i$$ Opinions ofAriftot1e> Roger Bacon> &c. [Book III. 



life of convex glafTes, both as magnifiers and as burn- 

 ing glafles, was not unknown to the ancients, though 

 the theory was not underftood. The magnifying 

 power of glafles, and fome other optical phenomena, 

 were alfo largely treated of by Alhazen, an Arabic phi- 

 lofopher of the twelfth century. Thefe obfervations 

 were followed by thofe of Roger Bacon, who demon- 

 ftrates by actual experiment, that a fmall fegmenr, of 

 a glafs globe would greatly afllft the fight of old per- 

 fons ; and from the hints afforded by thefe two philo- 

 fbphers, it is not unreafonable to conclude, that the 

 invention of fpectacles proceeded. Concerning the 

 a6bual author of this ufeful invention, we have no 

 certain information ; we only find, that it was gene- 

 rally known about the beginning of the fourteenth 

 century. 



In the year 1575, Maurolycus, a teacher of mathe- 

 matics at Meffina, publiflicd a treatife on optics, in 



tina, are exadlly correfpondent to the points of the object from 

 which they proceed. As however, from the great degree of con- 

 vergence which this contrivance will produce, the pencils of light 

 proceeding from the extreme points of the object will be made to 

 crofs each other before they rench the retina, the image on the re- 

 tina is always inverted. (See Plate XX. fig. 39.) 



19. The magnitude of the image painted on the retina will, 

 therefore, it is evident, depend on the greatnefs or obtufenefs of 

 the angle under which the rays proceeding 'from the extreme 

 points of the object enter the eye. For it is plain, that the more 

 open or obtufe the angle is, the greater is the tendency of thefe 

 rays to meet in a point and crofs each other : and the fooner they 

 crofs each other, after paffing the ctyftalline lens, the larger will be 

 the inverted image painted on the retina. (See Plate XX. fig. 40.) 

 The 'uij'ual angle, therefore, is that which is made by two right 

 lines drawn from the extreme points of any object to the eye ; and 

 on the meafure of that angle, the apparent magnitude of every 

 vifible object, wili depend. 



20. Tne prtftn ufed by opticians is a triangular piece of fie 

 glafs, which has, the power of feparating the rays of light. 



6 which 



