{ 387 j 
LXIT. Letter from Dr. Wotasron on the Periscovie 
Construction of Spectacles, 
To Mr. Tilloch. 
Sin.—lx the 180th number *of your Magazine (for April 
last), your correspondent Mr. Jones renewed his attack 
upon the periscopic construction of spectacles, maintaining 
as before, that the principle on which that form of glass is 
recommended for spectacles is not new, though al] his 
quotations prove that it was unknown to the authors on 
whose opinion he so confidently relies, and though it evi- 
dently is not even yet rightly understood by himself. . 
I have hitherto thought it wholly superfluous to make 
any answer. Those who understood the subject would 
certainly not expect any reply from me; those who did not, 
would not be benefited by any attempts of mine at further 
illustration ; and to Mr. Jones himself it is probable that 
my silence would be far more satisfactory than any ex- 
planation that I could give. 
I do hope however that the following Report of M. Biot 
wili gratify those who are best acquainted with the merits of 
the question by its fairness and perspicuity ; that the au- 
thority of one so justly celebrated as a mathematician will 
be received as conclusive by those who do not feel them- 
selves competent to decide on such subjects; and that, pos- 
sibly, even Mr. Jones himself, if his * duty to his profes- 
sional interest *” should again impel him to write upon the 
subject, may at least acknowledge that a philosopher of the 
first eminence in France probably writes without any pre- 
possession liable to warp his judgement; and that he may 
perhaps even feel persuaded that there must be some advan- 
tage in the periscopic construction which he has overlooked, 
when one so peculiarly skilled in optical science as M. Biot, 
gives such decided testimony to the superiority of this kind 
of spectacles. 
I hope you will find that have fairly translated the whole 
of the Report: but as it is possible that I may in some in- 
stances have misinterpreted the strict meaning of the au- 
* Sce vol. xli, p. 247, The liberality of Mr. Jones must be acknowledged 
in avowing himself the champion of the professional interest, in opposition 
to an intruder who has presumed to recommend, as an improvement, a 
mode of construction which is necessarily far more costly, on account of the 
thickness of glass that must be taken for the purpose, on account of the 
owed of this glass that must be ground away by hard labour, and more 
especially on account of the very small number of large glasses that can be 
arranged by the side of each other on a surface of small radius, so as to be 
ground at the same time by the same tool, 
be thor, 
