inflection. As not to mention those I have observ'd in
_Horn_, _Tortoise-Shell_, _transparent Gums_, and _resinous Substances_:
The _veins_ of Glass, nay, of melted _Crystal_, found, and much complained
of by Glass-grinders, and others, might sufficiently demonstrate the truth
of it to any diligent Observator.

But that, I presume, I have by this Example given proof sufficient (_viz.
ocular demonstration_) to evince, that there is such a modulation, or
bending of the rayes of light, as I have call'd _inflection_, differing
both from _reflection_, and _refraction_ (since they are both made in the
superficies, this only in the middle); and likewise, that this is able or
sufficient to produce the effects I have ascribed to it.

It remains therefore to shew, that there is such a property in the Air, and
that it is sufficient to produce all the above mentioned _Phaenomena_, and
therefore may be the principal, if not the only cause of them.

First, That there is such a property, may be proved from this, that the
parts of the Air are some of them more condens'd, others more rarified,
either by the differing heat, or differing pressure it sustains, or by the
somewhat heterogeneous vapours interspers'd through it. For as the Air is
more or less rarified, so does it more or less refract a ray of light (that
comes out of a denser medium) from the perpendicular. This you may find
true, if you make tryal of this Experiment.

Take a small Glass-bubble, made in the form of that in the second Figure of
the 37. _Scheme_, and by heating the Glass very hot, and thereby very much
rarifying the included Air, or, which is better, by rarifying a small
quantity of water, included in it, into vapours, which will expel the most
part, if not all the Air, and then sealing up the small neck of it, and
letting it cool, you may find, if you place it in a convenient Instrument,
that there will be a manifest difference, as to the refraction.

As if in this second Figure you suppose A to represent a small sight or
hole, through which the eye looks upon an object, as C, through the
Glass-bubble B, and the second sight L; all which remain exactly fixt in
their several places, the object C being so cized and placed, that it may
just seem to touch the upper and under edge of the hole L: and so all of it
be seen through the small Glass-ball of rarified Air; then by breaking off
the small seal'd neck of the Bubble (without at all stirring the sights,
objec