426 mSTORY OF LENSES AND MICROSCOPES [Ch. XII 



Turning now from the literature of this period to the work of Clau- 

 dius Ptolemaeus (70-147 a.d.) on optics, one is filled with admira- 

 tion for the exactness of knowledge displayed. He stated with a 

 clearness never since excelled the laws of refraction of light in passing 

 from transparent media of different density, and dealt with curved 

 as well as with plane surfaces (Sermo Quintus). It is almost incon- 

 ceivable that he should not have discovered the magnifying power of 

 curved bodies from their refractive action. A part of this discussion 

 is lost, but so far as known he did not make that discovery. 



In works dealing with the history of optics frequent reference is 

 made to Alhazen " On Appearances." This work is supposed to date 

 from about iioo a.d. It was translated from the Arabic by Risner 

 in 1572. Almost all of its sound teaching in optics conforms very 

 closely with that of Ptolemaeus whom Alhazen mentions. The struc- 

 ture and action of the eye is founded almost entirely on the work of 

 Galen. In using Alhazen one should note carefully what is said by 

 Risner in the preface, for it seems quite possible from his statement 

 that he might unconsciously have read into his translation knowledge 

 of optics which was a later acquisition; in a word in trying to make 

 clear the work of Alhazen, possibly a certain amount of later knowledge 

 was added to it. 



In passing it may be said in reading almost any of the ancient 

 writers and indeed aU large pubUcations of a single author, that they 

 are in the nature of cyclopedias, detaiUng the knowledge most in favor 

 at the time and often containing a certain amount of original matter. 

 The older the work the greater the proportion of original matter if 

 the author is of first rate abihty, because until recently there have 

 not been the periodicals and transactions 6i learned societies in which 

 to publish one's original contributions. 



The first clear and unmistakable statements from which dates 

 modern knowledge of lenses and their action are found in the works 

 of Roger Bacon; especially his Opus Majus, 1266-1267. Roger 

 Bacon's work is encyclopedic in many ways, and in many it is like 

 a modern monograph, giving full recognition of the opinions and work 

 of others. In his works (Opus Majus; Opus Tertium, etc.) lenses are 

 figured and discussed in detail. Bacon nowhere claims to be the 



