68' On the Form of spectacle- Gtasser.- 



The observation of Dr. W., tlmt only a portion of the gfssff 

 a little larger than the pupil of the eye is employed at once^ 

 is only just in as much as it relates to the mind being intent 

 on a point of an object, but mu so in regard to a general 

 view •;. for the refractive power of the lens- does most adnii- 

 rai)lv collect all the iniinitc number of pencils of rays or 

 cones i.=nto one assemblage at the pupil' of the eye, where 

 they cross o-f intersect eacli other: yet sirch is the ey-nuisite 

 3-ubtiltv of liszht-, that no confusion or irritation takes place,. 

 Man is thus blessed by assisted vision, as he is in vitality by 

 tlie respiration of air. Dr. W.'s inferring the fonn of a me- 

 niscus from the shape of a globe is manifestly erroneous, 

 and in respect to spectacles inappropriate : a glass globe or 

 sphere, without any sensible thickTie??:^ to a-n eye exactly 

 piseed in its centre admits all the incident rays to pass 

 throuoh it unrefractcd. If the eye deviates- from the centre, 

 a refraction will take place, and that in proportion to tlie 

 thickness of the sphere. Rays of incidence pass perfectly 

 unrefractcd throngh a true ground plane or parallel glass 

 to an eye before i-t ; and let the axis of the eye be ever so 

 much inclined, unless the glass be very thick, the object 

 will still -appear perfect^ ' and no refraction of the incident 

 ravs be observed. It is obvious, therefore^ the nearer a lens ap- 

 proaches to the figure of a plane, the more perfect it must be. 



The fisrure of a fneniscu*, which Dr. W. wi.-hes to adopt^ 

 is as diflerent tixnn a sphere as a plane,- Its figure is com- 

 posed of two positions of spheres, of different radii. Whca 

 wilh a positive focus, it is mathenu\iically demonstrable that 

 it has entirely the properties of a convex lens, and, with a 

 ncirative focus, the properties of a C(nicave one. When the 

 radius of the exterior curve is less than that of the interior, 

 it is a eon.vcx sort of lens, and magnili-js ; but when the ra- 

 dius of the interior curve is less than that of the other, a 

 concave lens, and diminishes. It has also been demonstrated,. 

 lh;'.t the nearer the form of the meniscus approaches to that 

 of a plano-convex or concave, the more perfect it will be, 

 and contain le.^s aberration. 



t slial! dispen-se here with proofs by algebraical and ana- 

 lytical forni-al:e, as any qualified reader will tltu-l them in the 

 optical works of [{nifger/.i, JMo/ineux, E:ilci-,, D-' yilci/iicrt, 

 Sw'iilt, Emerson, 37a.'-/ d«. and many others. 



The ravs of light issuing fsoni a near object to a spcctacle- 

 gla?s before the eye aire in diverpjing pencils or cones, and 

 the meniscus form of g!:m, of any certain positive focus,, 

 wilj rcirael tU«;ai to-.vards a state of pafallelisin into the eye. 



ncees- 



