stated Improvement of the Camera Olscura, d^c. 251 



of opening, E, whatever will change Nature's laws of 

 refraction so as to elongate the focus, or to produce two 

 different focuses in one lens ; and his previous explanation 

 of *• occasioning all pencils to pass as nearly as may be 

 at right angles to the surfaces of the lens," page go, is an 

 irrelevancy in optics, and is the error of reasoning I for- 

 merly imputed to Dr. VVollaston in his spectacle glass. It 

 is the angle that the rays make with the axis of the lens, of 

 whatever shape, that refraction is estimated from, as the 

 science teaches us ; but not from the geometrical positions 

 of pencils and surfaces. From the greater aberration that 

 the meniscus possesses, the images formed by it will be less 

 distinct, have less light, and be more distorted than by the 

 doulile convex lens. It is from the extended lateral distor- 

 tion, and bringing the meniscus nearer to the plane than 

 its exact focus, that I can assign a cause how Dr VVollas- 

 ton could have fallen into the error: had he placed the 

 concave side downwards, it would have been a better posi~ 

 tion, the images would have been more defined and en- 

 lightened; it was so applied in his spectacles, the convex 

 side being next to the object ; but in neither case will the 

 images be so perfect and vivid as by the double convex lens. 

 The meniscus in a camera is not a new application ; several, 

 some years back, were made, but not preferred. 1 can re- 

 fer to a machine now existing with one. I have caused 

 two lenses to be ground, one a double convex, the other a 

 meniscus, as Dr. VVollaston directs, of the same diameter, 

 nearly four inches, and focus twenty-two inches, which ex- 

 perimentally verify the correctness of my observations, and 

 which gny intelligent person may inspect, by application at 

 our manufactory, 30, Holborn. 



The following quotations may, to some of your readers, 

 better corroborate the truth of my remarks : 



" If the side were concave (of a piano) so that the leiig 

 became a meniscus, there is no proportion of the radii or 

 position of tlie lens, with regard to the radiant, but what 

 will give the aberration greater than the plano-convex in its 

 best position ; and since this was first observed by opti- 

 cians, the meniscus began to lose ground in the construc- 

 tion of optical instruments, and is now quite rejected." 

 Martin's Elements of 0[)tics, 1759, page 29. 



An oblique pencil of rays has its focus a little nearer the 

 Jens (double convex) than a direct pencil. Cor. fig. 9. 



This proposition holds good in a concave lens, and also 

 in a meniscus, as well as in a convex one. Emerson's 

 Optics, page 124, prop. 21. 



« When 



