An Explication of the Fifth Plate.

Figure 1. Is one of Sir Isaac Newton's Experiments, to shew the different Refrangibility of the Rays of Light, of the different Colours, Red, Orange, Yellow, Green, Blue, Indigo, Violet. Where DE is a Parallelogram of Pastboard, having the one half DG blue, and the other half FE red; both strongly illuminated by the same Candle: and having black Silk wrapped several times round it. MN is a Lens or double Convex Glass interpos'd, which gathers upon white Paper the blue Rays sooner at hi than the Red at HI: As appears by the Distinctness of the Colours and of the Silk at those and only those Distances. Where also at somewhat above 12 Feet from the Colours to the Images, the Distance between hi and HI is no less than an Inch and half.

Fig.2. Is another of Sir Isaac's Experiments to the same Purpose: Where XY represents the Sun: EG, a Window, with a small round Hole at F: within which is a Triangular Glass Prism ABC, by which the Rays of the Sun are differently refracted upon a white Wall or Paper MN; and become an Oblong Image PT; the Violet seen at P as most refracted; and the Red at T, as least refracted: And the intermediate Colours seen in intermediate Places, according to the different Degrees of their Refraction.

Fig.3. Is another of his Experiments, to shew that White is a Mixture of all Sorts of colour'd Rays; where DC is a Hole in the Window, which admits the Sun's Rays. EFG a Prism, casting its oblong colour'd Image upon a Lens, or double Convex Glass; which collects all those Rays into its Focus. In which Case, the Point of Concourse exhibits a perfect White Colour; tho' upon their Separation again, the oblong colour'd Image appears again, only in an inverted Position: as the crossing of the Rays in the Focus must of Necessity occasion.

Fig.4. Is the last Experiment improv'd; by shewing that the White Light made by the Mixture of all the Colours is but imperfectly so, when any of the several Colours are intercepted in their Passage to their Focus, or Place of Mixture.

Fig.5. Is the Experimentum Crucis, or determining Experiment. Where BF is the Hole that lets in a large Ray of Light: whose middle Part, after it has pass'd through the Prism ABC, is let through a lesser Hole at G, and forms an oblong colour'd Image at de: where another small Hole lets thro' one Colour only; which passing through the Second Prism abc it is refracted again, and cast upon NM. And here it is most remarkable, that the two Holes and second Prism are kept immoveable; and so the Rays Gg fall upon the second Prism in the very same Angle, whatever Colour they are of, and that by the Motion of the first Prism, all the Colours may successfully pass through the same Holes. Yet is the Refraction by the second Prism never then able to produce any Variety of Colours; but exhibits the Image always of that Colour alone, which falls upon it before the second Refraction.

Fig.6. Is a Figure for the Explication of the several Refractions and Reflections of Light, which cause the PhÆnomena of the Rainbow. Thus if the greatest Crowd of Rays enter in Parallel to BQ along or near to AN, the round Drop of Water LBGQ will refract Part of those Rays to F, whence Part of them will be reflected to G: And going there out of the Drop, will be thereby refracted to R, which double Refraction will so separate the several Colours, and make them go out in Angles so sensibly different, that as the Eye is placed a little higher or lower, it will see a different Colour; and that in Angles as AXR, of about 41 Degrees; and this is the Case of the primary Rainbow, which appears in about that Angle from the Axis BQ, or its Parallel AX. Thus also, if the same Line AN be now suppos'd to represent another Drop, and that some of the Rays at G are reflected a second time, and so pass out at H, and are there refracted to S; here will be a weaker Impression, but a like Refraction and Separation of the Colours as before; and the Eye placed a little higher or lower will also see different Colours, tho' in a contrary Order to the former; and that in an Angle, as AYS, of about 52 Degrees and a half; which is the Case of the secondary Rainbow.

Fig.7. Are the two Rainbows themselves, r presented as they appear in Nature. Where AEBF represents the Air full of spherical Drops of Rain, in such Parts as the Angles EOP, FOP are about 41 Degrees from the Axis OP, which Axis is the Line from the Sun's Center, through the Eye of the Spectator, to the Center of the Rainbow: And where CGDH represents the same Air, full of the like Drops, in such Parts where the Angles GOP, HOP are about 52 Degr. and a half. Where also the Rays SE, SF, SG, SH, coming from the Sun's Center, are represented as parallel, by reason of its vast Distance. These Rays, when they fall upon the higher Quadrant of the Drop, as at SE, SF, come to the Eye at O in about an Angle of 41 Degrees, after two Refractions, and one Reflection; and so cause the primary Rainbow: the Red is without, by the least refrangible Rays at F: and the blue within, by the more refrangible Rays at E. But when they fall upon the lower Quadrant of the Drop, as at SG, SH, they come to the same Eye at O, but in an Angle of about 52 Degrees and a half, after two Refractions, and two Reflections, and so cause the secondary Rainbow. Which is Blue without, by the more refrangible Rays at H; and Red within by the least at G. Where note, that because the Angles FOP, EOP, as well as those HOP, GOP, are ever the same, the same Colours must still be circular, or appear in the Surface of a right Cone, whose Axis is OP, and whose Sides are the Lines turned round thereon, as OE OF, and OG OH.