of the
former, or some other Wheel) to run round very swift also, by which two
cross motions the Glass cannot chuse (if care be us'd) but be wrought
into a most exactly spherical Surface.

But because we are certain, from the _Laws of refraction_ (which I I have
experimentally found to be so, by an Instrument I shall presently describe)
that _the lines of the angles of Incidence are proportionate to the lines
of the angles of Refraction_, therefore if Glasses could be made of those
kind of Figures, or some other, such as the most incomparable _Des Cartes_
has invented, and demonstrated in his Philosophical and Mathematical Works,
we might hope for a much greater perfection of Opticks then can be
rationally expected from spherical ones; for though, _caeteris paribus_, we
find, that the larger the _Telescope_ Object Glasses are, and the shorter
those of the _Microscope_, the better they magnify, yet both of them,
beside such determinate dimensions, are by certain inconveniences rendred
unuseful; for it will be exceeding _difficult_ to make and _manage_ a Tube
above an _hundred foot long_, and it will be as difficult to _inlighten_ an
Object less then an hundred part of an inch distant from the Object Glass.

I have not as yet made any attempts of that kind, though I know two or
three wayes, which, as far as I have yet considered, seem very probable,
and may invite me to make a tryal as soon as I have an opportunity, of
which I may hereafter perhaps acquaint the world. In the Interim, I shall
describe the Instrument I even now mention'd, by which the _refraction_ of
all kinds of Liquors may be most exactly measur'd, thereby to give the
curious an opportunity of making what further tryals of that kind they
shall think requisite to any of their intended tryals; and to let them see
that the laws of Refraction are not only notional.

The Instrument consisted of five Rulers, or long pieces placed
together, after the manner exprest in the second Figure of the first
_Scheme_, where AB denotes a straight piece of wood about six foot and
two inches long, about three inches over, and an inch and half thick,
on the back side of which was hung a small plummet by a line stretcht
from top to bottom, by which this piece was set exactly upright, and so
very firmly fixt; in the middle of this was made a hole or center, into
which one end of a hollow cylindrical brass Box CC, fashion'd as I