Further, in this Production of Colours there is no need of a determinate Light of such a bigness and no more, nor of a determinate position of that Light, that it should be on this side, and not on that side; nor of a terminating shadow, as in the Prisme, and Rainbow, or Water-ball: for we find, that the Light in the open Air, either in or out of the Sun-beams, and within a Room, either from one or many Windows, produces much the same effect: only where the Light is brightest, there the Colours are most vivid. So does the light of a Candle, collected by a Glass-ball. And further, it is all one whatever side of the coloured Rings be towards the light; for the whole Ring keeps its proper Colours from the middle outwards in the same order as I before related, without varying at all, upon 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 verticity 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 honourable Mr. Boyle. What may be said of those Ignes fatui that appear in the night, I cannot so well affirm, having never had the opportunity to examine them my self, nor to be inform’d by any others that had observ’d them: And the relations of them in Authors are so imperfect, that nothing can be built on them. But I hope I shall be able in another place to make it at least very probable, that there is even in those also a Motion which causes this effect. That the shining of Sea-water proceeds from the same cause, may be argued from this, That it shines not till either it be beaten against a Rock, or be some other wayes broken or agitated by Storms, or Oars, or other percussing bodies. And that the Animal Energies or Spirituous agil parts are very active in Cats eyes when they shine, seems evident enough, because their eyes never shine but when they look very intensly either to find their prey, or being hunted in a dark room, when they seek after their adversary, or to find a way to escape. And the like may be said of the shining Bellies of Gloworms; since ’tis evident they can at pleasure either increase or extinguish that Radiation.
It would be somewhat too long a work for this place Zetetically to examine, and positively to prove, what particular kind of motion it is that must be the efficient of Light; for though it be a motion, yet ’tis not every motion that produces it, since we find there are many bodies very violently mov’d, which yet afford not such an effect; and there are other bodies, which to our other senses, seem not mov’d so much, which yet shine. Thus Water and quick-silver, and most other liquors heated, shine not; and several hard bodies, as Iron, Silver, Brass, Copper, Wood, &c. though very often struck with a hammer, shine not presently, though they will all of them grow exceeding hot; whereas rotten Wood, rotten Fish, Sea-water, Gloworms, &c. have nothing of tangible heat in them, and yet (where there is no stronger light to affect the Sensory) they shine some of them so Vividly, that one may make a shift to read by them.
It would be too long, I say, here to insert the discursive progress by which I inquir’d after the proprieties of the motion of Light, and therefore I shall only add the result.
And, First, I found it ought to be exceeding quick, such as those motions of fermentation and putrefaction, whereby, certainly, the parts are exceeding nimbly and violently mov’d; and that, because we find those motions are able more minutely to shatter and divide the body, then the most violent heats or menstruums we yet know. And that fire is nothing else but such a dissolution of the Burning body, made by the most universal menstruum of all sulphureous bodies, namely, the Air, we shall in an other place of this Tractate endeavour to make probable. And that, in all extreamly hot shining bodies, there is a very quick motion that causes Light, as well as a more robust that causes Heat, may be argued from the celerity wherewith the bodyes are dissolv’d.
Next, it must be a Vibrative motion. And for this the newly mention’d Diamond affords us a good argument; since if the motion of the parts did not return, the Diamond must after many rubbings decay and be wasted: but we have no reason to suspect the latter, especially if we consider the exceeding difficulty that is found in cutting or wearing away a Diamond. And a Circular motion of the parts is much more improbable, since, if that were granted, and they be suppos’d irregular and Angular parts, I see not how the parts of the Diamond should hold so firmly together, or remain in the same sensible dimensions, which yet they do. Next, if they be Globular, and mov’d only with a turbinated motion, I know not any cause that can impress that motion upon the pellucid medium, which yet is done. Thirdly, any other irregular motion of the parts one amongst another, must necessarily make the body of a fluid consistence, from which it is far enough. It must therefore be a Vibrating motion.
And Thirdly, That it is a very short-vibrating motion, I think the instances drawn from the shining of Diamonds will also make probable. For a Diamond being the hardest body we yet know in the World, and consequently the least apt to yield or bend, must consequently also have its vibrations exceeding short.