8. But that _from all lucid_, shining and illuminate bodies, there is a _motion produced_ to the eye, and, through the eye, to the _optic_ nerve, and so into the _brain_, by which that apparition of _light_ or _colour_ is affected, is not hard to prove. And first, it is evident that the _fire_, the only lucid body here upon earth, worketh by _motion_ equally every way; insomuch as the motion thereof _stopped_ or inclosed, it is presently _extinguished_, and no more fire. And further, that that motion, whereby the fire worketh, is _dilation_, and _contraction_ of itself _alternately_, commonly called _scintillation_ or glowing, is manifest also by experience. From such _motion_ in the fire must needs arise a _rejection_ or casting from itself of that part of the _medium_ which is _contiguous_ to it, whereby that part also rejecteth the _next_, and so successively one part beateth back another to the very _eye_; and in the same manner the _exterior_ part of the eye presseth the _interior_, (the laws of refraction still observed). Now the interior coat of the eye is nothing else but a piece of the _optic_ nerve; and therefore the motion is still continued thereby into the _brain_, and by _resistance_ or reaction of the brain, is also a _rebound_ into the optic nerve again; which we _not conceiving_ as motion or rebound from _within_, do think it is _without_, and call it _light_; as hath been already shewed by the experience of a stroke. We have no reason to doubt, that the fountain of light, the _sun_, worketh by any other ways than the _fire_, at least in this matter. And thus all _vision_ hath its original from such _motion_ as is here described: for where there is no light, there is no sight; and therefore _colour_ also must be the same thing with _light_, as being the effect of the lucid bodies: their _difference_ being only this, that when the light cometh _directly_ from the fountain to the eye, or _indirectly_ by reflection from _clean_ and _polite_ bodies, and such as have _not_ any particular motion internal to alter it, we call it _light_; but when it cometh to the eye by reflection from _uneven_, _rough_, and coarse bodies, or such as are affected with internal motion of their own that may alter it, then we call it _colour_; colour and light differing only in this, that the one is _pure_, and the other _perturbed_ light. By that which hath been said, not only the truth of the third proposition, but also the whole manner of producing light and colour, is apparent. [Sidenote: The fourth proved.]