620 COSMOS. 



the Almagest. As he, like the Arabs, always calls Hip- 

 parchus, Abraxis, we may conclude that he also made use of 

 only a Latin translation from the Arabic. Next to Bacon's 

 chemical experiments on combustible explosive mixtures, his 

 theoretical optical works on perspective, and the position of 

 the focus in concave mirrors, are the most important. His 

 profound Opus majus contains proposals and schemes of 

 practicable execution, but no clear traces of successful optical 

 discoveries. Profoundness of mathematical knowledge can- 

 not be ascribed to him. That which characterizes him is 

 rather a certain liveliness of fancy, which, owing to the 

 impression excited by so many unexplained great natural 

 phenomena, and the long and anxious search for the solution 

 of mysterious problems, was often excited to a degree of 

 morbid excess in those monks of the middle ages who devoted 

 themselves to the study of natural philosophy. 



Before the invention of printing, the expense of copyists, 

 rendered it difficult, in the middle ages, to collect any large 

 number of separate manuscripts, and thus tended to produce 

 a great predilection for encyclopaedic works after the extension 

 of ideas in the thirteenth century. These merit special consider- 

 ation, because they led to a generalisation of ideas. There ap- 

 peared the twenty books de rerum natura of Thomas Cantipra- 

 tensis, Professor at Louvain (1230); The Mirror of Nature 

 (speculum naturale), written by Vincenzius of Beauvais (Bello- 

 vacensis) for St. Louis and his consort Margaret of Provence 

 (1250); The Book of Nature, by Conrad von Meygenberg, a 

 priest at Ratisbon (1349) ; and the Picture of the World (Imago 

 mundi) of Cardinal Petrus de Alliaco, Bishop of Cambray (1 41 0), 



majus (ed. Jebb, Lond. 1733), pp. 79, 288, and 404. It has been justly 

 denied (Wilde, Geschichte der Optik, th. i. s. 92-96) that the knowledge 

 derived from Alhazen, of the magnifying power of segments of spheres, 

 was actually the means of leading Bacon to construct spectacles. This 

 invention would appear to have been known as early as 1299, or to belong 

 to the Florentine, Salvino degli Arinati, who was buried, in 1317, in the 

 Church of Santa Maria Maggiore at Florence. If Roger Bacon> who 

 completed his Opus majus in 1267, speaks of instruments by means of 

 which small letters appear large, "utiles senibus habentibus oculos 

 .debiles," his words prove, as do also the practically erroneous considera- 

 tions which he subjoins, that he cannot himself have executed that 

 which obscurely floated before his mind as possible. 



