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_.