Now, because by the refraction in the Cornea, and some other parts of the eye, the sides of each Ray, which before were almost parallel, are made to converge and meet in a point at the bottom of the eye, therefore that side of the pulse which preceded before these refractions, shall first touch the Retina, and the other side last. And therefore according as this or that side, or end of the pulse shall be impeded, accordingly will the impressions on the Retina be varied; therefore by the Ray GACH refracted by the Cornea to D there shall be on that point a stroke or impression confus’d, whose weakest end, namely, that by the line CD shall precede, and the stronger, namely, that by the line AD shall follow. And by the Ray KCAI refracted to F, there shall be on that part a confus’d stroke or impression, whose strongest part, namely, that by the line CF shal precede, and whose weakest or impeded, namely, that by the line AF shall follow, and all the intermediate points between F and D will receive impressions from the converg’d Rays so much the more like the impressions on F and D by how much the nearer they approach that or this.

From the consideration of the proprieties of which impressions, we may collect these short definitions of Colours: That Blue is an impression on the Retina of an oblique and confus’d pulse of light, whose weakest part precedes, and whose strongest follows. And, that Red is an impression on the Retina of an oblique and confus’d pulse of light, whose strongest part precedes, and whose weakest follows.

Which proprieties, as they have been already manifested, in the Prisme and falling drops of Rain, to be the causes of the colours there generated, may be easily found to be the efficients also of the colours appearing in thin laminated transparent bodies; for the explication of which, all this has been premised.

And that this is so, a little closer examination of the Phænomena and the Figure of the body, by this Hypothesis will make evident.

For first (as we have already observed) the laminated body must be of a determinate thickness, that is, it must not be thinner then such a determinate quantity; for I have always observ’d, that neer the edges of those which are exceeding thin, the colours disappear, and the part grows white; nor must it be thicker then another determinate quantity; for I have likewise observ’d, that beyond such a thickness, no colours appear’d, but the Plate looked white, between which two determinate thicknesses were all the colour’d Rings; of which in some substances I have found ten or twelve, in others not half so many, which I suppose depends much upon the transparency of the laminated body. Thus though the consecutions are the same in the scum or the skin on the top of metals; yet in those consecutions in the same colour is not so often repeated as in the consecutions in thin Glass, or in Sope-water, or any other more transparent and glutinous liquor; for in these I have observ’d, Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, &c. to succeed each other, ten or twelve times, but in the other more opacous bodies the consecutions will not be half so many.

And therefore secondly, the laminated body must be transparent, and this I argue from this, that I have not been able to produce any colour at all with an opacous body, though never so thin. And this I have often try’d, by pressing small Globule of Mercury between two smooth Plates of Glass, whereby I have reduc’d that body to a much greater thinness then was requisite to exhibit the colours with a transparent body.

Thirdly, there must be a considerable reflecting body adjacent to the under or further side of the lamina or plate: for this I always found, that the greater that reflection was, the more vivid were the appearing colours.

From which Observations, is most evident, that the reflection from the under or further side of the body is the principal cause of the production of these colours; which, that it is so, and how it conduces to that effect, I shall further explain in the following Figure, which is here described of a very great thickness, as if it had been view’d through the Microscope; and ’tis indeed much thicker than any Microscope (I have yet us’d) has been able to shew me those colour’d plates of Glass, or Muscovie-glass, which I have not without much trouble view’d with it, for though I have endeavoured to magnifie them as much as the Glasses were capable of, yet are they so exceeding thin, that I have not hitherto been able positively to determine their thickness. This Figure therefore I here represent, is wholy Hypothetical.

[Schem. 6.]
Fig. 6.

Let ABCDHFE in the sixth Figure be a frustum of Muscovy-glass, thinner toward the end AE, and thicker towards DF. Let us first suppose the Ray aghb coming from the Sun, of some remote luminous object to fall obliquely on the thinner plate BAE, part therefore is reflected back by cghd, the first Superficies; whereby the perpendicular pulse ab is after reflexion propagated by cd, cd, equally remote from each other with ab, ab, so that ag + gc, or bh + hd are either of them equal to aa, as is also cc, but the body BAE being transparent, a part of the light of this Ray is refracted in the surface AB, and propagated by gikh to the surface EF, whence it is reflected and refracted again by the surface AB. So that after two refractions and one reflection, there is propagated a kind of fainter Ray emnf, whose pulse is not only weaker by reason of the two refractions in the surface AB, but by reason of the time spent in passing and repassing between the two surfaces AB and EF, ef which is this fainter or weaker pulse comes behind the pulse cd; so that hereby (the surfaces AB, and EF being so neer together, that the eye cannot discriminate them from one) this confus’d or duplicated pulse, whose strongest part precedes, and whose weakest follows, does produce on the Retina, (or the optick nerve that covers the bottom of the eye) the sensation of a Yellow.