changing the position of the light.

But above all it is most observable, that here are all kind of Colours
generated in a _pellucid_ body, where there is properly no such refraction
as _Des Cartes_ supposes his _Globules_ to acquire a _vertuity_ by: For in
the plain and even Plates it is manifest, that the second refraction
(according to _Des Cartes_ his Principles in the _fifth section of the
eighth Chapter of his Meteors_) does regulate and restore the supposed
_turbinated Globules_ unto their former uniform motion. This Experiment
therefore will prove such a one as our _thrice excellent Verulam_ calls
_Experimentum Crucis_, serving as a Guide or Land-mark, by which to direct
our course in the search after the true cause of Colours. Affording us this
particular negative Information, that for the production of Colours there
is not necessary either a great refraction, as in the Prisme; nor Secondly,
a determination of Light and shadow, such as is both in the Prisme and
Glass-ball. Now that we may see likewise what affirmative and positive
Instruction it yields, it will be necessary, to examine it a little more
particularly and strictly; which that we may the better do, it will be
requisite to premise somewhat in general concerning the nature of Light and
Refraction.

And first for Light it seems very manifest, that there is no luminous Body
but has the parts of it in motion more or less.

First, That all kind of _fiery burning Bodies_ have their parts in motion,
I think, will be very easily granted me. That the _spark_ struck from a
Flint and Steel is in a rapid agitation, I have elsewhere made probable.
And that the Parts of _rotten Wood_, _rotten Fish_ and the like, are also
in motion, I think, will as easily be conceded by those, who consider, that
those parts never begin to shine till the Bodies be in a state of