Vnlhe Form of Spcclade-Glasses. 69 



uecessan' to produce disunct vision in decayed sight prc- 

 tjiselv in the same nianuf-ras a double convex, or plaiio-coii- 

 A'C'.x. A meniscus, with a negative t'ociis, acts noways dir- 

 ferently from the double or plano-concave glasses, the rayi 

 of light beino diverged somewhat to counteract the effects of 

 loo great a convexity in the humours of the eye of a short- 

 sighted person. l\'rhaps it is hardly necessary to observe, 

 that imperfect vision in the optical sense consists, in the 

 long-sigiUed eye, in tiie rays of light not being sufficiently 

 converged by its humours to ni/Cet on the retina of the eye, 

 but fall bevond it ; and in a short-sighted eye by the rays 

 converging too much, so as to meet before they reach the 

 retina. 



Varying the geometrical figure of a lens does not consti- 

 tute any new^ optical pnuciplc; for any of the com'moiii 

 species of lenses may be cut into the form of a square, a 

 triangle, an oval, &c., all figuraLivdy various, but consisting 

 only of one optical pruiciple. 



The use of tlic mcnisctas has been abandoned by opticians, 

 by its containing, in comparison with other Jenses, the 

 greatest s])hericaf surface, and consequently prcKlucing the 

 greatest aberration. Reduchig the curvatures of the menis- 

 ■cus elongates the focus, and the same manner as in other 

 knses, and therefore reduces the aberration. Hence, in 

 spectacle-glasses that are aiot of short foci no perceptible 

 <lifierencc will be found to persons unacquainted with op- 

 tical experiments. There are various practical methods that 

 will point out to persons the aberration of lenses here spoken 

 of, and that the meniscus causes the greatest of any of the 

 otlier form of lenses; but the following 1 recommend as 

 the most easy and illustrative: 



Take a meniscus lens aljout the size of a spectacle-glass, 

 and with four iix'hcs positive focus, and take also a plano- 

 convex, or a double convex of the same diameter and focus, 

 in a room wnth one lighted candle ; at a distance, by night, 

 hold the convex glass near to the white wall or wainscot side 

 <)f the room; bersxeen it and the candle move the lens back- 

 ward and forward, till a clear image of the candle be formed, 

 \vhich will Ik; a distant inverted image of it. Do the sa;ne 

 with the meniscus, and there will be this difference observed 

 in the meniscus, that, encircling the vivid image of the 

 flame, there will be a faint white light, which is the circle 

 of the aberration. 'J'hese evidentb' show that the nnui-cus is 

 the w orst form of the two for a spectacle-glass or any other 

 purpose. 



E3 li 



