On the Manufacture of Optical Glass. [1829. 



9. Whilst the crucible is in the condition described, it is 

 clear that all those circumstances, as currents, bubbles, &c., 

 which tend to mix the glass, form abundant striae and veins of 

 enormous strength, and do harm unless they are continued in 

 activity until the mixture is nearly complete ; a state rarely, if 

 ever, acquired in the ordinary flint glass pot. But even if this 

 could be the case, there is a constant cause of deterioration 

 arising from the highly fluxing and dissolving quality given to 

 the glass by the oxide of lead. In this respect, flint glass far 

 surpasses crown or plate glass, and it is also during one stage 

 of its preparation more fluid : it consequently is continually 

 exerting a solvent power upon the crucible to a considerable 

 extent, occasioning that very irregularity in composition which 

 produces striae, whilst the comparative levity of the matter dis- 

 solved at the sides and bottom, and the ascending currents at 

 the hottest parts of the crucible, are constantly mixing this 

 deteriorating portion with the general mass. 



10. The difficulties which are thus introduced into the manu- 

 facture of flint glass fit for optical uses appeared to the com- 

 mittee, who, however, were none of them practical glass-makers, 

 to increase, as the scale upon which the inquiries were carried 

 on diminished : and the enormous expense of large experiments, 

 the time required for each, the number necessary to give 

 that experience which should render any one who undertook 

 the charge of this part of the inquiry an ordinary practical work- 

 man, and the uselessness of the resulting glass for any other 

 purpose than the one directly contemplated, compelled the 

 sub-committee to consider seriously on the possibility of making 

 other glasses than those ordinarily in use, which, at the same 

 time that they had the high dispersive power enabling them to 

 replace flint glass, might have also such fusibility as would 

 allow of their being perfectly stirred and mixed, and might be 

 retained, without alteration, in such vessels as could be procured 

 of any desired size. 



11. The borate of lead, and the borate of lead with silica, 

 were the substances which, after some trials, were found to 

 offer such reasonable hopes of success as to justify perseverance 

 in a series of experiments ; and the metal platina was looked 

 to as the material out of which to form the vessels intended to 

 be used. It was soon ascertained that the borate of lead could 



