on which produce the
colours.

From the phenomena thus briefly described, Newton deduced that
ingenious, though hypothetical, property of light called its "fits of
easy reflection and transmission." This property consists in supposing
that every particle of light from its first discharge from a luminous
body possesses, at equally distant intervals, dispositions to be
reflected from, and transmitted through, the surfaces of the bodies upon
which it is incident. Hence, if a particle of light reaches a reflecting
surface of glass _when in its fit of easy reflection_, or in its
disposition to be reflected, it will yield more readily to the
reflecting force of the surface; and, on the contrary, if it reaches the
same surface _while in a fit of easy transmission_, or in a disposition
to be transmitted, it will yield with more difficulty to the reflecting
force.

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 fall