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XIX. On a Periscopic Camera Obscura and Microscope. By 
William Hyde Wollaston, M. D. Sec. R. S. 
Read June 11, 1812. 
Although the views, which I originally had of the advantage 
to be derived from the periscopic construction of spectacles,* 
naturally suggested to me 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 ob- 
scura, where the portion of lens requisite for sufficient illumi- 
nation, 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. 
I have therefore had recourse to experiments, in order to 
determine by what construction the field of distinct represen- 
tation may be most extended ; and, I trust, the result will be 
acceptable to this Society. I shall take the same opportunity 
* Phil. Magaz. Vol. XVII. Nicholson’s Journal, VII. 143. 
