by any
prismatick Colour were in arithmetical Progression, as in the fifth
Observation. And the Diameter of the sixth Circle, when made by the
citrine yellow, and viewed almost perpendicularly was about 58/100 parts
of an Inch, or a little less, agreeable to the sixth Observation.

The precedent Observations were made with a rarer thin Medium,
terminated by a denser, such as was Air or Water compress'd between two
Glasses. In those that follow are set down the Appearances of a denser
Medium thin'd within a rarer, such as are Plates of Muscovy Glass,
Bubbles of Water, and some other thin Substances terminated on all sides
with air.

_Obs._ 17. If a Bubble be blown with Water first made tenacious by
dissolving a little Soap in it, 'tis a common Observation, that after a
while it will appear tinged with a great variety of Colours. To defend
these Bubbles from being agitated by the external Air (whereby their
Colours are irregularly moved one among another, so that no accurate
Observation can be made of them,) as soon as I had blown any of them I
cover'd it with a clear Glass, and by that means its Colours emerged in
a very regular order, like so many concentrick Rings encompassing the
top of the Bubble. And as the Bubble grew thinner by the continual
subsiding of the Water, these Rings dilated slowly and overspread the
whole Bubble, descending in order to the bottom of it, where they
vanish'd successively. In the mean while, after all the Colours were
emerged at the top, there grew in the center of the Rings a small round
black Spot, like that in the first Observation, which continually
dilated it self till it became sometimes more than 1/2 or 3/4 of an Inch
in breadth before the Bubble broke. At first I thought there had been no
Light reflected from the Water in that place, but observing it more
curiously, I saw within it several smaller round Spots, which appeared
much blacker and darker than the rest, whereby I knew that there was
some Reflexion at the other places which were not so dark as those
Spots. And by farther Tryal I found that I could see the Images of some
things (as of a Candle or the Sun) very faintly reflected, not only from
the great black Spot, but also from the little darker Spots which were
within it.

Besides the aforesaid colour'd Rings there would often appear small
Spots of Colours, ascending and descending up and down the sides of the
Bubble, by reason of some Inequalities in the subsiding of t