Books and Science in the Middle Ages 551 spectacles are mentioned. The lens made the later telescope, microscope, spectroscope, and camera possible, upon which so much of our modern science depends. The Arabic numerals began to take the place of the awkward Roman system of using letters. One cannot well divide XLVIII by VIII but he can easily divide 48 by 8. Roger Bacon knew of the explosive nature of a compound of sulphur, saltpeter, and charcoal, and a generation after his death gunpow- der began to be used a little for guns and artillery. A document is still preserved referring to the making of brass cannon and balls in Plorence in the year 1326. By 1350 powder works were in ex- istence in at least three German towns, and French and Eng- lish books refer now and then to its use. At least 'a hundred and fifty years elapsed, however, before gunpowder really began to supplant the old ways of fighting with bows and arrows and axes and lances. By the year 1500 it was becoming clear that the old stone castles w^ere insufficient protection against cannon, and a new type of unprotected castle began to be erected as residences of the kings and the nobility (see below, p. 570). Gunpowder has done away with armor, bows and arrows, spears and javelins, castles and walled towns. Arabic numerals Fig. 198. Effects of Cannon on a Medieval Castle
