Passy, May 23, 1785.
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BY Mr. Dollond's saying, that my double spectacles can only serve particular eyes, I doubt he has not been rightly informed of their construction. I imagine it will be found pretty generally true, that the same convexity of glass, through which a man sees clearest and best at the distance proper for reading, is not the best for greater distances. I therefore had formerly two pair of spectacles, which I shifted occasionally, as in travelling I sometimes read, and often wanted to regard the prospects. Finding this change troublesome, and not always sufficiently ready, I had the glasses cut, and half of each kind associated in the same circle, thus,
By this means, as I wear my spectacles constantly, I have only to move my eyes up or down, as I want to see distinctly far or
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Publication information: Book title: The Ingenious Dr. Franklin:Selected Scientific Letters of Benjamin Franklin. Contributors: Nathan G. Goodman - Editor, Benjamin Franklin - Author. Publisher: University of Pennsylvania Press. Place of publication: Philadelphia. Publication year: 1931. Page number: 54.
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