[ 464 j 
’ LXXV. Final Letter from Mr. Witt1am Jones on 
Dr. Woitaston’s Periscopic Spectacle Glass. 
To Mr. Tiiloch. 
Sirr,— Th your Journal of last month, I observe that Dr. 
Wollaston has continued his effort to impress the minds of 
your readers with a belief of his discovery of a new and 
improved form of the spectacle glass, and with an intima- 
tion that the eminent writers I quoted, as well as myself, did 
not rightly understand our old acquaintance the Meniscus, 
alens drawn from oblivion under his new appellation of 
Periscopic. This merits no reply from me. [t certainly 
would be more than “ superfluous” for him to make any 
answer to my former palpable refutation and exposure ; and 
really, in commiseration for his unfortunate cause, his *¢si- 
lence’? must be more agreeable to me than his contumacy. 
My dissatisfaction with him was not as an intruder merely 
presuming to recommend, but for first in a groundless man- 
ner depreciating the double convex or best form of Jens, 
advertising the old meniscus form as containing a newly 
discovered optical principle, and then by the name of a 
patent exacting a payment for a glass triple the price of the 
common superior or more perfect kind. In respect to the 
authority of a Mons. Biot’s advertisement in a French news- 
paper, which Dr. W. has imported and translated as a certi- 
ficate to gratify the “ best acquainted with the merits of the 
question ;”’ in my opinion, instead of supporting his case, it 
both disgraces and injures it. 
Mons. Biot, no doubt, is a reputable astronomer, and 
an able mathematician; but as a skilful practical optician 
I cannot give him equal credit. He ingenuously states 
that he proposed a trial of the glasses convex without and 
concave within to his friend Mons. Cauchoix, and requested 
his opinion on the subject. The boasted and surprising 
effects of a pair of these spectacles are afterwards stated ; 
and Mons. Biot, as a faithful friend to Dr. W., declares he 
will never wear any others: but there afterwards pops out an 
“< inconvenience” of an appearance of *f a variety of reflected 
images beside the principal object viewed, which occasioned 
some confusion.” 
Mons. Cauchoix, however, happily hit upon an expedient 
to remove this inconvenience, by diminishing the destructive 
concavity of the inner surface; or, in other words, to go 
cautiously back towards the double convex form he had 
vitiated.—So did Mr. Dollond’s workmen : but unhappily 
more; for, without waiting for a Ait, and regardless of posi- 
: tive, 
