huprovemcnt in the Foi'm of Spectacle Glasses. 327 



The muriatic acid becomes very strongiv oxygenated 

 when dissolved in hvperoxygenated sulphate of iron, which 

 thereby acquires a yellowish colour. 



The hvperoxygenation of iron increases its affinity for 

 acids in such a manner that callico-printers make no use of 

 an acetic solution of hypcroxygenated oxide of iron, as it 

 does not readily give up its acid by drying. 



LXI. On an Improveinent in the Form of Spectacle Glasses. 

 By Wm. Hyde VVollastox, M. D. F. R. S. * 



XT must have been remarked by persons who make use of 

 spectacles, especially those who require glasses of short 

 tbcal distance, that objects seen through them appear di- 

 stinct onlv^ when viewed through the central parts of the 

 glasses; that, when the direction of the sight E O, Fig. 1. 

 (Plate IX.) is considerably inclined to the surfaces, objects 

 appear distorted, and that this defect is greater in propor- 

 tion to the greater obliquity of that line. 



It is on this account that opticians have lately made and 

 recommended spectacle glasses of less diameter than those 

 formerly in use, thinking that the extreme parts of the field 

 of vision, which from indistinctness were of little use, might 

 be spared without nmch inconvenience. But this altera- 

 tion in the size of the glasses could hardly claim the merit 

 of an improvement, since for one defect it only substituted 

 another scarcelv less objectionable. 



It seems indeed rather extraordinary that during five 

 centuries which have elapsed since the invention of specta- 

 cles, neither theory nor accident should have produced any 

 considerable variation from their original construction. 



It was indeed conceived by Huygcns f, that the glasses, 

 instead of being equally curved on both sides, as is cus- 

 tomary, should have tlie curvatures of their opposite sur- 

 faces in the proportion of 6 to 1, because he had demon- 

 strated that such a form was best suited to the object-glasses 

 of telescopes. 



Dr. Smith also, in his Treatise on Optics (p. 259.), re- 

 peats this opinion of Hiiygens in the followmg cursory 

 manner: "And consequently this figure of a glass is the 



* Communicated by the author. 

 t Diopti'. Prop. 2S. 



X 4 best 



