That Roger Bacon was acquainted with the principle of the camera obscura is shown by his attempt at solving Aristotle's problem stated above, in the treatise De Speculis, and also from his references to Alhazen's experiments of the same kind, but although Dr John Freind, in his History of Physick, has given him the credit of the invention on the strength of a passage in the Perspectiva, there is nothing to show that he constructed any instrument of the kind.
Freind (1675-1728) in his Emmenologia gave a mechanical explanation of the phenomena of menstruation.
Their English contemporaries and successors, John Freind, William Cole, and Richard Mead, leaned also to mechanical explanations, but with a distrust of systematic theoretical completeness, which was perhaps partly a national characteristic, partly the result of the teaching of Sydenham and Locke.
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