Joseph Priestley on Roger Bacon

“Contemporary with Vitellio and Peccam was... Roger Bacon, a man of almost universal genius, and who wrote on almost every branch of science. He frequently quotes Alhazen on the subject of optics, and seems to have carefully studied his writings, as well as those of other Arabians, which were the fountains of natural knowledge in those days, and which had been introduced into Europe by means of the Moors in Spain. Notwithstanding the pains this great man took with the subject of opticks, it does not appear that, with respect to theory, he made any considerable advance upon what Alhazen had done before him.”

“Great as Bacon was, he was far from being free from the mistakes and prejudices of those who went before him. Even some of the most wild and absurd opinions of the antients have the sanction of his approbation and authority. He does not hesitate to assent to an opinion... that visual rays proceed from the eye; giving this reason for it, that every thing in nature is qualified to discharge its proper functions by its own powers, in the same manner as the sun, and other celestial bodies. He acknowledges, however, that the presence of light, as well as several other circumstances, is necessary to vision.”

“In his Opus Majus he demonstrates, that if a transparent body, interposed between the eye and an object, be convex towards the eye, the object will appear magnified. This observation our author certainly had from Alhazen... this writer [Bacon] gives us figures, representing the progress of rays of light through his spherical segment, as well as endeavours to give reasons why objects are magnified... From the writings of Alhazen and these observations and experiments of Bacon together, it is not improbable that some monks gradually hit upon the construction of spectacles, to which Bacon's lesser segment, not withstanding his mistake concerning it, was a nearer approach than Alhazen's... Whoever they were that pursued the discoveries of Bacon, they probably observed, that a very small convex glass, when held at a greater distance from a book, would magnify the letters more than when it was placed close to them, in which position only Bacon seemed to have used it. In the next place, they might try whether two of these small segments of a sphere placed together, or a glass convex on both sides, would not magnify more than one of them. They would then find, that two of these glasses, one for each eye, would answer the purpose of reading better than one; and lastly they might find, that different degrees of convexity, suited different persons. It is certain that spectacles were well known in the 13th century, and not long before. ...It would certainly have been a great satisfaction to us to have been able to trace the actual steps in the progress of this most useful invention, without which most persons who have a taste for reading must have had the melancholy prospect of passing a very dull and joyless old age; and must have been deprived of the pleasure of entertaining themselves by conversing with the absent and the dead, when they were no longer capable of acting their part among the living. Telescopes and microscopes are to be numbered among the superfluities of life when compared to spectacles, which may now be ranked almost among the necessities of it; since the arts of reading and writing are almost universal.”