as about three to four. Perhaps it may be a general Rule, That if any
other Medium more or less dense than Water be compress'd between the
Glasses, their Intervals at the Rings caused thereby will be to their
Intervals caused by interjacent Air, as the Sines are which measure the
Refraction made out of that Medium into Air.
_Obs._ 11. When the Water was between the Glasses, if I pressed the
upper Glass variously at its edges to make the Rings move nimbly from
one place to another, a little white Spot would immediately follow the
center of them, which upon creeping in of the ambient Water into that
place would presently vanish. Its appearance was such as interjacent Air
would have caused, and it exhibited the same Colours. But it was not
air, for where any Bubbles of Air were in the Water they would not
vanish. The Reflexion must have rather been caused by a subtiler Medium,
which could recede through the Glasses at the creeping in of the Water.
_Obs._ 12. These Observations were made in the open Air. But farther to
examine the Effects of colour'd Light falling on the Glasses, I darken'd
the Room, and view'd them by Reflexion of the Colours of a Prism cast on
a Sheet of white Paper, my Eye being so placed that I could see the
colour'd Paper by Reflexion in the Glasses, as in a Looking-glass. And
by this means the Rings became distincter and visible to a far greater
number than in the open Air. I have sometimes seen more than twenty of
them, whereas in the open Air I could not discern above eight or nine.
[Illustration: FIG. 3.]
_Obs._ 13. Appointing an Assistant to move the Prism to and fro about
its Axis, that all the Colours might successively fall on that part of
the Paper which I saw by Reflexion from that part of the Glasses, where
the Circles appear'd, so that all the Colours might be successively
reflected from the Circles to my Eye, whilst I held it immovable, I
found the Circles which the red Light made to be manifestly bigger than
those which were made by the blue and violet. And it was very pleasant
to see them gradually swell or contract accordingly as the Colour of the
Light was changed. The Interval of the Glasses at any of the Rings when
they were made by the utmost red Light, was to their Interval at the
same Ring when made by the utmost violet, greater than as 3 to 2, and
less than as 13 to 8. By the most of my Observations it was as 14 to 9.
And this Proportion seem'd very nearly the same in all Obliquities of my
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