the least thickness of thin Plates or Bubbles, that there is no manifest
Reflexion, but at many other thicknesses continually greater and
greater. For in the 15th Observation the Rays of the same Colour were by
turns transmitted at one thickness, and reflected at another thickness,
for an indeterminate number of Successions. And yet in the Superficies
of the thinned Body, where it is of any one thickness, there are as many
parts for the Rays to impinge on, as where it is of any other thickness.
Sixthly, If Reflexion were caused by the parts of reflecting Bodies, it
would be impossible for thin Plates or Bubbles, at one and the same
place, to reflect the Rays of one Colour, and transmit those of another,
as they do according to the 13th and 15th Observations. For it is not to
be imagined that at one place the Rays which, for instance, exhibit a
blue Colour, should have the fortune to dash upon the parts, and those
which exhibit a red to hit upon the Pores of the Body; and then at
another place, where the Body is either a little thicker or a little
thinner, that on the contrary the blue should hit upon its pores, and
the red upon its parts. Lastly, Were the Rays of Light reflected by
impinging on the solid parts of Bodies, their Reflexions from polish'd
Bodies could not be so regular as they are. For in polishing Glass with
Sand, Putty, or Tripoly, it is not to be imagined that those Substances
can, by grating and fretting the Glass, bring all its least Particles to
an accurate Polish; so that all their Surfaces shall be truly plain or
truly spherical, and look all the same way, so as together to compose
one even Surface. The smaller the Particles of those Substances are, the
smaller will be the Scratches by which they continually fret and wear
away the Glass until it be polish'd; but be they never so small they can
wear away the Glass no otherwise than by grating and scratching it, and
breaking the Protuberances; and therefore polish it no otherwise than by
bringing its roughness to a very fine Grain, so that the Scratches and
Frettings of the Surface become too small to be visible. And therefore
if Light were reflected by impinging upon the solid parts of the Glass,
it would be scatter'd as much by the most polish'd Glass as by the
roughest. So then it remains a Problem, how Glass polish'd by fretting
Substances can reflect Light so regularly as it does. And this Problem
is scarce otherwise to be solved, than by saying, that the Reflexion of
a Ray is effected, not by a single point of the reflecting Body, but by
some power of the Body which is evenly diffused all over its Surface,
and by which it acts upon the Ray without immediate Contact. For that
the parts of Bodies do act upon Light at a distance shall be shewn
hereafter.
150