438 



On a Periscopic Camera Obscura and Microscope. By William Hyde 

 Wollaston, M.D. Sec. R.S. Read June 11, 1812. [Phil. Trans. 

 1812, ;?. 370.] 



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 di- 

 stinctness 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 di- 

 stinctness of objects centrally situated. The radii of curvature here 

 preferred are in the proportion of 2 to 1 . 



A meniscus so formed is placed with its convex surface towards the 

 plane of representation, and with its concavity toward the objects. 



The aperture of the lens itself is made as large as the tools will 

 conveniently admit, but the pencil of rays admitted to it in each di- 

 rection is limited by a diaphragm, placed between it and the objects 

 at about one eighth of its focal length from the lens, and having an 

 opening about one tenth or one eleventh of the focal length in dia- 

 meter. 



By means of a camera thus constructed, the author observes, that 

 objects as far as 30 removed from its centre are represented with 

 considerable distinctness on the same plane as objects centrally 

 situated. 



The author also describes a microscope which he terms periscopic, 

 because it is formed on the same principle of endeavouring to place 

 all portions of a lens at right angles to the pencils transmitted through 

 it in different directions. In the microscope this is effected by placing 

 two plane convex lenses on opposite sides of the same aperture in a 

 piece of thin metal interposed between their plane surfaces. The 

 central opening being by that means nearly in the centre of curva- 

 ture, no rays are suffered to pass to the eye but what are at right 

 angles to both surfaces ; and a field of view of at least 20 is obtained 

 with a greater degree of illumination than can be admitted in the usual 

 construction. 



The paper is accompanied by a diagram for determining, by in- 

 spection, the various combinations of radii that will give to a menis- 

 cus any required focal length. 



