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.

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.

And secondly, this Yellow will appear so much the deeper, by how much the further back towards the middle between cd and cd the spurious pulse ef is remov’d, as in 2 where the surface BC being further remov’d from EF, the weaker pulse ef will be nearer to the middle, and will make an impression on the eye of a Red.

But thirdly, if the two reflecting surfaces be yet further remov’d asunder (as in 3 CD and EF are) then will the weaker pulse be so farr behind, that it will be more then half the distance between cd and cd.  And in this case it will rather seem to precede the following stronger pulse, then to follow the preceding one, and consequently a Blue will be generated.  And when the weaker pulse is just in the middle between two strong ones, then is a deep and lovely Purple generated; but when the weaker pulse ef is very neer to cd, then is there generated a Green, which will be bluer, or yellower, according as the approximate weak pulse does precede or follow the stronger.