_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 the Water.
And sometimes small black Spots generated at the sides would ascend up
to the larger black Spot at the top of the Bubble, and unite with it.
_Obs._ 18. Because the Colours of these Bubbles were more extended and
lively than those of the Air thinn'd between two Glasses, and so more
easy to be distinguish'd, I shall here give you a farther description of
their order, as they were observ'd in viewing them by Reflexion of the
Skies when of a white Colour, whilst a black substance was placed
behind the Bubble. And they were these, red, blue; red, blue; red, blue;
red, green; red, yellow, green, blue, purple; red, yellow, green, blue,
violet; red, yellow, white, blue, black.
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