XIX. On a periscopic camera obscura and microscope

Abstract

Although the views, which I originally had of the advantage to be derived from the periscopic construction of spectacles, naturally suggested to me by a corresponding improvement in the camera obscura , by substituting a meniscus for the double convex lens, I have hitherto deferred making it known to others, except as a subject of occasional conversation. Since in vision with spectacles, as in common vision, the pencil of rays received by the eye in each direction is small, the superiority of that form of glass, which disposes all parts of it most nearly at right angles with the visual ray, admits of distinct demonstration; but with respect to the camera obscura, where the portion of lens requisite for sufficient illumination, is of considerable magnitude, although it is evident that some improvement may be made in the distinctness of oblique images on the same principles, yet as the focus of oblique rays is far from being a definite point, the degree in which it may be-improved is not a fit subject of mathematical investigation.