• Instruments of the Antients. 347 



nihil aliud est quam subtilissimum vitrum. A mirror is nothing 

 else than a very thin, or fine, glass." — 3. Vincent de Bau- 

 vais, who flourished under the emperor Frederic II. A.D. 

 1240, in his Speculum Nalurale, lib. ii. cap. 78, says, that 

 " Inter omnia melius est speculum ex vitro et plurnbo : The 

 best mirror of all is of glass and lead." For, says he, in the 

 same chapter, " Quando sup erf unditur. plumbum vitro calido 9 

 efficitur altera parte terminalum valde radiosum: When 

 lead is poured on hot glass, a very brilliant surface is pro- 

 duced on the other side." — 4. The celebrated John Peckam*, 

 archbishop of Canterbury, in his Perspectiua communis , 

 pars 2. prop. 7, says : " Reflexio est a denso, quia densum ; 

 propter quod specula consueta vitrea sunt plumbo obducta : 

 Reflection is made from a dense body because of its density; 

 .on which account common glass mirrors are coated with lead/'* 

 He afterwards adds : " St ut quidam fabulantur, diapha- 

 veitas essct essentialis speculo, non fierent specula de ferto 

 et chalybe a diapkaneitate remotissimis, nee ctiam de mar- 

 more polito ; cujus iamen contrarinm videmns : Jf, as some 

 feign, transparency were essential to mirrors, they would 

 not be made of iron and steel, which are most remote from 

 transparency, nor even of polished marble ; of which, how- 

 ever, we see the contrary."— 5. My author next quotes from 

 the Ars Magna of Raymo?id Lully, who lived from the 

 year 1225 to 1315, the whole chapter De Speculo, which 

 is too long and perplexed with school metaphysics to be 

 here inserted. It may suffice to observe, that the subject 

 of it is mirrors coated with lead. — QP S. Isidore, of Seville, 

 who died A.D. 636, in his Etymol. lib, xvi. cap, 15, speak- 

 ing of glass, says, u Neque est alia speculis aptior materia ; 



* I cannot but observe that, as far as I can find, Abat takes no notice 

 of Roger Ea on, the countryman and cotemporary of Peckam, or Peccam\ 

 though in several parts of his' book he had fair opportunities of doing so. 

 Can this neglect be any way owing to the critique of Dr. S. ' Certain it 

 is, that in Mr. Bonnycastle s excellent translation of Bossut's Hist. Gener. 

 des Matbem. p. 189, we find this passage: — " Roger Ba on's Treatise on 

 Optics is particularly remarkable for the ingenious, just, and at the same 

 time new, ideas it oilers on the subjects of astronomical refraction, the ap- 

 parent magnitudes of objects, the extraordinary size of the sun and moon 

 near the horizon, the place of spherical foci, &c. Some English writers, 

 a little too much prejudiced in favour of their countryman, have fancied 

 that they discovered in this trratise that the author knew the use of 

 spectacles, and even of the telescope; but Mr. S., an Englishman of 

 more impartiality, and an irrefragable judge, has controverted this opi- 

 nion by an accurate and critical discussion of the passages that gave rise 

 to it,'.' Does not Roger Bacon s fame gain more by the just commenda- 

 tion of the learned Frenchman, than it loses by the hypei criticism of the 

 Uained Englishman > — Translator. 



A a 4 Nor 



