refraction is made out of the Glass into the
Plate of Air, or from the Sine of Incidence when the Refraction is made
out of the Plate of Air into the Glass.

_Obs._ 8. The dark Spot in the middle of the Rings increased also by the
Obliquation of the Eye, although almost insensibly. But, if instead of
the Object-glasses the Prisms were made use of, its Increase was more
manifest when viewed so obliquely that no Colours appear'd about it. It
wa