[Schem. 6.]
Fig. 3.

And to this end, we will calculate any two Rays: as for instance; let EF be a Ray cutting the Radius CD (divided into 20. parts) in G 16. parts distant from C, and ef another Ray, which cuts the same Radius in g 17. parts distant, these will be refracted to K and k, and from thence reflected to N and n, and from thence refracted toward P and p; therefore the Arch Ff will be 5.d 5′. The Arch FK 106.d 30′. the Arch fk 101.d 2′. The line FG 6000. and fg 5267. therefore hf. 733. therefore Fc 980, almost. The line FK 16024. and fk 15436. therefore Nd 196. and no 147 almost, the line Nn 1019 the Arch Nn 5.d 51′. therefore the Angle Nno is 34.d 43′. therefore the Angle Non is 139.d 56′. which is almost 50.d more than a right Angle.

It is evident therefore by this Hypothesis, that at the same time that ef touches f. EF is arrived at c. And by that time efkn is got to n, EFKN is got to d and when it touches N, the pulse of the other Ray is got to o. and no farther, which is very short of the place it should have arriv’d to, to make the Ray np to cut the orbicular pulse No at right Angles: therefore the Angle Nop is an acute Angle, but the quite contrary of this will happen, if 17. and 18. be calculated in stead of 16. and 17. both which does most exactly agree with the Phænomena: For if the Sun, or a Candle (which is better) be placed about Ee, and the eye about Pp, the Rays EFef at 16. and 17. will paint the side of the luminous object toward np Blue, and towards NP Red. But the quite contrary will happen when EF is 17. and ef 18. for then towards NP shall be a Blue, and towards np a Red, exactly according to the calculation. And there appears the Blue of the Rainbow, where the two Blue sides of the two Images unite, and there the Red where the two Red sides unite, that is, where the two Images are just disappearing; which is, when the Rays EF and NP produc’d till they meet, make an Angle of about 41. and an half; the like union is there of the two Images in the Production of the Secundary Iris, and the same causes, as upon calculation may appear; onely with this difference, that it is somewhat more faint, by reason of the duplicate reflection, which does always weaken the impulse the oftner it is repeated.

Now, though the second refraction made at Nn be convenient, that is, do make the Rays glance the more, yet is it not altogether requisite; for it is plain from the calculation, that the pulse dn is sufficiently oblique to the Rays KN and kn, as wel as the pulse fc is oblique to the Rays FK & fk. And therefore if a piece of very fine Paper be held close against Nn and the eye look on it either through the Ball as from D, or from the other side, as from B. there shall appear a Rainbow, or colour’d line painted on it with the part toward X appearing Red, towards O, Blue; the same also shall happen, if the Paper be placed about Kk, for towards T shall appear a Red, and towards V a Blue, which does exactly agree with this my Hypothesis, as upon the calculation of the progress of the pulse will most easily appear.

Nor do these two observations of the colours appearing to the eye about p differing from what they appear on the Paper at N contradict each other; but rather confirm and exactly agree with one another, as will be evident to him that examines the reasons set down by the ingenious. Des Cartes in the 12. Sect. of the 8. Chapter of his Meteors, where he gives the true reason why the colours appear of a quite contrary order to the eye, to what they appear’d on the Paper if the eye be plac’d in steed of the Paper: And as in the Prisme, so also in the Water-drop, or Globe the Phænomena, and reason are much the same.

Having therefore shewn that there is such a propriety in the prisme and water Globule whereby the pulse is made oblique to the progressive, and that so much the more, by how much greater the refraction is, I shall in the next place consider, how this conduces to the production of colours, and what kind of impression it makes upon the bottom of the eye; and to this end it will be requisite to examine this Hypothesis a little more particularly.

