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 putrefaction; and that is now generally granted by all, to be caused by the motion of the parts of putrifying bodies.  That the Bononian stone shines no longer then it is either warmed by the Sun-beams, or by the flame of a Fire or of a Candle, is the general report of those that write of it, and of others that have seen it.  And that heat argues a motion of the internal parts is (as I said before) generally granted.

But there is one Instance more, which was first shewn to the Royal Society by Mr. Clayton a worthy Member thereof, which does make this Assertion more evident then all the rest:  And that is, That a Diamond being rub’d, struck or heated in the dark, shines for a pretty while after, so long as that motion, which is imparted by any of those Agents, remains (in the same manner as a Glass, rubb’d, struck, or (by a means which I shall elsewhere mention) heated, yields a sound which lasts as long as the vibrating motion of that sonorous body) several Experiments made on which Stone, are since published in a Discourse of Colours, by the truly