PBRISCOPIC SPBCTACLKS. 
317 
be principle, on which that form of glass is recommended for 
pectacles, is not new, though all his quotations prove, that it 
vas unknown to the authors on whose opinion he so confidentljr 
dies, and though it evidently is not even yet rightly understood 
>y himself. 
I have hitherto thought it wholly superflous to make any 
inawer. Those who understood the subject would certain- 
ly not expect any reply from me j those who did not, wouldnot 
ae benefited by any attempts of mine at further illustration; 
ind to Mr. Jones himself, it is probable that my silence would 
be far more satisfactory than any explanation that I could give. 
I do hope, however, that the following report from M, Biot, 
will gratify those who are best acquainted with the merits of 
the question by its fairness and perspicuity ; that the authority, 
of one so justly celebrated as a mathematician, will be received 
as conclusive by those who do not feel themselves competent to 
decide on such subjects ; and that possibly even Mr. Jones 
liiimself, if his “ dultf to his professional intrrest*” should again 
i.mpel him to write upon the sul^ect, may at least acknowledge* 
..l»at a philosopher of the first eminence in France, probably 
txrrites without any prepossession liable to warp his judgment, * 
itnd that he may perhaps evert feel persuaded, that there must 
:^e some advantage in the periscopic construction, which he has 
•iverlooke*!, when one so peculiarly skilled in optical science as 
^V1. Biot gives such decided testimony to the superiority of this 
k:ind of spectacles. 
I hope you will find that I have fairly translated the whole of 
be report ; but as it is possible that I may in some irrttancefl 
\ 
* Seevot. 34. p. lOl. — ^The l?b<'rafr^ of Mr. Jones mnst beacknow- 
-eds«fd in avowing liinisetf the champion of the professional interest 
Lgainst an intriuler who has presnrtcd to recommend, as an improv c- 
nent, a inode of ronstnictien, which is necessarily far more costly, on 
1 ccouiit of tlie thickness of glass that must be employed, on account of 
le qaantity of this gla^ that mnst be ground away by liaid labour, 
nd more especially on account of the very small number of large 
Casses that tan be anranged by the side of each other on a snffa^e of 
41 bII radius, so as to bo ground at die same time with the same tool. 
havt 
