CX1V 



spectacles by aged persons occur even as early as 1299 and 1305. The pas- 

 sages of Roger Bacoii relate to the magnifying power of spherical segments 

 of glass. See Wilde, Gesch. der Optik, Th. i. S. 9396 ; and above, Note 

 284. 



(^ p. 315. In like manner, the above named physician and mathemati- 

 cian of the Margravate of Ansbach, Simon Marius, as early as 1608, after 

 receiving a description of the action of a Dutch telescope, is believed to have 

 constructed one himself. On Galileo's earliest observation of the moun- 

 tains in the moon, referred to in the text, compare Nelli, Vita di Galilei, Vol. 

 i. p. 200206 ; Galilei, Opere, 1744, T. ii. p. 60, 403, and (Lettera al Padre 

 Cristoforo Grienberger, in Materia delle Montuosita della Luna) p. 409 424. 

 Galileo found in the moon some circular districts, surrounded on every side 

 by mountains, like the form of Bohemia. " Eundem facit aspecturn lunse 

 locus quidam, ac faceret in terris regio consimilis Boemise, si montibus 

 altissimis, inque peripheriam perfecj^i circuli dispositis occluderetur undique" 

 (T. ii. p. 8). The measurements of the altitudes of the mountains were made 

 by the method of the tangent of the solar ray. Galileo, as Helvetius still 

 later, measured.the distance of the summit of the mountains from the boundary 

 of the illuminated portion, at the moment when the mountain summit first 

 caught the solar ray., I find no observation of the lengths of the shadows of 

 the mountains. He found the summits " incirca miglia quattro" in height, 

 and "much higher than our terrestrial mountains." The comparison is 

 curious, because, according to Riccioli, very exaggerated ideas of the height 

 of our mountains then prevailed ; and one of the principal or most celebrated 

 amongst them, the Peak of Teneriffe, was first measured with some degree of 

 exactness, trigonometrically, by Feuillee, in 1724. Galileo, like all other 

 observers up to the end of the 18th century, believed in the existence of many 

 seas in the moon, and of a lunar atmosphere. 



C 484 ) p. 316. I again find occasion (Kosmos, Bd. i. S. 434 ; Engl. trans. 

 Vol. i. note 159) to recal here the proposition laid down by Arago, "II n'y a 

 qu'une maniere rationelle et juste d'ecrire 1'histoire des sciences, c'eSt de 

 s'appuyer exclusivement sur des publications ayant date certaine : hors de la, 

 tout est confusion et obscurite." The singularly delayed publication of the 

 Frankischen Kalender's oder der Practica (1612), and of the astronomically 

 important memoir entitled " Mundus Jovialis anno 1609 detectus ope per- 

 spicilli Belgici (Feb. 1614)," may indeed have given occasion to the suspicion 

 that Marius had derived information from the Nuncius Sidereus of Galileo, 

 the dedication of which bears date in March 1610, or even from earlier com- 



