Saturn_ with an _Aperture_ of almost 3 inches, and with a single
Eye-glass of 2 inches _double convex_: but, when with the same Glass I
looked on the _Sun_ or _Venus_, I used both a smaller _Aperture_, and
shallower _Charge_. And though M. _Auzout_ seems to find fault with the
_English_ Glass of 36 foot, that had an _Aperture_ of but 23/4 inches
_French_; as also, with a 60 foot _Tube_, used but with an _Aperture_ of 3
inches; yet I do not find, that he hath seen Glasses of that length, that
would bear greater _Apertures_, and 'tis not impossible, but his _Theory_
of _Apertures_ may fail in longer Glasses.

* * * * *

_Of a means to illuminate an Object in what proportion one pleaseth; and of
the Distances requisite to burn Bodies by the _Sun_._

One of the means used by M. _Auzout_ to enlighten an Object, in what
proportion one pleaseth, is by some great _Object-Glass_, by him called a
_Planetary_ one, because that by it he shews the difference of Light, which
all the _Planets_ receive from the _Sun_, by making use of several
_Apertures_, proportionate to their distance from the _Sun_, provided that
for every 9 foot draught, or thereabout, one inch of _Aperture_ be given
for the _Earth_. Doing this, one sees (_saith he_) that the Light which
_Mercury_ receives, is far enough from being able to burn Bodies, and yet
that the same Light is great enough in _Saturn_ to see cleer there, seeing
that (to him) it appears greater in _Saturn_, than it doth upon our
_Earth_, when it is overcast with Clouds: Which (he adds) would scarce be
believed, if by means of this Glass it did not sensibly appear so; Whereof
he promises to discourse more fully in his {70} _Treatise of the usefulness
of great Optick-Glasses_, where he also intends to deliver several
Experiments, by him made, 1. Touching the quantity of Light, which a Body,
that is 10, 15 and 20 times, &c. remoter than _Saturn_, would yet receive
from the _Sun_. 2. Touching the quantity of Light, by which the _Earth_ is
illuminated even in the _Eclipses_ of the _Sun_, in proportion of their
bigness. 3. Touching the quantity of Light, which is necessary to burn
Bodies: he having found, that not abating the Light, which is reflected by
the Surfaces of the Glass (whereof he confesseth, he doth not yet exactly
know the quantity) there would be necessary about 50 times as much Light,
as we have here, for the burning of _Black_ Bodies; and neer 9 times m