4 
MR. FARADAY ON THE 
plete than at present ; for though glass has been made, and telescopes manufac- 
tured, yet I have no doubt that much more of improvement will be effected. 
It may be said that a long time has elapsed since the experiments were first 
instituted ; and that if any thing could be done, it should have been effected in 
so long a period. But be it remembered, that it is not a mere analysis, 
or even the developement of philosophical reasoning, that is required: it is 
the solution of difficulties, which, as in the cases of Guinand and Fraunhofer, 
required many years of a practical life to effect, if it was ever effected. It is the 
foundation and developement of a manufacturing process, not in principle only, 
but through all the difficulties of practice, until it is competent to give con- 
stant success : and I may be allowed to plead the acknowledged difficulty and 
importance of the subject as a reason, both why it may not yet have obtained 
perfection, and why it should still be pursued. 
My wish, however, to delay the account of the researches until I could 
have carried the experiments further, is overcome by the conviction that much 
more time must be expected to 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 experi- 
ments, have a right to an official 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 pro- 
bably he 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 ac- 
count 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 incurring the laborious prefatory experiments and investigations which 
we have had to undertake ; only introducing so much of the latter, and the princi- 
ples of the process, as are necessary to make the descriptions clear to a practical 
man, and enable hiinto avoid those circumstances which might otherwise occasion 
failure. That the paper may appear long and tedious I am aware ; but it should 
b< 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 
