eriment, and need not here repeat it.

There is yet another material Circumstance of this Experiment. For this
emerging Light being by a third Prism HIK [in _Fig._ 22. _Part_ I.][I]
refracted towards the Paper PT, and there painting the usual Colours of
the Prism, red, yellow, green, blue, violet: If these Colours arose from
the Refractions of that Prism modifying the Light, they would not be in
the Light before its Incidence on that Prism. And yet in that Experiment
we found, that when by turning the two first Prisms about their common
Axis all the Colours were made to vanish but the red; the Light which
makes that red being left alone, appeared of the very same red Colour
before its Incidence on the third Prism. And in general we find by other
Experiments, that when the Rays which differ in Refrangibility are
separated from one another, and any one Sort of them is considered
apart, the Colour of the Light which they compose cannot be changed by
any Refraction or Reflexion whatever, as it ought to be were Colours
nothing else than Modifications of Light caused by Refractions, and
Reflexions, and Shadows. This Unchangeableness of Colour I am now to
describe in the following Proposition.

_PROP._ II. THEOR. II.

_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.