hitherto been treated of in Opticks. For what hath been generally
agreed on I content my self to assume under the notion of Principles, in
order to what I have farther to write. And this may suffice for an
Introduction to Readers of quick Wit and good Understanding not yet
versed in Opticks: Although those who are already acquainted with this
Science, and have handled Glasses, will more readily apprehend what
followeth.
FOOTNOTES:
[A] In our Author's _Lectiones Optic _, Part I. Sect. IV. Prop 29, 30,
there is an elegant Method of determining these _Foci_; not only in
spherical Surfaces, but likewise in any other curved Figure whatever:
And in Prop. 32, 33, the same thing is done for any Ray lying out of the
Axis.
[B] _Ibid._ Prop. 34.
_PROPOSITIONS._
_PROP._ I. THEOR. I.
_Lights which differ in Colour, differ also in Degrees of
Refrangibility._
The PROOF by Experiments.
_Exper._ 1.
I took a black oblong stiff Paper terminated by Parallel Sides, and with
a Perpendicular right Line drawn cross from one Side to the other,
distinguished it into two equal Parts. One of these parts I painted with
a red colour and the other with a blue. The Paper was very black, and
the Colours intense and thickly laid on, that the Ph nomenon might be
more conspicuous. This Paper I view'd through a Prism of solid Glass,
14