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 Diameter of the
same Ring made by the outmost red, and the least Diameter thereof made
by the outmost violet: Contrary to what happens in the Colours of the
oblong Spectrum made by the Refraction of a Prism, where the red is most
contracted, the violet most expanded, and in the midst of all the
Colours is the Confine of green and blue. And hence I seem to collect
that the thicknesses of the Air between the Glasses there, where the
Ring is successively made by the limits of the five principal Colours
(red, yellow, green, blue, violet) in order (that is, by the extreme
red, by the limit of red and yellow in the middle of the orange, by the
limit of yellow and green, by the limit of green and blue, by the limit
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