Amidst these Thoughts I was forc'd from _Cambridge_ by the Intervening Plague, and it was more than two Years before I proceeded further. But then having thought on a tender way of polishing, proper for Metal, whereby, as I imagin'd, the Figure also would be corrected to the last; I began to try what might be effected in this kind, and by degrees so far perfected an Instrument (in the essential parts of it like that I sent to _London_,) by which I could discern _Jupiter_'s four Concomitants, and shew'd them divers times to two others of my Acquaintance. I could also discern the Moon-like Phase of _Venus_, but not very distinctly, nor without some niceness in disposing the Instrument. From that time I was interrupted, till this last Autumn, when I made the other. And as that was sensibly better than the first (especially for Day-Objects,) so I doubt not but they will be still brought to a much greater perfection by their Endeavours, who, as you inform me, are taking care about it at _London_. I have sometimes thought to make a _Microscope_, which in like manner should have, instead of an Object-glass, a reflecting piece of Metal. And this I hope they will also take into Consideration: For those Instruments seem as capable of improvement as _Telescopes_, and perhaps more, because but one reflective piece of Metal is requisite in them, as you may perceive in _Plate 3. Fig. 1._ where _AB_ representeth the Object Metal, _CD_ the Eye-glass, _F_ their common Focus, and _O_ the other Focus of the Metal, in which the Object is placed. But to return from this digression, I told you, that Light is not similar, or homogeneal, but consists of _difform_ Rays, some of which are more refrangible than others: So that of those, which are alike incident on the same Medium, some shall be more refracted than others, and that not by any virtue of the Glass, or other external Cause, but from a predisposition, which every particular Ray hath to suffer a particular degree of Refraction. I shall now proceed to acquaint you with another more notable deformity in its Rays, wherein the _Origin of Colours_ is unfolded: Concerning which I shall lay down the _Doctrine_ first, and then, for its Examination, give you an Instance or two of the _Experiments_, as a Specimen of the rest. The Doctrine you will find comprehended and illustrated in the following Propositions. 1. As the Rays of Light differ in degrees of Refrangibility, so they also differ in their disposition to exhibit this or that particular Colour. Colours are not _Qualifications of Light_, derived from Refractions, or Reflections of natural Bodies, (as 'tis generally believed) but _Original_ and _connate Properties_, which in divers Rays are divers. Some Rays are disposed to exhibit a red Colour and no other; some a yellow and no other, some a green and no other, and so of the rest. Nor are there only Rays proper and particular to the more eminent Colours, but even to all their intermediate Gradations.