in a mixture of all it appears white. But in Rays which are reflected in
other Angles, the Intervals of the Fits of the least refrangible being
greatest, make the Rings of their Colour in their progress from this
white Ring, either outwards or inwards, increase or decrease by the
greatest steps; so that the Rings of this Colour without are greatest,
and within least. And this is the reason why in the last Observation,
when the Speculum was illuminated with white Light, the exterior Rings
made by all Colours appeared red without and blue within, and the
interior blue without and red within.
These are the Ph nomena of thick convexo-concave Plates of Glass, which
are every where of the same thickness. There are yet other Ph nomena
when these Plates are a little thicker on one side than on the other,
and others when the Plates are more or less concave than convex, or
plano-convex, or double-convex. For in all these cases the Plates make
Rings of Colours, but after various manners; all which, so far as I have
yet observed, follow from the Propositions in the end of the third part
of this Book, and so conspire to confirm the truth of those
Propositions. But the Ph nomena are too various, and the Calculations
whereby they follow from those Propositions too intricate to be here
prosecuted. I content my self with having prosecuted this kind of
Ph nomena so far as to discover their Cause, and by discovering it to
ratify the Propositions in the third Part of this Book.
_Obs._ 13. As Light reflected by a Lens quick-silver'd on the backside
makes the Rings of Colours above described, so it ought to make the like
Rings of Colours in passing through a drop of Water. At the first
Reflexion of the Rays within the drop, some Colours ought to be
transmitted, as in the case of a Lens, and others to be reflected back
to the Eye. For instance, if the Diameter of a small drop or globule of
Water be about the 500th part of an Inch, so that a red-making Ray in
passing through the middle of this globule has 250 Fits of easy
Transmission within the globule, and that all the red-making Rays which
are at a certain distance from this middle Ray round about it have 249
Fits within the globule, and all the like Rays at a certain farther
distance round about it have 248 Fits, and all those at a certain
farther distance 247 Fits, and so on; these concentrick Circles of Rays
after their transmission, falling on a white Paper, will make
concentrick Rings of red upon the Paper, supposing the Light which
passes through one single globule, strong enough to be sensible. And, in
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