ON SPECTACLE GLASSE!?. 243 



and on the fouth it is terminated by mountains covered with 

 wood, the neareft of which appeared to me to be volcanic. 



At length I arrived at Bafnheim, remarkable for nothing but ^^^if^^^-^^ 

 the EngliQi gardens belonging to the lord of the manor; and Fuojice ftoncoo 

 thence to Coblentz. I found pumice Hone on the road, and ^^ ro-Jdandia 

 m itrata where pits had been dug in the ground. 



XIII. 



On an hnprovement in the Form of SpeStacle-GlaJfes, i»yWiLLi am 

 ' . HYDb Won. A ST ON, M D. F. R. S. Communicated by tht 



.Tmufl h^ve been remarked by perfons wbo make "^^ of J"^Frfcaion of 

 fpedlacles, efpecially thofe who require glaflTes of thort focal objeas out of 

 diftance, that obje6ls feen through them appear diftin6l, only <^enter are diT- ^ 

 when viewed through the central parts of the glalTes ; that 

 when the dire6lion of the fight E O, Fig. 1 . page 14-5 is con- 

 fiderably inclined to the furfaces, objects appear diftorted, and 

 that this dcfe<5l is greater in proportion to the greater obliquity 

 of that line. 



' It is on this account that opticians have lately made, and j^cwedy fonncr^ 

 recommended fpe^Sicle-glafles of lefs diameter than thofe make die glaffes 

 formerly in ufe, thinking that the extreme parts of the field Smaller j— 

 of vifion, which from indiftin<5inefs were of little ufe, might 

 be fpared without much inconvenience. But this alteration but to iJttIc 

 in the lize of the glafles could hardly claim the merit of an ^^^^» 

 improvement; fi nee for one defeat, it only fubftituted another 

 fcarcely lefs objed^ionable. 



It feems indeed rather extraordinary, that, during five 

 centuries which haveelapfed fince the invention of fpedlacles, 

 neither theory nor accident fhould have produced any con- 

 tiderable variation from the original conflrudion. ' 



It was indeed conceived by Huygens, that the glafies, in- Huyghens pro- 

 (lead of being equally curved on both fides, as is cufiomary, P"J5.^ that the ^ 

 (hould have the curvatures of their oppofite furfaces in the ture In fpefta- 

 proportion of fix to one, b6cauf<? he had demonfirated that *^'^ 8^*'^^^ ^o"'** 

 fuch a form was befl fuited to the objefl glafies of telefcopes. 



Dr. Smith alfo in his treatife on optics, (p. 258.) repeats Repeated bjr 

 this opinion of Huygens in the following curfory manner ; ^^^^^' 

 e , and 