First therefore, if we consider the manner of the progress of the pulse, it will seem rational to conclude, that that part or end of the pulse which precedes the other, must necessarily be somwhat more obtunded, or impeded by the resistance of the transparent medium, than the other part or end of it which is subsequent, whose way is, as it were, prepared by the other; especially if the adjacent medium be not in the same manner [Schem. 6.]
Fig. 4. enlightned or agitated. And therefore (in the fourth Figure of the sixth Iconism) the Ray AAAHB will have its side HH more deadened by the resistance of the dark or quiet medium PPP, Whence there will be a kind of deadness superinduc’d on the side HHH, which will continually increase from B, and strike deeper and deeper into the Ray by the line BR; Whence all the parts of the triangle, RBHO will be of a dead Blue colour, and so much the deeper, by how much the nearer they lie to the line BHH, which is most deaded or impeded, and so much the more dilute, by how much the nearer it approaches the line BR. Next on the other side of the Ray AAN, the end A of the pulse AH will be promoted, or made stronger, having its passage already prepar’d as ’twere by the other parts preceding, and so its impression wil be stronger; And because of its obliquity to the Ray, there will be propagated a kind of faint motion into QQ the adjacent dark or quiet medium, which faint motion will spread further and further into QQ as the Ray is propagated further and further from A, namely, as far as the line MA, whence all the triangle MAN will be ting’d with a Red, and that Red will be the deeper the nearer it approaches the line MA, and the paler or yellower the nearer it is the line NA. And if the Ray be continued, so that the lines AN and BR (which are the bounds of the Red and Blue diluted) do meet and cross each other, there will be beyond that intersection generated all kinds of Greens.

Now, these being the proprieties of every single refracted Ray of light, it will be easie enough to consider what must be the result of very many such Rays collateral: As if we suppose infinite such Rays interjacent between AKSB and ANOB, which are the terminating: For in this case the Ray AKSB will have its Red triangle intire, as lying next to the dark or quiet medium, but the other side of it BS will have no Blue, because the medium adjacent to it SBO, is mov’d or enlightned, and consequently that light does destroy the colour. So likewise will the Ray ANOB lose its Red, because the adjacent medium is mov’d or enlightned, but the other side of the Ray that is adjacent to the dark, namely, AHO will preserve its Blue entire, and these Rays must be so far produc’d as till AN and BR cut each other, before there will be any Green produc’d. From these Proprieties well consider’d, may be deduc’d the reasons of all the Phænomena of the prisme, and of the Globules or drops of Water which conduce to the production of the Rainbow.

[Schem. 6.]
Fig. 5.

Next for the impression they make on the Retina, we will further examine this Hypothesis: Suppose therefore ABCDEF, in the fifth Figure, to represent the Ball of the eye: on the Cornea of which ABC two Rays GACH and KCAI (which are the terminating Rays of a luminous body) falling, are by the refraction thereof collected or converg’d into two points at the bottom of the eye. Now, because these terminating Rays, and all the intermediate ones which come from any part of the luminous body, are suppos’d by some sufficient refraction before they enter the eye, to have their pulses made oblique to their progression, and consequently each Ray to have potentially superinduc’d two proprieties, or colours, viz. a Red on the one side, and a Blue on the other, which notwithstanding are never actually manifest, but when this or that Ray has the one or the other side of it bordering on a dark or unmov’d medium, therefore as soon as these Rays are entred into the eye and so have one side of each of them bordering on a dark part of the humours of the eye, they will each of them actually exhibit some colour; therefore ADC the production of GACH will exhibit a Blue, because the side CD is adjacent to the dark medium CQDC, but nothing of a Red, because its side AD is adjacent to the enlightned medium ADFA: And all the Rays that from the points of the luminous body are collected on the parts of the Retina between D and F shall have their Blue so much the more diluted by how much the farther these points of collection are distant from D towards F; and the Ray AFC the production of KCAI, will exhibit a Red, because the side AF is adjacent to the dark or quiet medium of the eye APFA, but nothing of a Blue, because its side CF is adjacent to the enlightned medium CFDC, and all the Rays from the intermediate parts of the luminous body that are collected between F and D shall have their Red so much the more diluted, by how much the farther they are distant from F towards D.