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soever, may be speedily made: which seems to him most easie, because, if it succeeds, with one and the same Tool may be ground an Object Glass of any length or breadth requisite, and that with very little or no trouble in fitting the Engin, and without much skill in the Grinder. He thinks it very exact, because to the very last stroke the Glass does regulate and rectifie the Tool to its exact Figure; and the longer or more the Tool and Glass are wrought together, the more exact will both of them be of the desired Figure. He affirms further, that the motions of the Glass and Tool do so cross each other, that there is not one point of eithers surface, but hath thousands of cross motions thwarting it, so that there can be no kind of Rings or Gutters made, either in the Tool or Glass.
5. A New Instrument, by which the Refraction of all kinds of Liquors may be exactly measured, thereby to give the Curious an opportunity of making Trials of that kind, to establish the Laws of Refraction, to wit, whether the Sines of the Angles of Refraction are respectively proportionable to the Sines of the Angles of Incidence: This Instrument being very proper to examine very accurately, and with little trouble, and in small quantities, the Refraction of any Liquor, not only for one inclination, but for all; whereby he is enabled to make accurate Tables. By the same also he affirms to have found it true, that what proportion the Sine of the Angle of the one inclination has to the Sine of its Angle of Refraction, correspondent to it, the same proportion have all the other Sines of Inclination to their respective Sines of Refractions.
Lastly, this Author despairs not that there may be found many Mechanical Inventions, to improve our Senses of Hearing, Smelling, Tasting, Touching, as well as we have improved that of Seeing by Optick Glasses.
London, Printed with Licence for John Martyn, and James Allestry, Printers to the Royal Society.
