ON PERISCOPIC SPECTACLES. QQ 



with double convex glafles of tlie fame diameter and focus, 

 I provided a pair of our own n-ianufa6tuTe, and as fold by us 

 at 3.y. 6d. Thefe two pair of fpectacles were attentively 

 compared together, by myfelf and feveral judicious and im- 

 partial perfons, in the manner as ftated by Dr. Wollafton of 

 •his. Therefult was as follows. 



The convex glades being applied as clofe as poffible to the Refult. With 



eyes, with the frame attached to the head, the print of a large '^^ '^''"^'^ ^°'^' 



■ i-n . ,. ,. vex lenfes. 



quarto page was viewed through them, at adiftance tor diltinci Extent of dif- 



vifion at their centres, the letters at the diftance of about 25 '^'"'^ vifion. 

 lines, appeared quite diftinft or well defined ; giving the axis 

 of the eyes a little obliquity to difcriminatemore lines, an indif- 

 tinftnefs or confufion of letters commenced, increafing to- 

 wards the extremity of fight, and from the lateral aberration 

 of the Jenfes, the letters were tinged with the prifmatic 

 colours. Keeping the head fixed in the fame pofilion, the with the perl f- 

 perifcopic glalTes were fubflituted. The extent of diftinft ^^opi'^s'^'^"* 

 letters without diftortion was nearly as great, but the coloured more colour, 

 letters were evidently nearer to the centre, and more numerous 

 than by the other glatfes. By inclining the axis of the eyes 

 ftill more than in the former cafe, or looking extremely afquint 

 through the glafles, a greater extent of lines was obferved, 

 but blended with colour and confufion. The optic nerves 

 felt a fenfible irritation, evidently from the fquinting pofilion 

 of the eyes, a refradion of many fuperfluous rays, and the 

 oonfequent increafed and unufual magnitude of Ihe images on 

 the retina. The pain in the eyes mentioned by Dr. Wollafton, 

 mufl have arifen only from this circumflance, and not from the 

 one he reprefented it to be. By a trial of the old menifcus Trial of anothar 

 glafs I before mentioned, which is of four inches focus, and menifcus. 

 correfponds with what he has a patent for, in comparifon with 

 one of the above piano convexes, the view of letters was flill 

 more extended, but illegible and with much colour, and like 

 the others towards the extremity, of no fort of ufe for the 

 purpofes of vifion. Now all this is conformable to the laws 

 of optics, and manifefts a properly different to that advanced 

 by Dr. Wollafion. 



Thefe feveral glaffes are alfo at the public fervice for in- 

 fpe6lion in our fliop in Holborn. 



By making the glafles of the above perifcopic fpe6lacles Concluding re^ 

 iiearly pianos. Dr. Wollafton's principle is (Jeflroyed, and my "^^'^^^' 



opinion 



