1829.] On the Manufacture of Qirtical Glass. 235 



elapse before I shall consider the investigation finished ; by the 

 consideration that a decided step has been made in the manu- 

 facture of glass for optical purposes ; and by the feeling that 

 the Royal Society which instituted, and the Government which 

 defrays the expenses of the experiments, have a right to an offi- 

 cial account of the present state of the investigation. Although 

 much useful information has been obtained respecting flint and 

 other glasses, yet as that train of research is very imperfect, 

 uncertain, and will probably be resumed, I shall confine my 

 present statement altogether to the heavy optical glass already 

 referred to. It will be impossible for me to describe all that 

 has been done on this subject ; but I shall endeavour to give 

 such an account of the glass, and the process by which it is 

 obtained in a homogeneous state, as shall enable other persons 

 to do what has been done at the Royal Institution, without in- 

 curring the laborious prefatory experiments and investigations 

 which we have had to undertake ; only introducing so much of 

 the latter, and the principles of the process, as are necessary 

 to make the descriptions clear to a practical man, and enable 

 him to avoid those circumstances which might otherwise occa- 

 sion failure. That the paper may appear long and tedious I 

 am aware ; but it should be remembered, that it can have no 

 other utility than as containing efficient instructions to the few 

 who may desire to manufacture optical glass ; and that to ren- 

 der whatever of this character it may have, imperfect, for the 

 sake of giving to it a more abbreviated and popular form, would 

 have been doing injustice to the objects and motives of those 

 who have instituted and supported the experiments. 



1. Process of Manufacture > $c. 



1. The general properties of transparency, hardness, and a 

 certain degree of refractive and dispersive power, which render 

 glass so valuable as an optical agent, are easily obtained : but 

 there is one condition essential in all delicate cases of its appli- 

 cation, which is not so readily fulfilled ; this is, a perfectly ho- 

 mogeneous composition and structure. Although every part 

 of the glass may in itself be as good as possible, yet without 

 this condition they do not act in uniformity with each other ; 

 the rays of light are deflected from the course which they ought 

 to pursue, and the piece of glass becomes useless. The streaks, 



