bigger than
those which were made by the blue and violet. And it was very pleasant
to see them gradually swell or contract accordingly as the Colour of the
Light was changed. The Interval of the Glasses at any of the Rings when
they were made by the utmost red Light, was to their Interval at the
same Ring when made by the utmost violet, greater than as 3 to 2, and
less than as 13 to 8. By the most of my Observations it was as 14 to 9.
And this Proportion seem'd very nearly the same in all Obliquities of my
Eye; unless when two Prisms were made use of instead of the
Object-glasses. For then at a certain great obliquity of my Eye, the
Rings made by the several Colours seem'd equal, and at a greater
obliquity those made by the violet would be greater than the same Rings
made by the red: the Refraction of the Prism in this case causing the
most refrangible Rays to fall more obliquely on that plate of the Air
than the least refrangible ones. Thus the Experiment succeeded in the
colour'd Light, which was sufficiently strong and copious to make the
Rings sensible. And thence it may be gather'd, that if the most
refrangible and least refrangible Rays had been copious enough to make
the Rings sensible without the mixture of other Rays, the Proportion
which here was 14 to 9 would have been a little greater, suppose 14-1/4
or 14-1/3 to 9.

_Obs._ 14. Whilst the Prism was turn'd about its Axis with an uniform
Motion, to make all the several Colours fall successively upon the
Object-glasses, and thereby to make the Rings contract and dilate: The
Contraction or Dilatation of each Ring thus made by the variation of its
Colour was swiftest in the red, and slowest in the violet, and in the
intermediate Colours it had intermediate degrees of Celerity. Comparing
the quantity of Contraction and Dilatation made by all the degrees of
each Colour, I found that it was greatest in the red; less in the
yellow, still less in the blue, and least in the violet. And to make as
just an Estimation as I could of the Proportions of their Contractions
or Dilatations, I observ'd that the whole Contraction or Dilatation of
the Diameter of any Ring made by all the degrees of red, was to that of
the Diameter of the same Ring made by all the degrees of violet, as
about four to three, or five to four, and that when the Light was of the
middle Colour between yellow and green, the Diameter of the Ring was
very nearly an arithmetical Mean between the greatest Di