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 _Laminæ_, 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.

Take two small pieces of ground and polisht Looking-glass-plate, each about
the bigness of a shilling, take these two dry, and with your fore-fingers
and thumbs press them very hard and close together, and you shall find,
that when they approach each other very near, there will appear several
_Irises_ or coloured Lines, in the same manner almost as in the
_Muscovy-glass_; and you may very easily change any of the Colours of any
part of the interposed body, by pressing the Plates closer and harder
together, or leaving them more lax; that is, a part which appeared coloured