The parts of bodies and their interstices or pores must not be less than of some definite bigness to render them coloured.
The transparent parts of bodies, according to their several sizes, reflect rays of one colour, and transmit those of another on the same ground that thin plates do reflect or transmit these rays.
The parts of bodies on which their colour depend are denser than the medium which pervades their interstices.
The bigness of the component parts of natural bodies may be conjectured by their colours.
Transparency he considers as arising from the particles and their intervals, or pores, being too small to cause reflection at their common surfaces; so that all light which enters transparent bodies passes through them without any portion of it being turned from its path by reflexion.
Opacity, he thinks, arises from an opposite cause, viz., when the parts of bodies are of such a size to be capable of reflecting the light which falls upon them, in which case the light is "stopped or stifled" by the multitude of reflections.
The colours of natural bodies have, in the Newtonian hypothesis, the same origin as the colours of thin plates, their transparent particles, according to their several sizes, reflecting rays of one colour and transmitting those of another.
Among the optical discoveries of Newton those which he made on the inflection of light hold a high place. They were first published in his "Treatise on Optics," in 1707.
III--The Discovery of the Law of Gravitation
From the optical labours of Newton we now proceed to the history of his astronomical discoveries, those transcendent deductions of human reason by which he has secured to himself an immortal name, and vindicated the intellectual dignity of his species.