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wards the center, and on the other fide red and yellow.

Now as all these things follow' from the properties of Light' by a mathematical way of rea- 9 . soning, so the truth of them may be manifested by Experiments." For in a dark Room, by .viewing these Rings through a Prism, by reflexion t • of the, several prismatick Colours, which an assistant causes to move to and fro upon a* Wall or Paper from whence they are reflected, whilst the Spectator's Eye, the Prism, and the Object- • glasses, (as in the 13th Observation,) are placed steady; the Position tof the Circles made successively by the several* Colours, will be found such, in respect of one another, as I have- described in the Figures a b x v, or a b x v, or a./3£T. And by the lame method the truth of the Explications of other Observations may be examined.

By what hath been said, the like Phænomena of Water and thin Plates of Glass may be. understood. But in small fragments of those Plates there is this farther observable, that where they lie flat upon a- Table, and are turned about their centers whilst they are , view'd through a Pr ism, they will in some postures exhibit Waves- of various Colours; and some of them exhibit these Waves in one or two positions only, but the * most of them do in all Positions exhibit them, and make them for the most part appear al- . « most all- over the' Plates. The reason is, that the Superficies of such Plates are not even, but • have many Cavities and Swellings, "which, how shallow* soever, - do a little vary the thickness of

. * the

the Plate. For at the several sides of those Cavities, for the Reasons newly described, • there ought to be produced-Waves' in' several postures ' of the Prism". Now though it be .but some very small and narrower parts of the Glass, by which these Waves for the' molt part are.caused; yet they . may seem to extend themselves over the whole Glass, because from the narrowels of those parts there are Colours of several Orders, that is, of several Rings, confusedly reflected,* which by Refraction of the Prism are unfolded, separated, and, according to" their degrees of Refraction, dispersed to several places, so as to constitute ib many several Waves, as there were divers orders of Colours promiscuously reflected from that part of the Glass.

These are the principal phenomena of thin Plates,or Bubbles, whose Explications depend oil the properties of Light, which I have heretofore deliver'd. And these you fee do necessarily follow from them, and agree with them, even to their very least circumstances; aud not only so, but do very much tend to their proof. Thus, by the 24th Observation it appears, that the Rays of several Colours, made as well by thin Plates or Bubbles, as by Refractions of,a Prism, have several degrees of Refrangibility; whereby those of each order, which at the reflexion from the Plate or Bubble are intermix'd with those of other orders, are separated from them by Refraction, and associated together so as to become visible by themselves like Arcs of Circles. For if the Rays were all alike refrangible, 'tis impossible that the whiteness, which

P 4 'to to the naked Sense appears uniform, should by Refraction have its parts transposed and ranged into those black and white Arcs.

It appears also that the 'unequal* Refractions pf difrorm Rays proceed not from any contingent irregularities; such as are Veins, an uneven Polish, or fortuitous position of the Pores" of Glass; unequal and casual Motions in the Air or Æther, the spreading, breaking, or dividing the same«Ray into many diverging parts; or the like. " For, admitting any such irregularities,1 it would be impossible for Refractions * to render, those Rings so very distinct, and well defined, as they do in the 24th Obiervation. It is necessary therefore that every Ray have its proper and constant degree of Refrangibility connate with it, according to which its refraction is ever justly and regularly perform'd; and that several Rays have several of thole degrees.

And what is said of their Refrangibility may be also understood of their Restexibility, that is,* of their Dispositions to be reflected, some at a greater, and others at a less thickness of thin Plates or Bubbles; namely, that' those Dispositions are also connate with the Rays, and immutable 5 as may appear by the 13th, 14th, and 15th Observations, compared with the fourth and eighteenth.

By the precedent Observations it appears also, that whiteness is a dissimilar mixture of all Colours, and that Light is a mixture of Rays endued with all those Colours. For, considering the multitude of the Rings of Colours in the 3d, 12th, and 24th Observations, it is. mamfest', that although in the 4th and 18th Observations there appear no more than eight or nine of those Rings, yet there are really a far greater number, which so much interfere and mingle" with one another, as after those eight or nine revolutions to dilute 'one another wholly, and constitute an even* and sensibly uniform

•whiteness. And consequently that whiteness must be allowed a mixture of all Colours, and

\ the Light which conveys it to the Eye must be a mixture of Rays endued with all those Colours. , *

■ But farther 5 by the 24th Observation it appears, that there is a constant relation between Colours and Refrangibility j the'most refrangi

w ble Rays being violet, the least refrangible red, and those of intermediate Colours having proportionably intermediate degrees of Refrangibi

* lity. And by the 13th, 14th, and 15th Observations, compared with the 4th or 18th, there appears to be the fame constant relation between Colour and Rerlexibility; the violet being in like circumstances, reflected at least thicknesses of any t thin Plate or Bubble, the red at greatest thicknesses, and the intermediate Colours at intermediate thicknesses.- Whence it follows, that the colorifick Dispositions of Rays are also connate with them, and immutable; and by consequence, that all the Productions and. Appearances of Colours in the World are derived, not from any physical Change caused in light by" Refraction or Reflexion, but only from the various Mixtures or Separations of' flays, by virtue of their different Refrangibility

or Inflexibility. And in this respect the Science of Colours becomes a Speculation as truly' mathematical as any other part of Opticks. I mean, so far as they depend on the "Nature of Light, and are not produced or altered by the Power of Imagination, or by sinking or pressing.the Eye, ' . ".

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