_PART III._
_Of the permanent Colours of natural Bodies, and the Analogy between
them and the Colours of thin transparent Plates._
I am now come to another part of this Design, which is to consider how
the Phaenomena of thin transparent Plates stand related to those of all
other natural Bodies. Of these Bodies I have already told you that they
appear of divers Colours, accordingly as they are disposed to reflect
most copiously the Rays originally endued with those Colours. But their
Constitutions, whereby they reflect some Rays more copiously than
others, remain to be discover'd; and these I shall endeavour to manifest
in the following Propositions.