Of a Means to Illuminate an Object in What Proportion One Pleaseth; And of the Distances Requisite to Burn Bodies by the Sun

His Theory of Apertures, though he seems to think it very authentick, yet to me it seems not so clear. For, the same Glass will endure greater or lesser Apertures, according to the lesser or greater Light of the Object: If it be for the looking on the Sun or Venus, or for seeing the Diameters of the Fix'd Stars, then smaller Apertures do better; if for the Moon in the daylight, or on Saturn, or Jupiter, or Mars, then the largest. Thus I have often made use of a 12 foot-Glass to look on 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. Auxout seems to find fault with the English Glasses of 36 foot, that had an Aperture of but 2½ 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. Auxout to enlighten an Object, in what proportion one pleaseth, is by some great Object-Glasses, 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 (faith 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 clear 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 Treatise. 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 near 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 nearer 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 be 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 nearer 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. In the above-mentioned French Tract there is also contained M. Auzout's Opinion of what he had found New in the Treatise of Signor Campani, which was spoken of in the first Papers of their Transactions, concerning both the Effect of the Telescope contrived after a peculiar way by the said Campani at Rome, and his