though every true Step made in this Philosophy brings us not immediately
to the Knowledge of the first Cause, yet it brings us nearer to it, and
on that account is to be highly valued.
_Qu._ 29. Are not the Rays of Light very small Bodies emitted from
shining Substances? For such Bodies will pass through uniform Mediums in
right Lines without bending into the Shadow, which is the Nature of the
Rays of Light. They will also be capable of several Properties, and be
able to conserve their Properties unchanged in passing through several
Mediums, which is another Condition of the Rays of Light. Pellucid
Substances act upon the Rays of Light at a distance in refracting,
reflecting, and inflecting them, and the Rays mutually agitate the Parts
of those Substances at a distance for heating them; and this Action and
Re-action at a distance very much resembles an attractive Force between
Bodies. If Refraction be perform'd by Attraction of the Rays, the Sines
of Incidence must be to the Sines of Refraction in a given Proportion,
as we shew'd in our Principles of Philosophy: And this Rule is true by
Experience. The Rays of Light in going out of Glass into a _Vacuum_, are
bent towards the Glass; and if they fall too obliquely on the _Vacuum_,
they are bent backwards into the Glass, and totally reflected; and this
Reflexion cannot be ascribed to the Resistance of an absolute _Vacuum_,
but must be caused by the Power of the Glass attracting the Rays at
their going out of it into the _Vacuum_, and bringing them back. For if
the farther Surface of the Glass be moisten'd with Water or clear Oil,
or liquid and clear Honey, the Rays which would otherwise be reflected
will go into the Water, Oil, or Honey; and therefore are not reflected
before they arrive at the farther Surface of the Glass, and begin to go
out of it. If they go out of it into the Water, Oil, or Honey, they go
on, because the Attraction of the Glass is almost balanced and rendered
ineffectual by the contrary Attraction of the Liquor. But if they go out
of it into a _Vacuum_ which has no Attraction to balance that of the
Glass, the Attraction of the Glass either bends and refracts them, or
brings them back and reflects them. And this is still more evident by
laying together two Prisms of Glass, or two Object-glasses of very long
Telescopes, the one plane, the other a little convex, and so compressing
them that they do not fully touch, nor are too far asunder. For the
Light which falls upon the farther Surface of the first Glass where the
Interval between the Glasses is not above the ten hundred thousandth
Part of an Inch, will go through that Surface, and through the Air or
_Vacuum_ between the Glasses, and enter into the second Glass, as was
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