178 DISSERTATION SECOND. £mm i. 



probable that be had never seen. The anatomical struc- 

 ture of the eye was known to him ; concerning the uses of 

 the different parts he had only conjectures to offer ; but on 

 seeing single with two eyes, he made this very important 

 remark, that, when corresponding parts of the retina are af- 

 fected, we perceive but one image. 



Prolixity and want of method are the faults of Alhazen. 

 Vitello, 1 a learned Pole, commented on his works, and has 

 very much improved their method and arrangement in a 

 treatise published in 1270. He has also treated more fully 

 of the subject of refraction, and reduced the results of his 

 experiments into the form of a table exhibiting the angles 

 of refraction corresponding to the angles of incidence, which 

 he had tried in water and glass. It was not, however, till 

 long after this period that the law which connects these an- 

 gles was discovered. The cause of refraction appeared to 

 him to be the resistance which the rays suffer in passing 

 into the denser- medium of water or glass, and one can see 

 in his reasoning an obscure idea of the resolution of forces. 

 He also treats of the rainbow, and remarks, that the alti- 

 tudes of the ->iin and bow together always amount to 42 de- 

 grees. He next considers the structure of the eye, of 

 which he has given a tolerably accurate description, and 

 proves, as Alhazen had before done, 2 that vision is not per- 

 formed by the emission of rays from the eye. 



Roger Bacon, distinguished for pursuing the path of true 

 philosophy in the midst of an age of ignorance and errour, 

 belongs 'o the same period ; and applied to the study of op- 

 ticks with peculiar diligence. It does not aj^ear, however, 

 that he added much to the discoveries of Alhazen and Pto- 



1 The name of this author is commonly written Vitcllio. He 

 may be supposed to have known best the orthography of his own 

 same. 



2 Alhazen, Opt. lib. 1. 



