126 On a perlicopic Camera Olscura 



may require some explanation. It has been already ob« 

 served, that by the common lens, any oblique pencil of rayi 

 is brought to a focus at a distance less than that of the 

 principal focus. But in the construction above described, 

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

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

 effect of the first surface is, to occasion divergence of parallel 

 rays, and thereby to elongate the focus ultimately produced 

 by the second surface, and since the degree of that diver- 

 gence 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 oblique as to increase the conver- 

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

 nearer to the lens than the centre of its second surface, that 

 oblique rays E/", after being refracted at the first surface, 

 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 y") falls -very nearly in 

 ihe same plane \\\\\\ that of an object centrally placed. 



In the use of spectacles by long-sighted persons, the 

 course of the rays in the opposite direction is so precisely 

 similar, that the same figure might serve to illustrate the 

 advantages of the periscopic construction. For the pur- 

 pose of seeing the extended page of a book (as at AB) with 

 least fatigue to the eye, that form of lens will be most be- 

 neficial, which renders the rays received from each part of 

 its surface parallel ; and this is effected by the exact coun- 

 terpart lo the preceding arrangement; for in this case the 

 opening E represents the place of the eye receiving parallel 

 rays from the lens in each direction, instead of transmitting 

 ihcm from a distance towards it. 



There is, however, this difi'erence between the cwo cases, 

 that in the camera obscura a much larger portion of the 

 lens is required to conspire in giving a distinct image of 

 any one ol)ject ; so that the conformation best adapted for 

 lateral objects would not be consistent with distinctness 

 at the centre; and hence arises a limit to the application of 

 the principle. On the common construction, the whole 

 lens is so formed as to give brilliancy and distinctness at 

 the centre alone, without regard to lateral objects. In 

 adopfing such a deviation from the customary form, as I 

 propose, in favour of a more extended,vievv, some diminu- 

 lion of the aperture is required in order to preserve the 

 *lesired distinctness at the centre. In my endeavours to 

 asceriaia the most eligible form of meniscus for this pur- 

 pose. 



