38 ON PERISCOPie SPECTACLES, 



X. 



All Exainination of Dr. Wollajion's EKperiment on his Perifcopic 

 Spedades. By Mr. William Jones, F. Am, P. S. 



To Mr. NICHOLSON. 

 SIR, 



TerSs"^"^' The inferences that Dr. Wollafton has thought it beft to 

 pubiifh in your laft month's Journal, inftead of a direft reply 

 to my refutation of his new principle of fpedacle glafTes, are 

 of themfelves fufficient to convince afty impartial perfon of the 

 validity of the obje6tions advanced by me in your preceding 

 Journal; and, notwithftanding an extraordinary experiment 

 he has therein related, as made only by himfelf, I fliould not 

 have thought it requifite to trouble your readers again, but 

 for the unfounded imputation fhat he has declared againft me, 

 that of having by an experiment deceived myfelf. I truft, Sir, 

 I may be allowed, in contradiction to this, to obferve, that, 

 after more than 20 years experience in the pra6tice of my pro- 

 feffion, fuch as daily adminiftering to decayed vifion, and 

 employment in the conftruftion of all kinds of optical in- 

 flruments, I Ihould not be acquainted with all the various 

 properties of lenfes, iingly or combined, and efpecially of fo 

 firaple and well known a form of lenfes as adopted by him, 

 is an idea, that I am confident he will not be able to imprefs 

 upon the minds of the public. I fuggefted no new experi- 

 ment, nor was any one wanting; the definitive laws I ad- 

 duced, were contained in the v/orks of the beft writers on 

 optics, and were fufficient to evince the want of originality 

 and improvement of his menifcus fliaped lens. In refpe6t to 

 the experiment by which he attempts to inforce a proof of an 

 advantage in his fpe6tacles, its value will be known by thq 

 following account of a repetition of it. 

 Experiment v/ith I am poflefled ctf" a pair of his perifcopic glatTes, mounted 

 a pair of perifco. ;„ a {mg\Q flcel frame, which coft 10*. 6d. The glatTes, I 

 compTre/whh a muft obferve, are different to his propofed form, having of 

 pair of the ufual each, the inner fide, or that next to the eye folittle incurvated, 

 conftrudtion. ^j^^^ ^^ ^i^^, perfon but an optician, they would be called piano 

 convexes. The focus is four inches, the fame as ufed by 

 Dr. Wollallon in his experiment. In a fimilar mounting^ 



with' 



