n distinctly, which could not at
all be seen through other parts of the Glasses where the Air was
interjacent. Although the Glasses were a little convex, yet this
transparent spot was of a considerable breadth, which breadth seemed
principally to proceed from the yielding inwards of the parts of the
Glasses, by reason of their mutual pressure. For by pressing them very
hard together it would become much broader than otherwise.

_Obs._ 2. When the Plate of Air, by turning the Prisms about their
common Axis, became so little inclined to the incident Rays, that some
of them began to be transmitted, there arose in it many slender Arcs of
Colours which at first were shaped almost like the Conchoid, as you see
them delineated in the first Figure. And by continuing the Motion of the
Prisms, these Arcs increased and bended more and more about the said
transparent spot, till they were compleated into Circles or Rings
incompassing it, and afterwards continually grew more and more
contracted.

[Illustration: FIG. 1.]

These Arcs at their first appearance were of a violet and blue Colour,
and between them were white Arcs of Circles, which presently by
continuing the Motion of the Prisms became a little tinged in their
inward Limbs with red and yellow, and to their outward Limbs the blue
was adjacent. So that the order of these Colours from the central dark
spot, was at that time white, blue, violet; black, red, orange, yellow,
white, blue, violet, &c. But the yellow and red were much fainter than
the blue and violet.

The Motion of the Prisms about their Axis being continued, these Colours
contracted more and more, shrinking towards the whiteness on either
side of it, until they totally vanished into it. And then the Circles in
those parts appear'd black and white, without any other Colours
intermix'd. But by farther moving the Prisms about, the Colours again
emerged out of the whiteness, the violet and blue at its inward Limb,
and at its outward Limb the red and yellow. So that now their order from
the central Spot was white, yellow, red; black; violet, blue, white,
yellow, red, &c. contrary to what it was before.

_Obs._ 3. When the Rings or some parts of them appeared only black and
white, they were very distinct and well defined, and the blackness
seemed as intense as that of the central Spot. Also in the Borders of
the Rings, where the Colours began to emerge out of the whiteness, they
were pretty distinct, which made th