_pupil_ of the eye, which though it appear, and be therefore
judged very large, is yet not above a quarter of the bigness it appears of,
by the _lenticular_ refraction of the _Cornea_.

The cause of all which _Phaenomena_ I imagine to be no other then this,
That the Parts of the Glass being by the excessive heat of the fire kept
off and separated one from another, and thereby put into a kind of sluggish
fluid consistence, are suffered to drop off with that heat or agitation
remaining in them, into cold Water; by which means the outsides of the drop
are presently cool'd and _crusted_, and are thereby made of a loose
texture, because the parts of it have not time to settle themselves
leisurely together, and so to lie very close together: And the innermost
parts of the drop, retaining still much of their former heat and
agitations, remain of a loose texture also, and, according as the cold
strikes inwards from the bottom and sides, are quenched, as it were, and
made rigid in that very posture wherein the cold finds them. For the parts
of the _crust_ being already hardened, will not suffer the parts to shrink
any more from the outward Surface inward; and though it shrink a little by
reason of the small parcels of some Aerial substances dispersed through
the matter of the Glass, yet that is not neer so much as it appears (as I
just now hinted;) nor if it were, would it be sufficient for to consolidate
and condense the body of Glass into a _tuff_ and close _texture_, after it
had been so excessively rarified by the heat of the glass-Furnace.

But that there may be such an expansion of the aerial substance contained
in those little _blebbs_ or bubbles in the body of the drop, this following
Experiment will make more evident.

* * * * *

Take a small Glass-Cane about a foot long, seal up one end of it
_hermetically_, then put in a very small bubble of Glass, almost of the
shape of an Essence-viol with the open mouth towards the sealed end, then
draw out the other end of the Pipe very small, and fill the whole Cylinder
with water, then set this Tube by the Fire till the Water begin to boyl,
and the Air in the bubble be in good part rarified and driven out, then by
sucking at the smalling Pipe, more of the Air or vapours in the bubble may
be suck'd out, so that it may sink to the bottom; when it is sunk to the
bottom, in the flame of a Candle, or Lamp, nip up the slender Pipe and let
it cool: wher