These Phaenomena, being so various, and so truly admirable, it will certainly be very well worth our inquiry, to examine the causes and reasons of them, and to consider, whether from these causes demonstratively evidenced, may not be deduced the true causes of the production of all kind of Colours.  And I the rather now do it, instead of an Appendix or Digression to this History, then upon the occasion of examining the Colours in Peacocks, or other Feathers, because this Subject, as it does afford more variety of particular Colours, so does it afford much better wayes of examining each circumstance.  And this will be made manifest to him that considers, first, that this laminated body is more simple and regular then the parts of Peacocks feathers, this consisting only of an indefinite number of plain and smooth Plates, heaped up, or incumbent on each other.  Next, that the parts of this body are much more manageable, to be divided or joyned, then the parts of a Peacocks feather, or any other substance that I know.  And thirdly, because that in this we are able from a colourless body to produce several coloured bodies, affording all the variety of Colours imaginable:  And several others, which the subsequent Inquiry will make manifest.

To begin therefore, it is manifest from several circumstances, that the material cause of the apparition of these several Colours, is some Lamina or Plate of a transparent or pellucid body of a thickness very determinate and proportioned according to the greater or less refractive power of the pellucid body.  And that this is so, abundance of Instances and particular Circumstances will make manifest.

As first, if you take any small piece of the Muscovy-glass, and with a Needle, or some other convenient Instrument, cleave it oftentimes into thinner and thinner Laminae, you shall find, that till you come to a determinate thinness of them, they shall all appear transparent and colourless, but if you continue to split and divide them further, you shall find at last, that each Plate, after it comes to such a determinate thickness, shall appear most lovely ting’d or imbued with a determinate colour.  If further, by any means you so flaw a pretty thick piece, that one part does begin to cleave a little from the other, and between those two there be by any means gotten some pellucid medium, those laminated pellucid bodies that fill that space, shall exhibit several Rainbows or coloured Lines, the colours of which will be disposed and ranged according to the various thicknesses of the several parts of that Plate.  That this is so, is yet further confirmed by this Experiment.