§65 Observations on the Camera Olscura, ^c. 



" VVlien paralli-l vays fall upon a plane side of a plano- 

 convex glass, the aberralion of the extreme ray, which is 

 ■f. of the thickness, is less than the Hke aberration cansed 

 by anv meniscus glass whose concave side is exposed to 

 the incident ray. 



*' When the said glasses have their convexities turned to 

 the incident rays, tlie aberration of the extreme ray in the 

 plano-convex, which is now but ^ of its thickness, is less 

 than the like aberration of any meniscus in this position. 



" The best of all double concave glasses has the semi- 

 diameters of its first and second .concavities as 1 to 6 ; and 

 consequently this is the best figure of a glass to help short- 

 sighted persons, as the double convex one of the like figure 

 is the best for spectacles." Smith's Optics, art. 661, 662, 

 665. 



" For since a meniscus, unless the surfaces of it are pa- 

 rallel to one another, has the same effect either tliat a con- 

 vex lens or a concave one would have, all the cases ot di- 

 verging or converging rays that are refracted by it will be 

 the same with those already explained in the instances of 

 convex or concave lenses." Rutherford's Plvilosophy, vol. i. 

 page 286. 



*' A plano-convex glass, with its j pi" ^e'' [• side towards 

 the incident parallel rays, has less aberration than any 

 meniscus with its \ conowe f *^*^^ exposed to parallel rays. 

 Whence it necessarily follows, that that meniscus is best, 

 which approaches nearest in shape to a plano-convex lens." 

 Harris's (of the Mint) Optics, 1776, page 67*. 



The sort of French angle of reduction that Dr. Wollas- 

 ton has given to obtain geometricaliv, but nearly, the radii 

 of a meniscus for a given focus, will be useless to the work- 

 man, as he already knows by a very short arithmetical 

 operation, how to obtain exactly such radii in half a mi- 

 nute's time, or a tenth part of the tiirie necessary to con- 

 struct that problem : by Gunter's sliding rule the tinte 

 would be still shorter. 



The combination of using two glasses in ordinary sim- 

 ple m.icroscopes, or hand magnifiers, to diminish the errors 

 arising from the spherical figure of one glass, was known 

 to Sir Isaac Newton, and to successive opticians. That late 



• So sensible have some optical glass-grinders been of the impracticability 

 and insufficiency of the meniscus glasses of very short foci for spectacles, 

 that I have in my possession some plano-convex and plano-concave glasses 

 actually fitted in the frames, and sold for the new ppiscuj)ic glasses. 



excellent 



