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 more for the burning of _White_ Bodies, than for the burning of _Black_ ones: and so observing the immediate proportions between these two, for burning bodies of _other_ Colors. Whence (he tells us) he hath drawn some consequences, touching the distance, at which we may hope, to burn Bodies here, by the means of _great Glasses_ and great _Looking-glasses_. So that (_saith he_) we must yet be seven times neerer the _Sun_, than we are, to be in danger of being burned by it. Where he mentions, that having given _Instructions_ to certain persons, gon to travel in _Hot Countries_, he hath among other particulars recommended to them, to try by means of great _Burning-glasses_, with how much less _Aperture_ they will burn _there_, than _here_, to know from thence, whether there by more Light _there_ than _here_, and how much; since this perhaps may be the only means of trying it, supposing, the same matters be used: although the difference of the Air already heated, both in _hot Countries_, and in the _Planets_, that are neerer than we, may alter, if not the quantity of Light, at least that of the Heat, found there. * * * * * _A further Account, touching Signor _Campani_'s Book and Performances about _Optick-glasses_._