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.
Now fourthly, if the thicker Plate chance to be cleft into two thinner Plates, as CDFE is divided into two Plates by the surface GH then from the composition arising from the three reflections in the surfaces CD, GH, and EF, 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 CDHG, and GHFE are varied.
And fifthly, if these surfaces CD and FE are further remov’d asunder, the weaker pulse will yet lagg behind much further, and not onely be coincident with the second, cd, 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 cd 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 shall be generated several consecutions of colours, whose order from the thin end towards the thick, shall be Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green; Yellow, Red, Purple, Blue, Green; Yellow, &c. and these so often repeated, as the weaker pulse does lose paces with its Primary, or first pulse, and is coincident with a second, third, fourth, fifth, sixth, &c. pulse behind the first. And this, as it is coincident, or follows from the first Hypothesis I took of colours, so upon experiment have I found it in multitudes of instances that seem to prove it. One thing which seems of the greatest concern in this Hypothesis, is to determine the greatest or least thickness requisite for these effects, which, though I have not been wanting in attempting, yet so exceeding thin are these coloured Plates, and so imperfect our Microscope, that I have not been hitherto successfull, though if my endeavours shall answer my expectations, I shall hope to gratifie the curious Reader with some things more remov’d beyond our reach hitherto.
Thus have I, with as much brevity as I was able, endeavoured to explicate (Hypothetically at least) the causes of the Phænomena I formerly recited, on the consideration of which I have been the more particular.
First, because I think these I have newly given are capable of explicating all the Phænomena of colours, not onely of those appearing in the Prisme, Water-drop, or Rainbow, and in laminated or plated bodies, but of all that are in the world, whether they be fluid or solid bodies, whether in thick or thin, whether transparent, or seemingly opacous, as I shall in the next Observation further endeavour to shew. And secondly, because this being one of the two ornaments of all bodies discoverable by the sight, whether looked on with, or without a Microscope, it seem’d to deserve (somewhere in this Tract, which contains a description of the Figure and Colour of some minute bodies) to be somewhat the more intimately enquir’d into.
Observ. [X]. Of Metalline, and other real Colours.
Having in the former Discourse, from the Fundamental cause of Colour, made it probable, that there are but two Colours, and shewn, that the Phantasm of Colour is caus’d by the sensation of the oblique or uneven pulse of Light which is capable of no more varieties than two that arise from the two sides of the oblique pulse, though each of those be capable of infinite gradations or degrees (each of them beginning from White, and ending the one in the deepest Scarlet or Yellow, the other in the deepest Blue) I shall in this Section set down some Observations which I have made of other colours, such as Metalline powders tinging or colour’d bodies and several kinds of tinctures or ting’d liquors, all which, together with those I treated of in the former Observation will, I suppose, comprise the several subjects in which colour is observ’d to be inherent, and the several manners by which it inheres, or is apparent in them. And here I shall endeavour to shew by what composition all kind of compound colours are made, and how there is no colour in the world but may be made from the various degrees of these two colours, together with the intermixtures of Black and White.
And this being so, as I shall anon shew, it seems an evident argument to me, that all colours whatsoever, whether in fluid or solid, whether in very transparent or seemingly opacous, have the same efficient cause, to wit, some kind of refraction whereby the Rays that proceed from such bodies, have their pulse obliquated or confus’d in the manner I explicated in the former Section; that is, a Red is caus’d by a duplicated or confus’d pulse, whose strongest pulse precedes, and a weaker follows: and a Blue is caus’d by a confus’d pulse, where the weaker pulse precedes, and the stronger follows. And according as these are, more or less, or variously mixt and compounded, so are the sensations, and consequently the phantasms of colours diversified.