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._