them near at hand, but also yield a violet Colour at both the edges of
every white Ring. And the reason is, that the Rays which enter the Eye
at several parts of the Pupil, have several Obliquities to the Glasses,
and those which are most oblique, if consider'd apart, would represent
the Rings bigger than those which are the least oblique. Whence the
breadth of the Perimeter of every white Ring is expanded outwards by the
obliquest Rays, and inwards by the least oblique. And this Expansion is
so much the greater by how much the greater is the difference of the
Obliquity; that is, by how much the Pupil is wider, or the Eye nearer to
the Glasses. And the breadth of the violet must be most expanded,
because the Rays apt to excite a Sensation of that Colour are most
oblique to a second or farther Superficies of the thinn'd Air at which
they are reflected, and have also the greatest variation of Obliquity,
which makes that Colour soonest emerge out of the edges of the white.
And as the breadth of every Ring is thus augmented, the dark Intervals
must be diminish'd, until the neighbouring Rings become continuous, and
are blended, the exterior first, and then those nearer the center; so
that they can no longer be distinguish'd apart, but seem to constitute
an even and uniform whiteness.
Among all the Observations there is none accompanied with so odd
circumstances as the twenty-fourth. Of those the principal are, that in
thin Plates, which to the naked Eye seem of an even and uniform
transparent whiteness, without any terminations of Shadows, the
Refraction of a Prism should make Rings of Colours appear, whereas it
usually makes Objects appear colour'd only there where they are
terminated with Shadows, or have parts unequally luminous; and that it
should make those Rings exceedingly distinct and white, although it
usually renders Objects confused and coloured. The Cause of these things
you will understand by considering, that all the Rings of Colours are
really in the Plate, when view'd with the naked Eye, although by reason
of the great breadth of their Circumferences they so much interfere and
are blended together, that they seem to constitute an uniform whiteness.
But when the Rays pass through the Prism to the Eye, the Orbits of the
several Colours in every Ring are refracted, some more than others,
according to their degrees of Refrangibility: By which means the Colours
on one side of the Ring (that is in the circumference on one side of its
center), become more unfolded and dilated, and those on the other side
more complicated and contracted. And where by a due Refraction they are
so much contracted, that the several Rings become narrower than to
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