Instruments of the Antients. 243 



one whose principal surface was concave, and a foot and a 

 half over, lie Bays that " this mirror greatly enlarged ob- 

 jects, and that it was as well worked, as well polished, and 

 as perfect, as if it had been made by one of our present 

 •skilful artists." 



85. 6i These, however, bavins; no immediate connexion 

 with my principal subject, I shall not now dwell on them, 

 but pass to other indications of the knowledge which the an- 

 tient inhabitants of our own continent had of mirrors. The 

 antients, in addition to what we have stated, possessed the 

 art of making mirrors of glass, as is evident from a passage 

 in Pliny, lib. xxxvi. cap. 26\ where speaking of glass, he 

 says, " Aliud flat u Jiguratnr, aliud tor no teritvr, alhid ur- 

 genti modo ccelatur, St/done quondam us ojpeinis vvhil't, si- 

 quidem etlani specula excogitaverat. Some of it is figured 

 by blowing, some turned in a lathe, some embossed like 

 silver; Sidon having been formerly famous for such manu- 

 factures, since even specula had been contrived there." If 

 we had more particular accounts of the glass-works and the 

 manufactures of Sidon, and their arts of forming and po- 

 lishing mirrors plane, concave or convex, of glass and 

 metal, we should perhaps find out some secrets, as well as 

 antient inventions, which we believe to be modern ; and 

 should see that the limits of their knowledge were not quite 

 so narrow as we arbitrarily prescribe. 



86. " But be this as it may, I shall now prove, that the an- 

 tients possessed more than sufficient knowledge of mir- 

 rors to have furnished Ptolemy with that of which we speak. 

 — We learn from experience, that the mirrors which we call 

 plane, are not really so when strictly examined. They are 

 almost always irregularly concave or convex, or both ; and 

 this is a necessary consequence of the manner of working 

 them. Everv intelligent artist, in preparing large object- 

 glasses for telescopes, has frequent occasion to be convinced 

 of this fact*. It is true that the concavities and convex- 

 ities in mirrors intended to be plane, are commonly por- 

 tions of very large spheres, and do not enlarge or diminish 

 objects; so that, as to sense, they produce the same effects 

 as if they were perfectly plane-. 



* I have heard that intelligent and philosophical artist, and worthy 

 man, Mr. Samuel Farley, explain (he cause of this inequality of plane 

 mirrors, and indeed the whole process of grinding and polishing glasses, 

 in a veiy satisfactory manner ; while he was performing with his hands 

 the work which he was describing. This method of illustrating by actual 

 performance, the nicer mechanical operations which occur in experimen- 

 tal philosophy, Mr. Farley practises in other instances; which renders 

 his lecture* uncommonly interesting and instructive. — 'translator. 



87. " Among 



