On a periscopic camera obscura and microscope

The same views which the author originally had of the periscopic construction of spectacles, naturally suggested to him a corresponding improvement in the camera obscura. But though it is evident that enlargement of the field of view, and some improvement in the distinctness of images obliquely situated, might be made by causing each part of a lens to be as nearly as may be at right angles to each pencil of rays that are to be collected to a focus, it was not clear to what extent this principle could be carried where the portion of lens is necessarily large, and consequently includes parts that may require a different form for preserving distinctness of other pencils of rays. He has, accordingly, made trial of different forms of meniscus for this purpose, and describes the construction which appears to possess the greatest advantage with least detriment to the brilliancy and distinctness of objects centrally situated. The radii of curvature here preferred are in the proportion of 2 to 1.