n slowly between them, and the Circles thereby became less and
the Colours more faint: Insomuch that as the Water crept along, one half
of them at which it first arrived would appear broken off from the other
half, and contracted into a less Room. By measuring them I found the
Proportions of their Diameters to the Diameters of the like Circles made
by Air to be about seven to eight, and consequently the Intervals of the
Glasses at like Circles, caused by those two Mediums Water and Air, are
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