REMARKS ON SPECTACLES. 197 



meniTcus caufes thegreatefl of any of the other forms of lenfes ; j^^^g^';''^^^''"^^ 

 but, the following, 1 would recommend as the mod eafy and certain the valve 

 illuftrative. ofDr.Wolla- 



Take a menifcus lens about the fize of a fpe6lacle-glafs, fpe^adcj^"^ 

 and with four inches pofitive focus, and take alfo a piano con- 

 vex or a double convex, of the fame diameter and focus, in a 

 room with one lighted candle, at a diftance by night, hold the 

 convex glafs near to the white wall or wainfcot fide of theroom, 

 between itandthecandle;movethelens backwards andforwards 

 till a clear image of the candle is formed, which will beadiftin^ 

 inverted image of it; do the fame with a menifcus, and there 

 will be this difference obferved by the menifcus, that incir- 

 cling the vivid image of the flame, there will be a faint white 

 light, which is the circle of the aberration ; and, evidently 

 fliews, that it is the worft form of the two for a fpedacle- 

 glafs, or any other purpofe. 



Two convex double glalTes placed together in one cell, 

 contain lefs aberration than one glafs of the fame diameter 

 and focus ; and two piano convex-glafles with their convex 

 fides placed together in one cell, give ftill lefs aberration. 

 It is lofs of light only that can be objected to. They are too 

 weighty to be adopted in fpedacles, but in eye pieces of large 

 telefcopes for viewing celeflial obje6ts, they have been ufed * 



with great advantage. To engravers, miniature painters, 

 and other artifls, they are moft ufeful, as by (hort foci and 

 large apertures, they give them the moft diflin6l view of a 

 large furface placed before them. 



For the fatisfadion of an intelligent perfon, who may be 

 difpofed to have an ocular proof of the properties of glalTes 

 as herein advanced, I have confl:ru6ted a frame, containing a 

 i double convex, piano convex, a menifcus, two piano con- 

 I vexes, with their convex fides to each other, all of the fame 

 diameter, and the foci about four inches, and by which may 

 be feen that the greateft peripheretical indifiin6lnefs is with 

 the menifcus glafs. This apparatus will be fliewn by ap- 

 plication at our manufaftory in Holborn, 



The menifcus as a figure for a fpe6tacle glafs I conlider very 

 objedionable. To afford a large field of view, its diameter 

 muft be confiderable; which for a (hort focus will increafe 

 thicknef-?, protuberance outwards, and weight; and, in con- 

 cave glaflTes occafion the frames to be made thicker; theglalTes 



Will 



