An Intimation Given in the Journal des Scavans,of a Sure and Easie Way to Make All Sorts of Great Telescopical Glasses, Together with a Generous Offer of Furnishing Industrious Astronomers with Them

An Intimation given in the Journal des Scavans, of a sure and easy way to make all sorts of great Telescopical Glasses, together with a generous offer of furnishing industrious Astronomers with them. The Usefulness of great Glasses for Telescopes, and the care and pains hitherto taken to perfect this Invention is sufficiently known; but the difficulty of the work doth so much increase in great Glasses of that kind, that it hath not been surmounted hitherto. Monsieur Borelli, one of the Royal Academy of the Sciences of Paris, whose addiction to Natural Philosophy, and chiefly to Chymistry, hath been known long since, hath found out a sure and very easy method to work all sorts of such great Glasses, which hath never failed him. He hath already carried the Experience of his Secret to extraordinary bignesses, having made one of them very good of two hundred foot, wrought on both sides on the same rule: Which shews, that if he had wrought it flat on both sides, the glass would have been of four hundred foot. This easiness of making great Glasses, and the desire of procuring some advancement to Astronomical discoveries, have induced him to make presents of them in divers places to several persons capable to make use of them: And the same motive doth now invite him to make the like offer not only to the Astronomers that are dispersed up and down in the Kingdom of France, but also to those that are in foreign Countries, especially in those parts, where there is some established Academy or Society for Astronomical Observations; offering in this case to every one of such Societies three very good Glasses, one of ten or twelve foot for a Chamber; another of twenty five or thirty foot for ordinary observations, and a third of sixty or eighty foot, to make new discoveries with. Private persons that are not in a condition to make Engins for great Glasses, may, at least, make use of Glasses of fourteen or twenty foot, which he is willing to send them, therewith regularly to observe the Eclipses of the Satellites of Jupiter which happen almost every day, and afford so fair a way for establishing establishing the Longitudes over all the Earth. For, besides that these Eclipses are very frequent, the Emersion and Immersion of these Satellites, especially in the shadow of Jupiter, is so momentary and so sensible, that they may be observed with the greatest exactness, being altogether exempt from those essential inconveniences that accompany the Eclipses of the Sun and Moon, which also are rare, and whose beginning and end are always doubtful by reason of a certain ambiguous light. The Longitudes of places at Sea, Capes, Promontories, and divers Islands being once exactly known by this means, would doubtless be of great help and considerable usefulness to Navigation. Since Monsieur Borelli hath found this way of working Glasses, he entrusted the secret of it to a person of the Academy above-mentioned; and he purposeth to publish the same hereafter, with some other considerable Observations touching the same Glasses. A Letter from Liege concerning Mr. Newton's Experiment of the coloured Spectrum; together with some Exceptions against his Theory of Light and Colours. Hon'ld Sir, Mr. Gascoigne having received your obliging Letter of Jan. 18, with fresh directions from Mr. Newton; but wanting convenience to make the Experiment according to the said instructions, he has requested me to supply his want. In compliance with his request I have made many Trials; the issue whereof I here acquaint you with: next, with some exceptions, grounded on Experiments, against Mr. Newton's new Theory of Light and Colours. The vertical angle of my Prism was 60 deg; the distance of the Wall, whereon the coloured Spectrum appeared, from the Window, about 18 foot: The diameter of the Hole in the Window-shuts in length the line a, which upon occasions I contrasted to half the said diameter; but still with equal success as to the main of the Experiment. The refractions on both sides the Prism, were as near as I could make them, equal,