_All homogeneal Light has its proper Colour answering to its Degree of
Refrangibility, and that Colour cannot be changed by Reflexions and
Refractions._
In the Experiments of the fourth Proposition of the first Part of this
first Book, when I had separated the heterogeneous Rays from one
another, the Spectrum _pt_ formed by the separated Rays, did in the
Progress from its End _p_, on which the most refrangible Rays fell, unto
its other End _t_, on which the least refrangible Rays fell, appear
tinged with this Series of Colours, violet, indigo, blue, green, yellow,
orange, red, together with all their intermediate Degrees in a continual
Succession perpetually varying. So that there appeared as many Degrees
of Colours, as there were sorts of Rays differing in Refrangibility.
_Exper._ 5. Now, that these Colours could not be changed by Refraction,
I knew by refracting with a Prism sometimes one very little Part of this
Light, sometimes another very little Part, as is described in the
twelfth Experiment of the first Part of this Book. For by this
Refraction the Colour of the Light was never changed in the least. If
any Part of the red Light was refracted, it remained totally of the same
red Colour as before. No orange, no yellow, no green or blue, no other
new Colour was produced by that Refraction. Neither did the Colour any
ways change by repeated Refractions, but continued always the same red
entirely as at first. The like Constancy and Immutability I found also
in the blue, green, and other Colours. So also, if I looked through a
Prism upon any Body illuminated with any part of this homogeneal Light,
as in the fourteenth Experiment of the first Part of this Book is
described; I could not perceive any new Colour generated this way. All
Bodies illuminated with compound Light appear through Prisms confused,
(as was said above) and tinged with various new Colours, but those
illuminated with homogeneal Light appeared through Prisms neither less
distinct, nor otherwise colour'd, than when viewed with the naked Eyes.
Their Colours were not in the least changed by the Refraction of the
interposed Prism. I speak here of a sensible Change of Colour: For the
Light which I here call homogeneal, being not absolutely homogeneal,
there ought to arise some little Change of Colour from its
Heterogeneity. But, if that Heterogeneity was so little as it might be
made by the said Experiments of the fourth Proposition, that Change was
not sensible, and therefore in Experiments, where Sense is Judge, ought
to be accounted none at all.
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