196 REMARKS ON SPECTACLES. ^ 



Remarks on a concave Icn?, and diminifbes. It has alfo been demonftraled, 



Jenfes} to al- ,1,1 1 r r •/• 1 /• 



certain the valve "1^* '"^ nearer the form of a menilcus approaches to that or a 

 of Dr. Wolla- plano convex or concave, the more perfeft it will be, and 



fton's patent 1 1 r 1 • 



fpcdlaclcs. produce lets aberration. 



I ftiall difpenfe here with the proofs by algebraical and 

 analytical formulas, as any qualified reader will find them in 

 the optical works of Huygem, Molineux, Euler, D*Alembert, 

 Smith, Emerfon and Martin ; and many others. 



The rays of light ifluing from a near objed to a fpedacle 

 glafs before the eye, are in diverging pencils or cones, and 

 the menifcus form of glafs of any certain pofitive focus, will 

 refract them towards a flate of parallelifm into the eye, necef- 

 fary to produce ditlind vifion, in decayed fight, pre- 

 cifely in the fame manner as a double convex or piano convex 

 glafs of the fame focus would do. A menifcus with a negative 

 focus, ads no ways diflferent from the double or piano concave 

 glatfes; the rays of light being divergent fomewhat to coun- 

 teract the eflfeds of too great a convexity in the humours of 

 the eye of a ftiort fighted perfon. Perhaps, it is hardly 

 neceffary to obferve, that imperfe6l vifion in the optical fenfe, 

 confifis in the long fighted eye, in the rays of light not being 

 fufficiently converged by its humours, to meet on the retina 

 of the eye, but falling beyond it ; and in a fliort fighted eye 

 by the rays converging too much, fo as to meet before they 

 reach the retina. 



Varying the geometrical figure of a lens, does not con- 

 ftitute any new optical principle, for any of the common 

 fpecies of lenfes, may be cut into the form of a fquare, a 

 triangle, an oval, &c.*all figuratively various, but confifting 

 only of one optical principle. 



The ufe of the menifcus has been abandoned by Opticians, 

 by its containing in comparifon with other lenfes, the greatefi: 

 fpherical furface, and confequently producing the greatell 

 aberration. Reducing the curvatures of the menifcus elongates 

 the focus, in the fame manner as in other lenfes, and there- 

 fore reduces the aberration. Hence in fpedacle-glalfes that 

 are not of fliort foci, no preceptible diflference will be found 

 » to perfons unacquainted with optical experiments. 



There are various pradtical methods that will point out to 

 perfons the aberration of lenfes here fpoken of, and that the 



minifcus 



