1829.] On the Manufacture of Optical Glass. 241 



be readily formed from dry materials, and that silica might be 

 added with great advantage to the resulting glass ; a range of 

 proportions between the three ingredients being permissible, 

 which gave much command over the properties of hardness, 

 colour, weight, refractive and dispersive power, &c., and yet 

 remained within the required range of fusibility. Platinum also 

 was ultimately found to answer perfectly the purpose of retain- 

 ing the glass ; for though at first it was continually liable to 

 failure, yet it was ultimately ascertained that neither the glass 

 nor any of the substances entering into its composition, separate 

 or mixed, had the slightest action upon it. Finally, it was found 

 that several kinds of glass formed of these materials, were in 

 their physical properties fitted to replace flint glass in the con- 

 struction of telescopes, in some cases apparently even with 

 advantage ; since which time the experiments have been unre- 

 mittingly pursued. 



12. The great proportion of oxide of lead in these glasses 

 rendered attention to very minute points essential ; for other- 

 wise striaB were inevitably formed, and even the destruction of 

 the apparatus involved. For this reason, after a certain num- 

 ber of trials upon composition had been made, one unvarying 

 set of proportions were adopted, and the attention given al- 

 together to the discovery and establishment of a process which 

 should yield constantly good results. This, as far as it has 

 been carried into effect and proved, it is now my object to 

 describe. 



13. The glass with which I have principally worked is a sili- 

 cated borate of lead, consisting of single proportionals of silica, 

 boracic acid, and oxide of lead. The materials are first puri- 

 fied, then mixed, fused, and made into a rough glass, which is 

 afterwards finished and annealed in a platina tray. 



14. Purification of Materials. Oxide of Lead. The oxide 

 of lead at first used was litharge ; but this source occasioned 

 frequent destruction of the platinum trays, in consequence of the 

 existence of particles of metallic lead, which alloying with the 

 platina, rendered it fusible. When red lead was substituted 

 for litharge, the same effect took place, due to the presence of 

 particles of carbonaceous and reducing matter. Both these 

 substances also contained so much iron and other impurities, as 

 to give a deep colour to the glass, far beyond what was expected 



