The application of the theory of alternate fits of transmission and reflection to explain the colours of thin plates is very simple.

Transparency, opacity and colour were explained by Newton on the following principles.

Bodies that have the greatest refractive powers reflect the greatest quantity of light from their surfaces, and at the confines of equally refracting media there is no reflection.

The least parts of almost all natural bodies are in some measure transparent.

Between the parts of opaque and coloured bodies are many spaces, or pores, either empty or filled with media of other densities.

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.

In the year 1666, Newton was sitting in his garden at Woolsthorpe, reflecting on the nature of gravity, that remarkable power which causes all bodies to descend towards the centre of the earth.  As this power does not sensibly diminish at the greatest height we can reach he conceived it possible that it might reach to the moon and affect its motion, and even hold it in its orbit.  At such a distance, however, he considered some diminution of the force probable, and in order to estimate the diminution, he supposed that the primary planets were carried round the sun by the same force.  On this assumption, by comparing the periods of the different planets with their distances from the sun, he found that the force must decrease as the squares of the distances from the sun.  In drawing this conclusion he supposed the planets to move in circular orbits round the sun.