Page:Micrographia - or some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon.djvu/120

Rh pulse a b is after reflexion propagated by c d, c d, equally remote from each other with a b, a b, so that a g + g c, or b h + h d are either of them equal to a a, as is also c c, but the body B A E being transparent, a part of the light of this Ray is refracted in the surface A B, and propagated by g i k h to the surface E F, whence it is reflected and refracted again by the surface A B. So that after two refractions and one reflection, there is propagated a kind of fainter Ray e m n f, whose pulse is not only weaker by reason of the two refractions in the surface A B, but by reason of the time spent in passing and repassing between the two surfaces A B and E F, e f which is this fainter or weaker pulse comes behind the pulse c d; so that hereby (the surfaces A B, and E F 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 e f is remov'd, as in 2 where the surface B C being further remov'd from E F, the weaker pulse e f 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 C D and E F are) then will the weaker pulse be so farr behind, that it will be more then half the distance between c d and c d. 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 e f is very neer to c d, then is there generated a Green, which will be bluer, or yellower, according as the approximate weak pulse does precede or follow the stronger.

Now fourthly, if the thicker Plate chance to be cleft into two thinner Plates, as C D F E is divided into two Plates by the surface G H then from the composition arising from the three reflections in the surfaces C D, G H, and E F, there will be generated several compounded or mixt colours, which will be very differing, according as the proportion between the thicknesses of those two divided Plates C D H G, and G H F E are varied.

And fifthly, if these surfaces C D and F E are further remov'd asunder, the weaker pulse will yet lagg behind much further, and not onely be coincident with the second, c d, but lagg behind that also, and that so much the more, by how much the thicker the Plate be; so that by degrees it will be coincident with the third c d backward also, and by degrees, as the Plate grows thicker with a fourth, and so onward to a fifth, sixth, seventh, or eighth; so that if there be a thin transparent body, that from the greatest thinness requisite to produce colours, does, in the manner of a Wedge, by degrees grow to the greatest thickness that a Plate can be of, to exhibit a colour by the reflection of Light from such a body, there Rh