And the Colours at one end of every Arc must be in a contrary order to
those at the other end, by reason that they cross in the intermediate
white; namely, their ends, which verge towards [Greek: Ux], will be red
and yellow on that side next the center, and blue and violet on the
other side. But their other ends which verge from [Greek: Ux], will on
the contrary be blue and violet on that side towards the center, and on
the other side red and yellow.
Now as all these things follow from the properties of Light by a
mathematical way of reasoning, so the truth of them may be manifested by
Experiments. For in a dark Room, by viewing these Rings through a Prism,
by reflexion of the several prismatick Colours, which an assistant
causes to move to and fro upon a Wall or Paper from whence they are
reflected, whilst the Spectator's Eye, the Prism, and the
Object-glasses, (as in the 13th Observation,) are placed steady; the
Position of the Circles made successively by the several Colours, will
be found such, in respect of one another, as I have described in the
Figures _abxv_, or abxv, or _[Greek: abxU]_. And by the same method the
truth of the Explications of other Observations may be examined.
By what hath been said, the like Ph nomena of Water and thin Plates of
Glass may be understood. But in small fragments of those Plates there is
this farther observable, that where they lie flat upon a Table, and are
turned about their centers whilst they are view'd through a Prism, they
will in some postures exhibit Waves of various Colours; and some of them
exhibit these Waves in one or two Positions only, but the most of them
do in all Positions exhibit them, and make them for the most part appear
almost all over the Plates. The reason is, that the Superficies of such
Plates are not even, but have many Cavities and Swellings, which, how
shallow soever, do a little vary the thickness of the Plate. For at the
several sides of those Cavities, for the Reasons newly described, there
ought to be produced Waves in several postures of the Prism. Now though
it be but some very small and narrower parts of the Glass, by which
these Waves for the most part are caused, yet they may seem to extend
themselves over the whole Glass, because from the narrowest of those
parts there are Colours of several Orders, that is, of several Rings,
confusedly reflected, which by Refraction of the Prism are unfolded,
separated, and, according to their degrees of Refraction, dispersed to
several places, so as to constitute so many several Waves, as there were
divers orders of Colours promiscuously reflected from that part of the
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