70 HISTORY OF SCIENCE 



should be imputed rather to the age than to any want of skill or 

 industry on the part of the compilers. Yet the comparatively small 

 superiority evinced by these tables over those drawn up by Ptolemy, 

 shows that during the eleven centuries between their respective dates 

 no very great advance had been made in astronomy. 



We find the thirteenth century of our era comparatively poor in men 

 of scientific genius, but characterized by at least one invention which 

 has proved of the very highest importance to the progress of science. 

 The invention is that of spectacles or glass lenses, which can be traced 

 with distinctness no further back than the end of the above-mentioned 

 century. Possibly the writings of Roger Bacon may have led to the 

 actual production of 'segments of glass spheres in order to effect the 

 enlargement of objects, so clearly indicated in the extract we have 

 given from Roger Bacon's works. Much learned discussion has taken 

 place upon allusions in ancient authors to the magnifying power of 

 glass globes filled with water, etc., but the use of glass lenses to aid the 

 vision seems to date only from about the time above stated. Italy 

 appears entitled to boast of the honour of this invention. In the dis- 

 course of an Italian friar, preached in 1305, the hearers are reminded 

 that spectacles had been invented only twenty years before, and that 

 they are an admirable contrivance. 



About the same period there came into use an invention destined to 



FIG. 26. MARINER'S COMPASS. 



exercise a remarkable influence on the progress of knowledge, by so- 

 enlarging the resources of navigators that ships began to venture on 

 voyages to greater and greater distances, until at length the passage to 

 India by doubling the Cape of Good Hope was found, and Columbus 

 discovered the New World. We need hardly say that we here allude 

 to the Manner's Compass, which is almost too well known to require 

 any description. It consists essentially of a slender bar of steel, which, 

 after having been drawn several times along a loadstone, is poised on 

 a pivot on which it can turn freely in the horizontal plane. It will rest 

 only in a definite direction, which, for the present,, may be described as 

 pointing nearly north and south. The power possessed by the load- 

 stone of attracting iron was known in ancient times, as proved by allu- 



