_The Cause of Reflexion is not the impinging of Light on the solid or
impervious parts of Bodies, as is commonly believed._
This will appear by the following Considerations. First, That in the
passage of Light out of Glass into Air there is a Reflexion as strong as
in its passage out of Air into Glass, or rather a little stronger, and
by many degrees stronger than in its passage out of Glass into Water.
And it seems not probable that Air should have more strongly reflecting
parts than Water or Glass. But if that should possibly be supposed, yet
it will avail nothing; for the Reflexion is as strong or stronger when
the Air is drawn away from the Glass, (suppose by the Air-Pump invented
by _Otto Gueriet_, and improved and made useful by Mr. _Boyle_) as when
it is adjacent to it. Secondly, If Light in its passage out of Glass
into Air be incident more obliquely than at an Angle of 40 or 41 Degrees
it is wholly reflected, if less obliquely it is in great measure
transmitted. Now it is not to be imagined that Light at one degree of
obliquity should meet with Pores enough in the Air to transmit the
greater part of it, and at another degree of obliquity should meet with
nothing but parts to reflect it wholly, especially considering that in
its passage out of Air into Glass, how oblique soever be its Incidence,
it finds Pores enough in the Glass to transmit a great part of it. If
any Man suppose that it is not reflected by the Air, but by the outmost
superficial parts of the Glass, there is still the same difficulty:
Besides, that such a Supposition is unintelligible, and will also appear
to be false by applying Water behind some part of the Glass instead of
Air. For so in a convenient obliquity of the Rays, suppose of 45 or 46
Degrees, at which they are all reflected where the Air is adjacent to
the Glass, they shall be in great measure transmitted where the Water is
adjacent to it; which argues, that their Reflexion or Transmission
depends on the constitution of the Air and Water behind the Glass, and
not on the striking of the Rays upon the parts of the Glass. Thirdly,
If the Colours made by a Prism placed at the entrance of a Beam of Light
into a darken'd Room be successively cast on a second Prism placed at a
greater distance from the former, in such manner that they are all alike
incident upon it, the second Prism may be so inclined to the incident
Rays, that those which are of a blue Colour shall be all reflected by
it, and yet those of a red Colour pretty copiously transmitted. Now if
the Reflexion be caused by the parts of Air or Glass, I would ask, why
at the same Obliquity of Incidence the blue should wholly impinge on
those parts, so as to be all reflected, and yet the red find Pores
enough to be in a great measure transmitted. Fourthly, Where two Glasses
touch one another, there is no sensible Reflexion, as was declared in
the first Observation; and yet I see no reason why the Rays should not
impinge on the parts of Glass, as much when contiguous to other Glass as
when contiguous to Air. Fifthly, When the top of a Water-Bubble (in the
17th Observation,) by the continual subsiding and exhaling of the Water
grew very thin, there was such a little and almost insensible quantity
of Light reflected from it, that it appeared intensely black; whereas
round about that black Spot, where the Water was thicker, the Reflexion
was so strong as to make the Water seem very white. Nor is it only at
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.
Now if Light be reflected, not by impinging on the solid parts of
Bodies, but by some other principle; it's probable that as many of its
Rays as impinge on the solid parts of Bodies are not reflected but
stifled and lost in the Bodies. For otherwise we must allow two sorts of
Reflexions. Should all the Rays be reflected which impinge on the
internal parts of clear Water or Crystal, those Substances would rather
have a cloudy Colour than a clear Transparency. To make Bodies look
black, it's necessary that many Rays be stopp'd, retained, and lost in
them; and it seems not probable that any Rays can be stopp'd and
stifled in them which do not impinge on their parts.
And hence we may understand that Bodies are much more rare and porous
than is commonly believed. Water is nineteen times lighter, and by
consequence nineteen times rarer than Gold; and Gold is so rare as very
readily and without the least opposition to transmit the magnetick
Effluvia, and easily to admit Quicksilver into its Pores, and to let
Water pass through it. For a concave Sphere of Gold filled with Water,
and solder'd up, has, upon pressing the Sphere with great force, let the
Water squeeze through it, and stand all over its outside in multitudes
of small Drops, like Dew, without bursting or cracking the Body of the
Gold, as I have been inform'd by an Eye witness. From all which we may
conclude, that Gold has more Pores than solid parts, and by consequence
that Water has above forty times more Pores than Parts. And he that
shall find out an Hypothesis, by which Water may be so rare, and yet not
be capable of compression by force, may doubtless by the same Hypothesis
make Gold, and Water, and all other Bodies, as much rarer as he pleases;
so that Light may find a ready passage through transparent Substances.
The Magnet acts upon Iron through all dense Bodies not magnetick nor red
hot, without any diminution of its Virtue; as for instance, through
Gold, Silver, Lead, Glass, Water. The gravitating Power of the Sun is
transmitted through the vast Bodies of the Planets without any
diminution, so as to act upon all their parts to their very centers
with the same Force and according to the same Laws, as if the part upon
which it acts were not surrounded with the Body of the Planet, The Rays
of Light, whether they be very small Bodies projected, or only Motion or
Force propagated, are moved in right Lines; and whenever a Ray of Light
is by any Obstacle turned out of its rectilinear way, it will never
return into the same rectilinear way, unless perhaps by very great
accident. And yet Light is transmitted through pellucid solid Bodies in
right Lines to very great distances. How Bodies can have a sufficient
quantity of Pores for producing these Effects is very difficult to
conceive, but perhaps not altogether impossible. For the Colours of
Bodies arise from the Magnitudes of the Particles which reflect them, as
was explained above. Now if we conceive these Particles of Bodies to be
so disposed amongst themselves, that the Intervals or empty Spaces
between them may be equal in magnitude to them all; and that these
Particles may be composed of other Particles much smaller, which have as
much empty Space between them as equals all the Magnitudes of these
smaller Particles: And that in like manner these smaller Particles are
again composed of others much smaller, all which together are equal to
all the Pores or empty Spaces between them; and so on perpetually till
you come to solid Particles, such as have no Pores or empty Spaces
within them: And if in any gross Body there be, for instance, three such
degrees of Particles, the least of which are solid; this Body will have
seven times more Pores than solid Parts. But if there be four such
degrees of Particles, the least of which are solid, the Body will have
fifteen times more Pores than solid Parts. If there be five degrees, the
Body will have one and thirty times more Pores than solid Parts. If six
degrees, the Body will have sixty and three times more Pores than solid
Parts. And so on perpetually. And there are other ways of conceiving how
Bodies may be exceeding porous. But what is really their inward Frame is
not yet known to us.