Page:Philosophical Transactions - Volume 001.djvu/73

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His Theory of Apertures, though he seems to think it very authentick, yet to me it seems not so cleer. 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. 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 tor 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