Ch. XII] HISTORY OF LENSES AND MICROSCOPES 427 



inventor of lenses. He expounds the principles on which they act, 

 referring back to Ptolemrcus for the laws of refraction so clearly set 

 forth by him. And he discusses over and over again the marvelous 

 things which lenses enable one to do. Nearly all of the things men- 

 tioned by Bacon we know are possible from our own experience. 

 He tells us that much of his private fortune was spent in obtaining 

 apparatus of all kinds, for he insisted that the final test in science 

 is experiment. 



He pointed out that convex lenses made it possible for old men 

 to read the smallest letters, and within thirty-two years from that 

 time, i.e., in 1299, we have in a manuscript this notable sentence: 

 " I am so affected by years that I cannot read or write without the 

 glasses they call spectacles, lately found out for the benefit of old 

 men when their eyesight gets weak" (Carpenter-Dallinger, p. 118, 

 Harting, III, p. 16). 



§ 691. Spectacles. — It is rather surprising that the use of spec- 

 tacles became so general in so short a time after Bacon had sent his 

 manuscript to Pope Clement IV. The part on optics (Perspectiva) 

 was copied many times and widely distributed among the libraries. 

 It is referred to by many writers, e.g. Porta, Maurolycus, Kepler, 

 Scheiner, etc. Apparently, then, it was available for any one who was 

 interested greatly in optics. For over 300 years from the time of 

 Roger Bacon practically all of the work in optics was in the hands of 

 the spectacle makers, and to them we owe both the telescope and the 

 microscope and the lenses for the camera obscura and the magic 

 lantern (§ 701-704). 



§ 692. Concave spectacles. — Roger Bacon knew concave as well as 

 convex lenses, but he did not refer to the use of concave lenses for 

 people with short sight (myopia) so far as I have been able to find out. 

 Just who made this discovery has never been shown. However, 

 within 200 years from the first statements in the Opus Majus con- 

 cerning the use of convex glasses for old men, mention becomes more 

 and more common of concave glasses for the myopes, and from 

 1568 onward convex and concave spectacles have a fixed place 

 as aids to vision. See Barbaro, § 705a, and the works of Pansier, 

 p. 29-31, and Bock, p. 44. 



