the u/tra-murine reflects the blue light much more copiously than theCinnaber doth. The fame Experiment may be tried successfully with red Lead and Indigo, or with any other two colour'd Bodies, if due allowance be made for the different strength or weakness of their Colour and Light.

And as the reason of the Colours of natural Bodies is evident by these Experiments, so it is farther confirmed and put past dispute by the two first Experiments of the first Part, whereby 'twas proved in such Bodies that the reflected Lights which differ in. Colours do differ also in degrees of Refrahgibility. For thence it's certain, that some Bodies reflect the more refrangible, others the less refrangible Rays more copiously.

And that this is not only a true reason of these Colours, but even the only reason, may appear farther from this Consideration, that the Colour of homogeneal Light cannot be changed by the Reflexion of natural Bodies. *

For if Bodies by Reflexion cannot* in the least change the Colour of any one fort of Rays, they cannot appear colour'd by any other means than by reflecting those which either are of their own Colour, or which by mixture must produce it.

But in trying Experiments of this kind care must be had that the Light be sufficiently homogeneal. For if Bodies be illuminated by the prdinary prismatick Colours, they will appear neither of their own Day-light Colours, nor of the Colour of the Light cast on them, but of

some some middle Colour between both, as I have found by Experience; Thus œd Lead (for instance) illuminated with the ordinary prifmarick green will not appear either red or green, but orange or yellow, or between yellow and green, accordingly as the green Light by which 'tis illuminated is more or less compounded. For because red Lead appears red when illuminated with white Light, wherein all forts of Rays are equally mixed, and in the green Light all forts of Rays are not equally mixed, the Excess of the yellow-making, green-making and blue-making Rays in the incident green Light, will cause those Rays to abound ib much in the reflected Light, as to draw the Colour from red towards their Colour. And because the red Lead reflects the red-making Rays most: copiously in proportion to their number, and next after them the orange-making and yellow-making Rays; these Rays in the reflected Light will be more in proportion to the Light than they were in the in* cident green Light, and thereby will draw the reflected Light from green towards their Colour. And therefore the red Lead will appear neither red nor green, but of a Colour between both.

In transparently colour'd Liquors 'tis observable, that their Colour uses to vary with their thickness. Thus, for instance, a red Liquor in a conical Glass held between the Light and the Eye, looks of a pale and dilute yellow at the bottom where 'tis thin, and a little higher where 'tis thicker grows orange, and where 'tis still thicker becomes red, and where 'tis thickest the red is deepest and darkest. For it is to be eoncetv'd that sy ch a Liquor: stops the, indigomaking and violet-making Rays most easily, the blue-making Rays more difficultly, the greenmaking Rays still more difficultly, and the redmaking most difficultly: And that if the thickness ot the Liquor be only so much as suffices to stop a competent number of the violet-making and indigo-making Rays, without diminishing much the number of the rest, the rest must (by Prop. 6. Part 2.) compound a pale yellow. But if the Liquor be so much thicker as to stop also a great number of tfie-blue-making Rays, and, some of the green-making, the rest must compound an orange j and where it is so thick as to stop also a great number of the green-making and a considerable number of the yellow-making, the rest must begua to compound a red, and this red must grow deeper and darker as the yellow-making and orange-making Rays are more and more stopp'd by increasing the thickness of the Liquor, ib that few Rays besides the red-making can get through.

Of this kind is an Experiment lately related to me by Mr. Haliey, who, in diving deep into the Sea in a diving vessel, found in a clear Sunshine Day, that when he was funk many Fathoms deep into the water, the upper part of his Hand on which • the Sun shone directly through the Water and through a small Glass Window in the Vessel appeared of a red Colour, like that of a Damask Rose, and the Water below and the under part of his Hand illuminated by Light reflected from the Water

below below look'd green. For thence it may be gathers, that the Sea-Water reflects back the violet and blue-making Rays most easily, and lets the red-making Rays pass, most freely and copiously to great Depths. For thereby the Sun's direct Light at all great Depths, by reason of the predominating red-making Rays, must appear red j and the greater the Depth is, the fuller and intenser must that red be. And at such Depths as the violet-making Rays scarce penetrate unto, the blue-making, green-s-making, and yellowmaking Rays being reflected from below more copiously than the red-making ones, must com- pound a green.

Now, if there be two Liquors of full Colours, suppose a red and a blue, and both of them so thick as suffices to make their Colours sufficiently full; though either Liquor be sufficiently transpattent apart, yet will you not be able to see through both together. For, if only the red-making Rays pass through one Liquor, and only the bluemaking through the other, no ' Rays can pals through both. This Mr. Hook tried casually with Glass Wedges filled with red and blue Liquors, and was surprized at the unexpected Event, the reason of it being then unknown; which makes me trust the more to his Experiment, though I have not tried it my self. But he that would repeat it, must take -care the Liquors be of very good and full Colours.

Now, whilst Bodies become coloured by reflectwhich they do not reflect or transmit. For, if Gold be foliated and held between your Eye and the Light, the Light looks of a greenish blue, and therefore massy Gold lets into its Body the blue-making Rays to be reflected to and fro 'within it till they be stopped and stifled, whilst it reflects the yellow-making outwards, and thereby looks yellow. And much after the fame manner that Leaf Gold is yellow by reflected, and blue by transmitted Light, and massy Gold is. yellow in all positions of the Eye; there are some Liquors, as the Tincture of Lignum Nepb'iticum, and some sorts of Glass which transmit one sort of Light most copiously, and reflect another sort, and thereby look of several Colours, according to the Position of the Eye to the Light. But, if these Liquors or Glasses were so thick /and massy that no Light could get through them, I question not but they would like all other opake Bodies appear of one and the fame Colour in all positions of the Eye, though this I cannot yet affirm by Experiencei For all colour'd Bodies, so far as my observation reaches, may *be seen through if made sufficiently thin, and therefore are in some measure transparent, and differ only in degrees of Transparency from tinged transparent Liquors j these Liquors, as well as those Bodies, by a sufficient Thickness becoming opake. A transparent Body which looks of any Colour by transmitted Light, may also look of the same Colour by reflected Light, the Light of that Colour being reflected by the farther Surface of the Body, or by the Air beyond it. And

then