J44 ^^ SPECTACLE GLASSES. 



" and confequently this figure of a glafs is the bell for tpet- 



tacles, as the double concave of like figure is the beft to help 



ftiort-fighted perfons/' 



This is gooa for g^t although it may be very true that fuch a form of glafs 



Unkt °bit not ^^^ ^^^ Calculated for the objed glafs of a teleicope, previous 



for fpeftacks. to the celebrated difcovery of the achromatic objecl-glafs by 



the late by Mr. Dollond, yet whatever advantages might at 



any time be expe6led from the telefcopic objeft-glafs fo fiiaped, 



thefe were not to be obtained by a fimilarconftrudlonin fpec- 



tacles, as may eafily be feen by coniidering the different ufes 



of the refpedlive inftruments. 



Telefcopic vifion In a telefcope in the firft placci our view is neceflarily 



is nearly in the ^^^^^^^ ^^ ^^^^ diftance on each fide of the axis ; and 



wis: ■' r L 1 



fecondly, every part of the obje6l glafs contributes to the 



dif^in6lnefs of any objed viewed. 



and the whole Ifc is under thefe circumftances alone that the proportion of 



ITc. ""Radiant *'^^ curvatures above-mentioned, might be proper for a fingle 



point. objed glafs, as being capable of colleding into the focus tlie 



rays that fall on every part of it parallel to the axis; 

 fn fpeaades it By Ipec^acles on the contrary, obje6ts are to be viewed if 

 is very difierent. pQ(]^i3ie j^ every diredion in w'hich they might be feen by the 

 naked eye, which is often far removed from the centers of the 

 glades ; confequently a conftitution that is calculated to re- 

 •prefent corre6lly central objects alone, cannot be the mofl: ad- 

 vantageous, 

 A fmall part of j^ thefe alfo, the portion of the glafs employed at once is 

 ImptTedat' " ^arcely larger than the pupil of the eye, fo that any endea- 

 ©nee. vour to procure the concurrence of all parts of a glafs in any 



one effe6t is evidently fuperfiuous, and may alfo be (hewn to 

 be prejudicial. 

 Whence im- It is therefore propofed to remedy the imperfe6lions ob- 



bc7oTht fr'Tm^ fervable in the fpeaacle glafles hitherto generally ufed, upon 

 different con- a principle fuggefted by this latter confideration, which prefents 

 fiderations. ^^ opportunity by a different confiru6tion of rendering objects 



in all diredlions diftind. 

 iJadTconcave '^^^ alteration requifite for this purpofe, is extremely 

 l»cxttheeye all fimple, and eafily intelligible. Suppofing an eye to be placed 

 the rays may be -^^ ^^^ center of any hollow globe of glafs, it is plain that 

 through nearly obje6ls would then be feen perptndicularli/ through its furface' 

 perpcadicular. ]„ every dire61ion. Confequently the more nearly any 



fpedack* 



