ibct. r] DISSERTATION SECOND. 179 



lemy, with whose writings, particularly those of the former, 

 he seems to have been well acquainled. In some things he 

 was much behind the Arabian optician, as he supposed 

 with the ancients that vision is performed by rays emitted 

 from the eye. It must, however, be allowed, that the ar- 

 guments employed on both sides of this question are so 

 weak and inconclusive, as very much to diminish the merit of 

 being right, and the demerit of being wrong. What is most 

 to the credit of Bacon, is the near approach he appears to 

 have made to the knowledge of lenses, and their use in as- 

 sisting vision. Alhazen had remarked, that small objects, 

 letters, for instance, viewed through a segment of a glass 

 sphere, were seen magnified, and that it is the larger segment 

 which magni6es the most. The spherical segment was sup- 

 posed to be laid with its base on the letters, or other mi- 

 nute objects which were to be viewed. Bacon recommends 

 the smaller segment, and observes, that the greater, though 

 it magnify more, places the object farther off than its natu- 

 ral position, while the other brings it nearer. This shows 

 sufficiently, that he knew how to trace the progress of the 

 rays of light through a spherical transparent body, and un- 

 derstood, what was the thing least obvious, how to determine 

 the place of the image. Smith, in his Opticks, endeavours 

 to show, that these conclusions were purely theoretical, and 

 that Roger Bacon had never made any experiments with 

 such glasses, notwithstanding that he speaks as if he bad 

 done so. l This severe remark proceeds on some slight in- 

 accuracy in Bacon's description, which, however, does not 

 seem sufficient to authorize so harsh a conclusion. The 

 probability appears rather to be, as Molineux supposed, 

 that Bacon had made experiments with such glasses, and 

 was both practically and theoretically acquainted with then 1 



1 Smith's Opticks. vol. II. Remarks, 6 76. 



