some little Interval. I have sometimes observed the Diameter of that
Spot to be between half and two fifth parts of the Diameter of the
exterior Circumference of the red in the first Circuit or Revolution of
Colours when view'd almost perpendicularly; whereas when view'd
obliquely it hath wholly vanish'd and become opake and white like the
other parts of the Glass; whence it may be collected that the Glasses
did then scarcely, or not at all, touch one another, and that their
Interval at the perimeter of that Spot when view'd perpendicularly was
about a fifth or sixth part of their Interval at the circumference of
the said red.
_Obs._ 9. By looking through the two contiguous Object-glasses, I found
that the interjacent Air exhibited Rings of Colours, as well by
transmitting Light as by reflecting it. The central Spot was now white,
and from it the order of the Colours were yellowish red; black, violet,
blue, white, yellow, red; violet, blue, green, yellow, red, &c. But
these Colours were very faint and dilute, unless when the Light was
trajected very obliquely through the Glasses: For by that means they
became pretty vivid. Only the first yellowish red, like the blue in the
fourth Observation, was so little and faint as scarcely to be discern'd.
Comparing the colour'd Rings made by Reflexion, with these made by
transmission of the Light; I found that white was opposite to black, red
to blue, yellow to violet, and green to a Compound of red and violet.
That is, those parts of the Glass were black when looked through, which
when looked upon appeared white, and on the contrary. And so those which
in one case exhibited blue, did in the other case exhibit red. And the
like of the other Colours. The manner you have represented in the third
Figure, where AB, CD, are the Surfaces of the Glasses contiguous at E,
and the black Lines between them are their Distances in arithmetical
Progression, and the Colours written above are seen by reflected Light,
and those below by Light transmitted (p. 209).
_Obs._ 10. Wetting the Object-glasses a little at their edges, the Water
crept in 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
115