interfere with one another, they must appear distinct, and also white,
if the constituent Colours be so much contracted as to be wholly
co-incident. But on the other side, where the Orbit of every Ring is
made broader by the farther unfolding of its Colours, it must interfere
more with other Rings than before, and so become less distinct.
[Illustration: FIG. 7.]
To explain this a little farther, suppose the concentrick Circles AV,
and BX, [in _Fig._ 7.] represent the red and violet of any Order, which,
together with the intermediate Colours, constitute any one of these
Rings. Now these being view'd through a Prism, the violet Circle BX,
will, by a greater Refraction, be farther translated from its place than
the red AV, and so approach nearer to it on that side of the Circles,
towards which the Refractions are made. For instance, if the red be
translated to _av_, the violet may be translated to _bx_, so as to
approach nearer to it at _x_ than before; and if the red be farther
translated to av, the violet may be so much farther translated to bx as
to convene with it at x; and if the red be yet farther translated to
[Greek: aY], the violet may be still so much farther translated to
[Greek: bx] as to pass beyond it at [Greek: x], and convene with it at
_e_ and _f_. And this being understood not only of the red and violet,
but of all the other intermediate Colours, and also of every revolution
of those Colours, you will easily perceive how those of the same
revolution or order, by their nearness at _xv_ and [Greek: Yx], and
their coincidence at xv, _e_ and _f_, ought to constitute pretty
distinct Arcs of Circles, especially at xv, or at _e_ and _f_; and that
they will appear severally at _x_[Greek: u] and at xv exhibit whiteness
by their coincidence, and again appear severally at [Greek: Yx], but yet
in a contrary order to that which they had before, and still retain
beyond _e_ and _f_. But on the other side, at _ab_, ab, or [Greek: ab],
these Colours must become much more confused by being dilated and spread
so as to interfere with those of other Orders. And the same confusion
will happen at [Greek: Ux] between _e_ and _f_, if the Refraction be
very great, or the Prism very distant from the Object-glasses: In which
case no parts of the Rings will be seen, save only two little Arcs at
_e_ and _f_, whose distance from one another will be augmented by
removing the Prism still farther from the Object-glasses: And these
little Arcs must be distinctest and whitest at their middle, and at
their ends, where they begin to grow confused, they must be colour'd.
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