stated Improvement of the Camera Olscura, t^Jc, 249 



will, bv experiment, be rendered very manifest, and which 

 have been clearly demonstrated by that learned optician 

 Mr. Benjamin Mariin, in his Elements of Optics, by Dr. 

 Smith, and by others. 



The siibseqaent paragraph, at page 91, describes Dr. 

 Wollaston'd proposed improvement: the substance, in his 

 own words, is as follows : 



" Ihe lens is a meniscus, with the curvatures of its sur- 

 faces about in the proportion of two to one, so placed that 

 its concavity is presented to the objects, and its convexity 

 towards the plane on which the images are formed. The 

 aperture of the lens is four inches, its focus about twenty- 

 two. There is al^o a circular opening, two inches in dia- 

 meter, placed at about one-eighth of the focal length of the 

 lens from its concave side, as the means of determining the 

 quantity and direction of rays that are to be transmitted. 



*' The advantage of this construction over the common 

 camera obscura is such, that no one who makes the com- 

 parison can doubt of its superiority ; but the causes of 

 this njav require some explanation. It lias been already 

 observed, tliat by the common lens, any oblique pencil of 

 rays is brnuorht to a focus at a distance less than that of the 

 principal focus. But in the construction above described, 

 the focal distance of oblique pencils is not merely as great, 

 but is greater than that of a direct pencil. For since the 

 cBtct of the first surface is to occasion divergence of 

 parallel rays, and therebv to elongate the focus ultimately 

 produced by the second surface, and since the degree of that 

 divergence is increased by obliquity of incidence, the focal 

 length resulting from the combined action of both surfaces 

 will be greater than in the centre, if the incidence on the 

 second surface be not so o!)Iique as to increase the conver- 

 gence. On this account, the opening E is placed so much 

 uearer to the lens than the centre of its second surface, 

 that oblique rays !E/, after being refracted at the first sur- 

 face, are transmitted through the lens nearly in the direction 

 of its shorter radius, and hence are made to converge to a 

 point so distant that the image (at/) falls very nearly in 

 ihe same plane with that of an object centrally placed." 



The radii of curvatures lor a meniscus of twenty-two 

 inches focus, being as two to one, is not essential. The 

 theory of Dioptrics shows that the greater the proportion, 

 pr the nearer that the radius of one side approaches to in- 

 finity or the side to a piano, the more perfect the lens will 

 ^)c. Dr. Wollaston has not stated the diameter of the con- 

 vex lens, but the reader must suppose it to be four inches, 



like 



