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HISTORICAL VIEW OF THE

realised. The principle of the composition of gun-powder is distinctly pointed out in his writings, thoughhe never brought it into practice, actuated, as it is said,by motives of humanity. He describes clearly thediving bell, and various self-moving machines. Heattained considerable skill in medicine ; and wrote onvarious parts of moral, philosophical, and metaphysicallearning.

His discoveries in optics have been the subject ofmuch discussion; he certainly appears to have under-stood the theory of lenses. Alhazen had remarked thatsmall objects, such as letters, viewed through a segmentof a glass sphere, were seen magnified, and that thelarger the segment of the sphere, the greater will be thedegree of magnification. The spherical segment wassupposed to be laid with its flat side on the letters orobject. Bacon discusses the comparative advantages ofsegments, including a greater or less portion of thespherical surface ; but though his arguments have beenconsidered by some to involve certain errors, yet, uponthe whole, what he has said is admitted to give a moregeneral view of the subject than had hitherto beentaken, and to prove sufficiently that he knew how totrace the progress of the rays of light through a spheri-cal transparent body, and understood how to determinethe place of the image. Dr. Smith, in his Optics , hasendeavoured to show that these conclusions were purelytheoretical, and that Bacon had never made any actualexperiments on the subject. This has been controvertedby Mr. Molyneux, who contends that Bacon was notonly acquainted with the properties of lenses theoreti-cally, but that he also applied them practically.

His knowledge, however, was clearly such, that ifhe did not himself reduce it to practice in the actualformation and use of lenses, he made known principleswhich could hardly have remained long without suchapplication. The use of lenses, especially as spectacles,was certainly a very early invention, but in an ignorantage we cannot expect to find the origin of discoveries