70 Proofs from old English Books, that the 



which contradict the rules not of perspective only, but even 

 of common sense and probability*. And, considering the 

 imperfection, both of optics and of the graphical art, in the 

 days of Bacon, together with the errors of transcribers, 

 would it not be extremely unjust not to make proportiona- 

 lly greater allowances for the faults of his diagrams? Thus, 

 it does not appear to me, that the doctor had any satisfac- 

 tory authority for substituting superimpositi for suppositi, 

 or for drawing any unfavourable inference from the faults 

 of Bacon's figures. 



22. I may add, that Bacon says, in the sequel of the 

 same passage, that " this instrument is useful to old men, 

 and to those who have weak eyes ; for they may see the let- 

 ters, however small, in sufficient magnitude." Now what 

 man would have said " this instrument is useful to old men, 

 &c. (hoc instrumentum est utile senibus, 8cc), unless he had 

 witnessed its effects ? This would have been to talk of a 

 thing as real, which had no existence. If he had only 

 thought of it, without having seen or made it, he would 

 have said, such an instrument ivould be useful to old men, 

 &c. And, had this been Bacon's language, it would surely 

 have been a proof that he knew something of the theory, 

 contrary to what Dr. S. would allege in his observations 

 on this and other passages of his great author. But do not 

 Bacon's very excuseable errors in theory only make it the 

 more probable that he is accurate as to the fact ; and that 

 he either invented reading-glasses or spectacles, or had at 

 least experienced their effects ? 



23. Dr. S., before he proceeds to examine the pretensions 

 of Bacon to an acquaintance with the dioptric telescope, 

 translates from the Opus Majusf a whole chapter as fol- 

 lows: — De visione fracia major a sunt, &c. " Greater 



* In fig. 67. tab. vii. of Wolfius's excellent Eirm. Diopt. the engraver 

 has represented a lens as in absolute contact with an eye. Mr. Gibson, 

 a very ingenious mechanician at Hampstead, whose late brother at Kelso 

 was one of the first opticians in Great Britain, and who has himself paid 

 particular attention to optics, complains, that, except Dr. Brook Taylor, 

 he scarcely knows a writer on perspective free from gross errors. You, 

 Mr. Tillcch, know to your cost, that, except Mr. Lowry, there are few 

 artists who engrave figures in ordinary books, which are always correct 

 in point of perspective. Even the diagrams in Agnesi's Analytical In- 

 stitutions are not entirely free from the errors of the wood-cutter. Yet 

 I believe it would be hard to say, whether that work does most credit to 

 the fair author ; to Mr. Co'ison, the translator; to Mr. Hellins, the editor j 

 to Mr. Taylor, the printer; or to Mr. Baron Maseres, to whose munifi- 

 cence the mathematicians of this country are indebted for its publication. 



f Dr ; Jebb's c 'ition, London 1733, P- 3 57> as quoted in the Com- 

 plcat System of Optics, Remaiks, i;z, 113., • 



5 things 



